Amateur Astronomy
Everything I wish someone had told me when I got started
There is a lot of information out there on Amateur Astronomy… a whole lot. There’s so much that it’s hard to know where to start, so that is what we’re going to do here - show you one way to get started and where to find additional information. We’ll keep things in a logical order that build on each other and provide you with a good foundation to follow up on anything that catches your interest in this hugely broad hobby. We’ll also provide you with a list of things you might like to get interested in.
Amateur Astronomy can be very formal and technical: Expensive optics, GPS enabled robo-scopes, plate solving programs, imaging equipment with incredible resolution, and lists of unbelievably faint and obscure objects. One of the great things about the hobby is that you can be just as much an Amateur Astronomer as anybody else if all you have is an economical 80mm Achromatic refractor and a star-finder app on your phone.
All you really need is a clear night, a star chart, a blanket or comfy chaise, and maybe a pair of binoculars.
Here we are not going to cover astro-imaging, the nuclear chemistry of stars, the physics of black holes, or the complex math behind celestial mechanics. There are much better resources available for those kinds of details and we’ll include a few references for those too.
What we are going to do is give you a good introduction to what there is to see in the sky and equip you with the basic skills to locate and observe anything there is… within the limitations of your equipment, your eyes, and that particular night’s sky. :-)
You won’t find anything new here - everything here has been developed and published all over the place by professional and amateur astronomers over the past 200 years or more. What you will find here is one place to keep you focused and get you back on track when you wander off into the internet search weeds.
One more thing: Even the basics can cover a lot of information. Don’t be intimidated. Enjoying the sky does not require a mastering of all we’re going to present. You choose what works for you. Just get out there.
It’s best to look at these in order but if you’re already familiar with some of it, then jump in anywhere. There’s really no wrong way to learn all this… but we think this is a pretty good one. :-)
BTW - This is a work in progress. We’ll start with an outline and then provide the details as we go. If a topic you’re interested in is not yet filled in, please come back later and check.
Once we do get everything filled in we’ll still be modifying the content per your feedback and adding to or changing the references when we find something we like better. We’ll also be adding images from our Club members.
Let’s get started.
Fred Rains
Outreach Coordinator
Birmingham Astronomical Society of Alabama
Goals and Outline
Our Goals:
1. Show you some of the things you can see and give you an idea of what they’ll really look like.
2. Show you three ways to find things.
3. Help you understand the limitations of the sky, your eyes, and equipment.
4. Show where to go next.
Outline:
1. Start with Safety
2. The Planisphere
3. Things to See
4. Magazine and Internet Images vs an Eyepiece
6. How the Sky Changes Hourly, Daily, and Yearly
7. Constellations and Asterisms
10. The Meridian, Ecliptic, and Zenith
11. How to Read a Star Map Using Alt/Az or Polar Coordinates
12. FOV, Magnification, and More Star Hopping
13. Putting It All Together Using an Inclinometer
14. Putting It All Together With an Equatorial Mount
15. Tips On How To See Something Once You Find It
16. What's In The Sky Right Now?
19. Types of telescopes, eyepieces, reversed images, and finders
21. How to Rate The Sky ( Cloud Cover, Transparency, Seeing, and Brightness )
22. Three Twilights and When Night Begins
23. When Does the Moon Rise and Set
24. A Close-Enough Sidereal Time Estimate
25. How Far South/North Does Your Horizon Go?
26. Putting It All Together to Plan a Night of Observing
1. Start with Safety
You should start every observing session with a consideration for safety. This is a very safe hobby - but there are a few things you need to keep in mind.
1. Tell someone where you’re going, when you’ll be back, and check in with them.
2. Secure items in your vehicle: You don’t want a 15 pound counterweight bouncing around if you have to swerve to avoid an animal
3. Don’t trip over equipment: Make sure power cords and external batteries are out of the way. A white observation chair is a really good idea. Make sure your truck’s tailgate is up or has a red light on it - walking into that in the dark will leave a mark.
4. Wind, cold, heat, and lightning: Check the weather and dress accordingly. If you are going to be observing from a higher altitude remember it gets 3 degrees colder for every 1000’ of elevation. The temperature in the desert can get very cold at night and very hot during the day.
If you can hear thunder you can get struck by lightning. You are about to get struck by lightning If you suddenly realize the hair on your arms is standing up and you smell ozone. Immediately get in a car or building or squat down and low as you can with your feet touching. Do not get under a tree!
Have plenty of water, coffee, and snacks. Take some sunglasses, sunscreen, and a floppy hat. Take a pop-up canopy if there’s no shade nearby
5. Are you awake enough to drive home? Take a nap!
6. Animals and bugs: Have mosquito spray - I like the picaridin based sprays. They will not harm optics or plastics like DEET will. Lemon eucalyptus based repellents get good ratings.
Be mindful of larger animals in your part of the country - in our region the only thing that may bother you are wild pigs. If they come around then get in your vehicle until they leave.
7. Most people are nice… most. A little paranoia protects you from a lot of dumb. Try to observe with friends.
8. Heavy equipment - make sure someone is there to help you, or take something smaller.
9. If you’re going to be doing solar observing during the day, be sure you understand, and everyone around you understands solar viewing safety and the permanent damage that the Sun will do to your eyes.
10. Have a basic first aid kit - a few bandages and ointments for cuts and bruises. A couple of aspirin is a good idea.
11. Communications - do you know where you are and could you tell someone how to find you?
12. Powering your equipment off of your car can lead to a dead battery.
13. Some places you go for observing can be pretty remote. Did you gas up or charge up?
14. Did you bring your phone? Is it charged? Do you have a charging cord for the car? Reception?
15. Be familiar with your surroundings: our Club’s dark-ish spot is located on top of a bluff and there is an honest-to-goodness 200 foot cliff about 40 yards away
2. How to use a Planisphere
The quickest way to get started is with a Planisphere. If you’re lucky enough to still have a nearby bookstore you can usually get one of these in the Science/Astronomy section. If not, you can get them on the internet here and here. If you can’t wait to get one, you can print out an excellent monthly star chart here that has all the information that the Planisphere has but is only good for one month. The Planisphere is good for any day of the year from now on. Pretty slick for technology that’s been around since the 17th century.
Naturally there are apps for your phones, tablets, and laptops. A good one that you can get for free is Stellarium. There are several others and you can search for “planetarium apps” to see some options.
A planisphere has an outer cardboard or plastic stationary section and a round inner section that can be rotated. The outer section is marked off with the days of the year and the inner section is marked with the time of day/night. You simply line up the time of day with the day of the year and you see the stars and constellations in the sky at that time. The planisphere will have North South East and West printed on the stationary section. Note that East and West appear to be reversed - not so. The planisphere is made to be viewed while looking up at the sky. If you hold it over your head and point it North, you will see that East and West are in their correct spots.
Using nothing more than a planisphere you can learn the names of the seasonal bright stars and begin to learn the different shapes of the constellations. The brighter stars are designated with larger dots on the planisphere and you can begin to learn the different “magnitudes” of brightness. The planisphere also shows an outline of the Milky Way and a line called the “ecliptic” where all the planets and the Moon will be found. We’ll talk more about the ecliptic later. You can also determine the rise and set times of the Sun for any day of the year and the rise and set times of any constellation during the year. Again - a lot of information from a simple device made up of two pieces of cardboard. A video showing you how to use a planisphere is found here. One other thing you’ll need is a red penlight or a regular flashlight covered with several layers of red cellophane held by a rubber band. You can also use brake light repair tape. You want your eyes to be dark adapted which means the pupils have opened up nice and wide. A bright white light will dilate your pupils again and it will be a few minutes before they open back up and you can see faint objects ( in my case it could take up to 30 minutes to open back up). The eyes are not as sensitive to red light that is just bright enough for you to
read the planisphere.
Taking the planisphere out on clear nights and getting familiar with how the sky moves, being able to locate and name the brighter stars and constellations, and learning the stories and legends passed down through the ages - this may be all you ever want to do and, by itself, would be well worth the effort. It will also give you a good head start on the rest of what we’re about to cover.
3. Some of the Things to See
Our solar system consists of a star ( our Sun). Planets and moons, a rocky asteroid belt, and an outer cloud of left-over frozen stuff from the Solar System’s creation revolve around the Sun.
Our Sun and Solar System are part of the Milky Way galaxy - an enormous rotating island whirlpool of millions of stars, star systems, gas, dust, rocks, and ice that is all gravitationally bound together. All of this is traveling through space along with other island universes, each one separated from the other by incredible distances. Distances at this scale are measured by the speed of light (3 X 10^10 Meters/Sec). E.g, it takes light 8 light minutes to travel from the Sun to Earth. The Milky Way is almost 100,000 light “years” wide. Our Solar System is located about 1/2 of the way out from the center of the Milky way. Our Milky Way and all the other galaxies make up the Universe. The best information available is that the Universe is approximately 14 billion years old.
A more detailed description can be found here.
The Planets
Our planet ( Earth) revolves around the Sun once every 365.25 days or so. As it is moving around the Sun it is also rotating once every 24 hours. When a particular location on the planet is facing the Sun it is daytime for that location and when it is facing away from the Sun it is night. During the night you can look out into space and away from the Sun and see the different classes of objects. The stars you see are primarily in our local neighborhood of the Milky Way - about 300 light years away from us. Our Solar System has 8 recognized planets - there used to be a ninth - Pluto - but the definition of planet was redefined and Pluto did not make the cut. The four “inner” planets are: Mercury, Venus, Earth, and Mars. They are closest to the Sun and are rocky. The four “outer” planets, Jupiter, Saturn, Uranus, and Neptune, are “gas giants”. Mercury, Venus, Mars, Jupiter, Saturn, and on a real clear night - Uranus, can be seen in the sky with the naked eye. They look like stars but they don’t “twinkle” and they move in relation to the stars around them. The word “planet” comes from a Greek word for “wanderer”.
More details can be found here.
The Moon
A moon is a natural object in space that orbits another natural object in space like a planet or large asteroid. They are also called satellites. Some planets, e.g. Jupiter, have multiple moons and in some cases you can observe their movement from hour to hour through a small telescope. Earth has only one moon. “The” Moon is the natural satellite that orbits Earth. You could spend the rest of your life just observing the Moon and its craters, rilles,
mountains, and shadows.
More details can be found here.
Stars come in different colors
Most people think all the stars in the night sky are white. If you look closely you’ll find that some are actually yellow, red, and so white that they look blue. This has to do with how hot the star is at its surface. The hotter it is, the whiter/bluer it is. The cooler it is, the more red/yellow it is. On a clear dark night you can see approximately 2,000 stars with just your eyes. Binoculars and telescopes show many many more.
More details about stars can be found here and here.
Multiple Stars and color contrasting Multiple Stars
Many stars - some say most - that look like a single point of light to us, are not singular but are actually two or more stars that are gravitationally bound to each other, and from our perspective, very very close to one another. There are a few stars that aren’t actually close to each other but just appear that way because of the way they line up. These are called “optical doubles”. If viewed through a telescope, some of these star systems can be “resolved” as individual stars. Some of these multiple star systems are made up of stars that vary in brightness and some also vary in color due to the variations in the star’s temperatures. Some of these contrasting colors are striking and beautiful. It’s also fun to see just how close some of these star systems can be and still be resolved by your telescope.
More information about multiple stars can be found here and here.
Star Clusters
An “open cluster” of stars contains a few dozen to a few hundred individual stars in a loosely formed and gravitationally bound group that can take on interesting shapes in a telescope (e.g., NGC 457 looks like ET the Extraterrestrial from the old movie.) A “globular cluster” can have a few hundred thousand stars that are compacted into a dense ball. These look like a spoonful of sugar in a telescope and can contain several different colorful stars. Globular clusters are some of the oldest objects in the Universe.
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More information on star clusters can be found here.
Nebulae
Enormous clouds of gas and dust occupy certain regions of galaxies and can form a “nebula”. The plural of nebula is “nebulae” (pronounced, “ neh’-byu-lee”). Sometimes these clouds can be illuminated by nearby stars. These are called “reflection nebulae’. In some cases the gas clouds will glow when they are “charged” by nearby stars - similar to how a flourescent light works. These are called “emission nebulae”. Some emission nebulae are small and spherical and are called “planetaries” because their tiny discs look like a planet. Sometimes the clouds of dust and gas will prevent you from seeing what is behind them and cause dark patches in space. These are called “dark nebulae”. Our Milky Way Galaxy has many such nebulae and some of them take on identifiable shapes like dumbbells, horseheads, the North American Continent, and tiny smoke rings. These can be seen with telescopes. When a nebula condenses due to gravitational attraction it can become so compact that nuclear fusion occurs and the result is a new star.
More information about nebulae can be found here.
Galaxies
We talked about Galaxies a little at the first of this section. They are huge with millions of stars and they are all separated by incredible distances, but at least one of the distant ones can be seen from a dark spot with just your unaided eyes.You’ll need a telescope or a pair of binoculars to see others.
Our own Milky Way can sometimes be seen if the night is clear and dark enough. You are looking edgewise through it and the millions of stars look like a glowing cloud that stretches all the way across the sky. If you look more closely at it through a telescope you can begin to see all those stars and other things that are discussed above. The center of our Milky Way is in the constellation of Sagittarius.
More information on galaxies can be found here.
4. Magazines and Internet Images vs an Eyepiece
Until I can get some of our wonderful club imagers to duplicate what you see in the eyepiece… there are sketches! As long as there has been a telescope there have been gifted individuals who make sketches of what they see in the eyepiece. And some of them are very good. These are very close if not exactly what you will see through the eyepiece. Great examples of this art can be found here. Compare them to some of the images of the same objects in the previous section, Some of the things to see.
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5. Which Way is North?
The majority of people in this world do not know which way is north. Don’t feel bad if you’re one of them - all of us have gotten too used to looking at glowing screens and listening to young ladies in our cell phones tell us where to turn.
Why is knowing where North is located so important? Because you will be using charts that reference North, South, East, and West and, although you can simply look at the stars, it sure is nice to know where to begin looking. North is also the zero point for a more specific reference system that shows you where to look a lot more accurately. We’ll cover that in a bit.
If it’s clear, the best way to find North is in the daytime. The Sun rises generally in the East and sets generally in the West. If you point your right arm toward the East and your left arm to the West you are then facing North and South is directly behind you. Pick a good landmark to the North you can reference at night. In some articles you’ll also find references such as North-North-East, Southwest,
etc. These are the points in between North, East, South, and West. There are 32 of these points and they define the “Compass Rose” used by ancient mariners to navigate the oceans. And they worked very well.
Today we use a magnetic compass that has 360 divisions called “degrees”. Looking North and moving 90 degrees clockwise along the horizon will take you to due-East. Continue moving 90 more degrees along the horizon and you are due-South. 90 more degrees and you are looking due-West. And 90 more degrees and you are back to where you started looking due North.
One thing you need to keep in mind is that a magnetic compass doesn’t always point to “true north”. The classic magnetic compass with its little red tipped arrow will point to “magnetic north” which can vary significantly from the true north you need for navigation using a geographic map. Because of the Earth’s magnetic field there is a “magnetic deviation” that must be taken into consideration if you are using a magnetic compass. This magnetic deviation is different for different locations on the planet and can be significant. To make it more interesting the local deviation itself changes slowly over the years as the Earth’s magnetic field evolves. You can learn more about magnetic deviation here. Also remember that the accuracy of a magnetic compass will be affected by nearby metal objects such as automobiles or even belt buckles.
Most cell phones have a built-in compass app that is usually very accurate. These apps will have the ability to switch between magnetic north and true north. Nearby metals objects will affect these apps also.
At night you have Polaris (Alpha Ursis Minoris) in the constellation of Ursa Minor, the Little Bear. Part of this constellation is called the Little Dipper and Polaris is the star at the tip of the Little Dipper’s handle. Polaris is the current “Pole Star”. This is a bright star that happens to be inline with the Earth’s rotational axis and points to “true north”. It appears to be stationary during the night as the other stars rotate around it in a counter-clockwise motion. The exact point in space that lines up with the Earth’s rotational axis and can be seen from the Northern Hemisphere is called the “North Celestial Pole”, or NCP. Currently Polaris is very close to the NCP. If you extend your arm and hold your hand out, the distance between Polaris and the NCP is less than the width of the finger nail on your little finger. The NCP actually moves very slowly through the sky from century to century. It will actually be closer to Polaris by the year 2100. A good article on finding Polaris is found here. There is corresponding South Celestial Pole, SCP, that can be seen from the Southern Hemisphere but there is currently no bright star nearby. Go here to see how to find the SCP. The movement of the two Celestial Poles is known by the intimidating name “Precession of the Equinoxes” - which basically describes how the Earth wobbles. More information on this can be found here.
6. How the Sky Changes Hourly, Daily, and Yearly
Yes, it’s the planet that’s rotating and revolving, but it’s handy to think of the sky doing the moving. The Sun rises in the general direction of the East in the morning and sets in the general direction of the West in the evening. More about the “general” part in a minute.
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Our Solar System is like a very thin, giant disc with the Sun at the center and the planets at different distances away from the center. From your high school geometry class, the disk is called a plane. This plane is called the “Plane of the Ecliptic”, or simply “The Ecliptic”. There is no real definition of “above” and “below” the Ecliptic so, for our discussion, we’ll call “above” the side of the Ecliptic that contains the North Celestial Pole, NCP, we talked about earlier. And “below” will be the other side of the Ecliptic. If you were to look down at the Solar System from above the Solar system you would see the Sun at the center and the planets revolving in a counterclockwise motion around it. This is why the stars appear to move a little farther to the West each night from our perspective here on Earth. If you saw a particular star straight overhead at 10PM local time, that same star would be a little farther to the West the next night at 10PM. And in about three months that same star would be setting at 10PM. And six more months after that, that same star would be rising in the East at 10PM. And three months after that it would be back straight overhead at 10PM exactly one year after you first observed it. All of this is due to the Earth orbiting the Sun. Every 12 months or so the Earth returns to the same spot in it’s orbit and that night you can see the stars that are away from us in that particular area of the Milky Way. Each night over the course of a year you can see all the stars that surround our Solar System in the Milky Way. Most of these stars are relatively close, around 300 light years away - some closer some farther. It’s estimated you can see 6000 stars with just your eyes over the course of the year. More information on all of this is found here.
The moon appears to move a little farther to the East each day. If you were looking down at the Earth’s north pole from out in space, you would see the moon revolving around the earth in a counterclockwise orbit. It takes 27 days and 8 hours for the Moon to make a complete orbit around the Earth. Occasionally the Moon’s orbit takes it directly between the Earth and Sun and you have a Solar Eclipse. Occasionally the Moon’s orbit takes it into the shadow of the Earth and you have a Lunar eclipse. Except during a Lunar eclipse, one side of the Moon is always fully illuminated by the Sun, but depending on where the moon is in its orbit, we see only a portion of the illuminated side. These are called the moon’s phases. A crescent moon we see only a sliver just before sunset or sunrise, a gibbous moon is a little more than half illuminated, a full moon is fully illuminated, and a new moon is not illuminated at all. For additional details you can look here.
We mentioned that the Sun rises “generally” in the East and sets “generally” in the west. We say generally because the Sun rises due East and sets due West only twice a year due the Earth’s axis being slightly tilted in relation to its orbit around the Sun. The other times the spot where the Sun rises is slowly moving North or South on the horizon. The sunrise reaches its northernmost point at the Summer Solstice, usually June 21st, and this is when the longest day of the year occurs in the Northern Hemisphere. ( This is also the Winter Solstice in the Southern Hemisphere) On this date in the Northern Hemisphere the Sun rises and sets then starts a slow movement to the South. It passes due East at the Autumn Equinox. The length of the days and nights are very nearly equal at an Equinox. The Sun continues south until it reaches the Winter Solstice, around December 21st. This is the shortest day and longest night in the Northern Hemisphere.
It then starts northward again and the cycle repeats. This is hard to get your head around at first… and at second. For now hit the “I believe” button and go here to get additional info.
7. Constellations and asterisms
A constellation is a group of stars in an area of the sky that has a pattern that reminds you of an object or animal or person. The origins of some constellations go back thousands of years and include Ursa Major ( The Big Bear), Ursa Minor ( The Little Bear), Hercules the Hero, Orion the Hunter, and Aquarius the Water Bearer.
The International Astronomical Union was organized in 1919 to, among other things, create a common set of naming guidelines and to establish a forum for issues within the worldwide Astronomical community. Part of the initial efforts included standardized boundaries for today’s 88 recognized constellations. You will see faint outlines on star maps that define the area in the sky, from our perspective here on Earth, associated with a particular constellation. This gives the amateur astronomer a starting point for finding things, kind of like knowing what State a City is in. Many times the recognizable features that make up a constellation are outlined on star maps. Some examples are shown here. As you gain time under the stars you’ll become familiar with these patterns and find it a lot easier to quickly locate things. More info can be found here.
An asterism is a grouping of stars that is part of a constellation or multiple constellations and has another identifiable shape and name. Examples of this include: The Big Dipper (The Plough in Great Britain) in Ursa Major, The Little Dipper in Ursa Minor, The Summer Triangle which consists of three bright stars from three different constellations, The Keystone in Hercules, The Northern Cross in Cygnus, The Teapot in Sagittarius, the great square in Pegasus, The Big W in Cassiopeia), and The Coathanger in Vulpecula. More about asterisms can be found here.
8. What Time / Day Is It?
First let’s discuss 12 hour time vs 24 hour time. Most clocks (digital and analog) will show the time displayed in hours from 1 to 12. You use the same hours for both night and day and refer to them as “AM” or “PM”. AM means “before mid-day” and PM means “after mid-day”. After 12 midnight it becomes 12AM and a new day begins. The hours between 12 midnight and 12 noon are called AM. After 12 Noon it becomes 12PM and the hours stay PM until 12 Midnight and it starts all over again.
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To avoid the AM and PM considerations some clocks simply use a 24 hour system. 1AM is called just “1” and 1PM is now called 13. 2PM is called 14, all the way to midnight which is 24 (its also 0 for the next day…). With a 24 hour format clock, any hour from 1 to 12 is before noon and any hour from 12 to 24 is after noon. See this site for an illustration. If you know the time in 24 hour format but you're more comfortable thinking in AM and PM, just subtract 12 from any hour between 12 and 24 to get the time in PM in a 12 hour format. Again - much easier to do than to explain. E.g. 20:00, sometimes called “twenty hundred hours” would be 20 -12 = 8PM on a 12 hour clock.
We are all familiar with our local time. The one quoted on the local news channels and shown on the signs outside banks and schools.
If you have traveled across the country you have run across “time zones” and had to move your clocks an hour or so one way or the other. Most of our cell phones these days do this automatically for us. A standardized time system across the world is a relatively recent thing. Before the late 19th Century each town and city had its own local time. This was based on the 24 hour day but it was centered around local noon. When you went from city to city you needed to find a local clock tower to know how to set your watch. In the early 1880’s it was decided that the world would be split into 24 time zones divided by lines of longitude. This was based on the movement of the Sun by 15 degrees every hour. Greenwich, England was chosen as the starting point and it was designated as the Prime Meridian. As the time zones move eastward from the Prime Meridian the clocks are moved ahead by one hour because a location to the east would see a sunrise more or less one hour before a location 15 degrees to the west. Your local time would be referenced to one of these time zones, such as 12 Noon CST for 12 noon Central Standard Time. Central Standard Time is 6 hours behind Greenwich time ( or “Greenwich Mean Time”). Greenwich Mean Time is also known as simply GMT and sometimes “Coordinated Universal Time”, or sometimes “Zulu” in the military. Universal Time, or simply UT. UT is the time listed in many astronomical lists so anyone anywhere can derive their own local time by knowing how many hours ahead or behind of UT their time zone is. This does get a little more complicated when you introduce Daylight Savings Time. This is the time of year when many, but not all, locals move the clocks ahead one hour on the same day to maximize the use of the daylight.
All these time zones do create a problem: if you could head east and travel around the world in one hour and when you started it was 12 noon on the first of the month at Greenwich and as you travel eastward you advance your clock one hour for each time zone, when you get back to Greenwich it would be 1:00 PM but you have added a whole calendar day. This was solved by “The International Date Line”. This is a line running roughly north and south 180 degrees from Greenwich. If you cross this line going from West to East you subtract one day. Going East to West you add one day. This convention keeps the calendar straight for travelers. The line is not according to International Law and it zig zags around according to some political boundaries. There is a Nautical International Date Line that is set by International Treaty and is used by the militaries of the world. More information can be found here.
Leap Years and Leap Seconds have to do with the fact that the Earth revolves around the Sun in 365.24 days and not 365 days like our calendars show. If we didn’t have Leap Years where we add an extra day (usually - there’s always a “usually”) in February, the calendars would slowly get out of synch with the seasons. A Leap Second is something similar but used to keep very accurate clocks in synch. You can read more about all this here and here.
Julian dates have a starting point in the distant past and are continuously counted from that starting point. They are in a format that is easily used in calculations and are at the heart of much of your goto astronomical equipment.
In general Epochs are very large periods of time during which something significant has occurred. Usually when an amateur astronomer refers to an epoch we are referring to the date when the location of astronomical objects were made. You’ll read more about that in the section on Equatorial Mounts and Polar Coordinates. These coordinates change very slowly and are updated every 50 years. We are currently in epoch J2000. Some older reference books that I have use epoch J1950 and are still plenty accurate enough for finding objects.
Sidereal Time refers to where the Earth is in its orbit around the Sun. We talked about this in the section on how the sky appears to move. If you know your sidereal time you can determine what objects will be available for viewing that evening. Refer here for more info.
A pretty good summary of all this is given here
9. Azimuth and Altitude
Azimuth is a measurement along the horizon. It divides the horizon into 360 “degrees”. True North is 0 degrees in Azimuth and it increases as you turn to the East until you make a full 360 degree circle and return to North. East is at 90 degrees, South is at 180 degrees, West is at 270 degrees, and back to the North at 0 degrees (which is the same as 360 degrees).
Altitude is also measured in degrees. The horizon is always at 0 degrees in altitude regardless of which direction you are looking. Straight up is 90 degrees from the horizon and If you continue straight back to the horizon behind you, you will have gone 180 degrees. Using Azimuth and Altitude you can find any location in the sky as we’ll go over shortly. A more detailed explanation of Azimuth and Altitude is found here.
An explanation of RA is found here. And here.
If you really want to get into the weeds look into International Celestial Reference Frame, or ICRF
10. The Meridian, Ecliptic, and Zenith
These are three locations in the sky that you need to get familiar with and you’ll use a lot.
The Meridian is a line in altitude that goes up from due North on the horizon, travels directly overhead to the South, and down to due South on the horizon.
The Zenith is the point directly above you in the sky.
The Ecliptic is a line where you’ll find the planets and the moon will not be too far off from it. From the Northern Hemisphere the Ecliptic is found running East to West on the South side of your zenith. In the Southern Hemisphere this line will be on the northern side of your Zenith. If you see a bright object on the Ecliptic that isn’t on your star chart… it’s a planet. More info is shown here.
11. How To Read a Star Map Using Alt/Az or Polar Coordinates
There are three basic maps: the planisphere, paper maps, and planetarium software.
We’ve already discussed one of them: the planisphere, where you have two pieces of either cardboard or plastic connected so that one of the pieces rotates within the other. The outer piece has a cutout that represents the sky. The inner piece has a star map and there are markings for the 24 hours in a day on this piece. The outer piece has markings for the days of the year . You can rotate the inner section until the time of day and day of the year line up and what you have in the cut out are the stars and constellations in the sky on that day and at that time. In effect you have an analog computer - pretty slick for something that has been around since the seventeenth century. You hold the planisphere so that the compass direction shown on the outer section points down and agrees with the direction you are facing. What you see on the planisphere are the stars and constellations above the horizon in that direction. The stars are represented by different sized dots - the larger the dot, the brighter the star. Again, this is 600 year old technology and it is still one of the best ways to discover the sky… It’s also a lot easier to do than to explain. See here for a video.
You can also print out basic monthly star maps here. These are very useful in learning your way around.
The planisphere and monthly star maps are great introductions to star hoping.
We discussed earlier how you can use Altitude and Azimuth to locate any object in the sky IF you have a planetarium app that gives you that information for the specific day and time of day you are observing. These numbers constantly change as the sky appears to rotate overhead. The Planisphere we just discussed can be adjusted accordingly to show you relative locations but a paper map needs another coordinate system that remains constant - this is where Polar Coordinates comes in. With alt/az coordinates you turn to a certain point on the horizon and then go straight up by a specific angle to locate an object; to use “polar coordinates” imagine that Polaris is the center of a bullseye with concentric circles extending outward and all the way to the south. These concentric circles show an object’s declination, or the angular distance in degrees from polaris. The circle extending outward from the Earth’s equator is 0 degrees and increases to 90 degrees at Polaris. It decreases southward to -90 degrees at the southern pole. Each degree, north and south, is made up of 60 arcminutes, and each minute is made up of 60 arcseconds. You add the “arc” to show you’re measuring an angle instead of time. This measurement is called Declination, or Dec. Every object in the sky has its own Dec. Now imagine there are lines extending out of Polaris like the spokes of a wheel. There are 24 of these spokes labeled from 0 to 24. If you’re facing polaris the spokes will increase , 0,1,2,3, etc., clockwise. The zero point for these 24 lines passes through the constellation of Pisces - although it’s called the First Point of Aries. Confused? It’s better with illustrations like the ones shown here. The spokes are measured in hours - and it’s no coincidence that this is just like the 24 hours in a day. Each hour is divided into 60 minutes, each minute into 60 seconds. This measurement is called Right Ascension, or RA. In this instance you’re really talking about minutes and seconds of time but before we get too far in the weeds lets only talk about the minutes and seconds of angle. See here for a good explanation of where Right Ascension came from.
Every object in the sky has its own RA. You follow that line until it crosses the object’s Dec to find the object. With these two coordinates, right ascension and declination, you can locate any object in the sky and, unlike an objects alt/az coordinates that change every minute, polar coordinates don’t change ( they actually do but its so slow the locations are seldom( every 50 years or so) updated. Polar Coordinates provide a convenient way to make maps of the sky. One of these that I particularly like is the Orion DeepMap 600. This is a folding map that shows the entire sky. It shows the RA as you move left or right, and it shows the DEC as you move up or down the map. It also has a listing of 600 popular objects on the back. This list shows an object’s RA and DEC and it’s easy to use these to find the object on the map and see where it lies in relation to the rest of the sky. Similar to the planisphere, you will have to turn the map one way or the other to get it lined up with how the sky looks at the time. Easy to do. Polar Coordinates also give you a way to construct an “equatorial mount” for your telescope. We’ll go more into detail on that in the “Put it all together” section.
There are also several atlas format books that show separate and more detailed maps for individual sections of the sky. These are also conveniently laid out in RA and DEC. You can search on “sky atlas” to find several options available. One of the more popular ones is the Pocket Sky Atlas.
This brings us to the planetarium apps for computers and cell phones. These can be incredibly powerful and provide an amazing amount of information. Some of these can be linked to the sensors in your tablet or phone and will show you what’s in the sky by simply pointing your device at the sky. Nice. They can zoom in and out and can show the Alt/Az coordinates or the Polar coordinates. Some can be connected to the more advanced telescopes and actually control the scope. Some of these can have a confusing learning curve but are well worth the effort. And a few are free downloads. Simply search for “planetarium apps” for more info. Two of my favorites are SkySafari and Stellarium.
You can also print out specific constellations, shown with RA and DEC coordinate grid, right off the internet here
12. FOV, Magnification, and More Star Hopping
We need to quickly touch on a couple of things and we can begin to put all this together.
There is a common malady called “Aperture Fever” where an amateur astronomer buys bigger and bigger telescopes in the hopes to see more and more. “Seeing more” means seeing fainter and fainter objects and also seeing smaller and smaller objects and finer and finer details on all objects. In general this is true and the larger the aperture (size of the objective lense or primary mirror) of a telescope, the more you can “see”. But there are some practical limitations to what you can expect to “see” and these are primarily due to the sky itself. We’ll discuss this more in the section on “How to Rate the Sky” but for now it is rare that the turbulence in the atmosphere will let you see anything at greater than 300 times magnification, 300X - regardless of the size of the scope. As a matter of fact it is an excellent night when you can observe at 300X. Most nights about 200X is the most you’re going to get. Above that the image will be brighter in a larger telescope but the image will degrade and get fuzzy with lack of detail. Be wary if you see an advertisement for a particular telescope that promises incredible magnifications above 300X.
There is a very good rule of thumb that says the maximum magnification to expect from a telescope is 50X to 60X the diameter in inches of a telescope’s objective lense or primary mirror. Always add to that, “up to 300X on great nights”. The good news is that, for most objects, the max magnification you’ll need is 150X.
To calculate the magnification of a telescope you divide its focal length by the focal length of the eyepiece. The focal length is the distance from the objective lense or the primary mirror to where an image is formed. The focal length of an eyepiece is the same thing and will always be much shorter than the focal length of the telescope.
Example: you have a 6 inch Newtonian Telescope ( a reflector) with a focal length of 1250mm (48”). You are using a 25mm focal length eyepiece. 1250/25 = 50X
Besides magnification you need to know the Field of View (FOV). This is the measure of “how much” sky you see when looking through an eyepiece. FOV is measured in degrees/arcminutes/arcseconds like the measures for the Altitude and Azimuth. (Remember that when minutes and seconds are used to measure angles they are technically called arcminutes and arcseconds to distinguish them from time - but folks usually leave out the “arc”. An eyepiece that yields a low magnification will show you the most FOV. The higher the magnification, the smaller the FOV. A typical FOV for a low magnification eyepiece is one degree (10). They can go much higher than that and they can go lower than that but for many telescopes the one degree max FOV is a good working number.
The 25mm eyepiece in the example above has a FOV of 1 degree. To visualize this you can extend your arm in front of you and hold up your hand to where you can see the finger nail on your little finger. That fingernail has an area of approximately 1 degree. A full moon has an area of 1/2 degree.
We’ll go over this in more detail in the calculations section.
( NOTE: you don’t need any math to enjoy the sky but a basic understanding of these concepts really helps)
Finally we get to “Star Hoping”. To star hop you use a paper map or planetarium app and locate the object you’d like to observe. You find a bright, easy to locate star near the object and, in our example above, you would imagine a one degree FOV and you see how many FOV’s you’d have to move the telescope to get to the object you’re looking for. There is a real good explanation and illustration of this here.
13. Putting It All Together Using an Inclinometer
An inclinometer (angle finder) is a device that measures angles. You can find these in the small tool section of the nearby big box hardware store usually near the bubble levels and tape measures. There are mechanical ones like this and there are digital ones like this. You can use one of these along with your planetarium app to easily find things in the sky.
Your planetarium app will give you an object’s location in RA/DEC and also in Alt/Az. First, be sure your telescope mount is level, this is important to assure that when you move the telescope in altitude that it goes straight up from the horizon. Then, using a star chart, point your telescope in the general direction of the object you’re looking for. Get the object’s altitude from the planetarium app, place the inclinometer on the scope optical tube, move the scope to the correct altitude, and pan left and right until you find the object.
Use an eyepiece with a FOV of at least one degree - the wider eyepiece FOV, the better. Also, remember that the object is moving (relative to us🙂) - update the altitude number for the object every couple of minutes if needed.
14. Putting It All Together With an Equatorial Mount
There are two basic equatorial mounts: the german equatorial and the fork equatorial. For simplicity we’ll discuss the german mount. The same principles apply to both.
There are some really nice german equatorial mounts with incredibly precise engraved setting circles, 10k step encoders, and carefully machined internal gearing. Most have digitally controlled drives with unfaltering goto capabilities, even in windy conditions… we will not be using one of those for illustration... We will use the EQ1 in the discussions. Even though the EQ1 is a very basic piece of equipment (read that cheap), and even though its setting circles are difficult to read, and even though its tolerances are such that it wobbles in a gentle breeze, it still provides the basic functionality of the most expensive equatorial mounts on the market. It will find anything the other mounts will, but instead of dead center, sub-arcsecond positioning of an object in your eyepiece, the EQ1 will get you within a couple of FOVs - good enough. And you don’t have to wait on a GPS to sync up or master complex handsets or know the exact date, time, and location to use the EQ1 - all you need are the polar coordinates of a visible star and the coordinates of the objects for the night. And if you can understand and use an EQ1 you won’t have any trouble with the rest of the mounts.
The EQ1
The EQ1 consists of a tripod and a mount that has a main axis that can be adjusted in altitude to point to Polaris. A telescope is attached such that it can rotate around the main axis in RA and it can also swing outward from Polaris in DEC. There are RA and DEC setting circles inscribed on the mount. Here’s the instruction manual.
1. If using a reflector you first check the collimation - this shouldn’t be needed for a refractor. Collimation can be done in the daylight and is not as hard to do as some folks would have you believe. You can use a laser, cheshire, or a simple collimation cap. I have an Orion StarBlast 4.5 on my EQ1 and I get good results with the collimation cap. More information on collimation of reflectors can be found here.
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2. Align your finder with your telescope. This can also be done during the day. Find an object a few blocks away and get it in your telescope eyepiece field of view. Cell towers, street lights, etc. work well for this. Adjust your finder until it is centered on the object. Turn the telescope to another object using the finder and check to be sure the object is also in the telescope eyepiece field of view. You’re done.
3. Polar align the mount.
This is a good time to go here and get familiar with the EQ1 controls shown in fig. 1 of the Orion Instruction Manual.
Roughly level the mount - for this method it doesn’t have to be perfectly level. We will go over another method below where it does need to be as level as possible but close-enough works for this one . Turn the telescope in Dec until Dec setting circle is at 900 and tighten the Dec lock screw. With many telescopes the Dec is permanently set at the factory and can not be changed. This is great. Then loosen the RA lock and position the telescope so the counterweight shaft is pointing straight down. Then tighten the RA lock screw. Again, close-enough to straight down works. Then, using the mount’s azimuth adjustment and latitude adjustments only, place the Polaris in the center of the telescope eyepiece. Tighten the altitude and azimuth locks. ( Sometimes these adjustments will move slightly when you tighten the alt and az lock screws - you may need to readjust to allow for this.) The mount is now polar aligned.
We’re lucky because the North Celestial Pole (NCP) happens to be very close to Polaris here in the Northern Hemisphere at this time in history ( See this article on Precession of the Equinoxes - basically the North Star changes every 3000 years or so because the Earth
wobbles).
Read this article on the Kochab method for a more accurate positioning of Polaris in the eyepiece - for a basic mount like the EQ1, getting Polaris in the center of the eyepiece works. (NOTE: This process assumes you have a clear view of Polaris at night. There are other ways of doing this in the daytime or if your view of Polaris is obstructed or if you are in the Southern Hemisphere. We’ll go over one of them at the end and provide references for others.)
4. Now, look up the RA and DEC of a bright star visible that night, then loosen the RA and DEC lock screws and point the telescope at that bright star ( closer to the Celestial Equator, the better - This would be a star with a DEC near 0 degrees). Once the star is in the center of the telescope’s eyepiece, adjust the mount’s RA setting circles to match the star’s RA. Check the DEC setting circle to verify it is in agreement with the star’s DEC. One thing I do is after I get the RA and DEC of the star I’ve chosen, I go ahead and move the scope in DEC to the star’s DEC. Then all you have to do is swing the telescope in RA to the star. You most likely will have to make a small adjustment with the Slow Motion controls but it saves a little time. You’re now good to go. To move to another object select its polar coordinates and move the mount to those coordinates. First remember that if you’re using a mount that does not have a motor to keep the RA moving as the sky moves, that you’ll need to simply be sure the RA setting circle is reset to the current target before moving to the next one. Again, the DEC setting circle does not move and is good. Do this each time before you move to the next target.
A good article on this is from the BBC Sky at Night website. The following images from an EQ1 mount can be found there.
This is a good spot to talk about “resolution” and “close enough”. Resolution is the realistic accuracy of the setting circles and close enough tells you how accurate you need to be with your numbers.
The first image is of the EQ1 DEC setting circle.
The larger tic marks are each 10 degrees, the smaller tic marks are each 5 degrees, and if you look closely there are also tiny tick marks that are each 2.5 degrees. To further complicate things, the metal index mark points too low and is too wide - bottom line is the best you can hope for is to get within 2.5 degrees of arc resolution using this DEC setting circle above.
The RA setting circle is shown below:
Here the larger tic marks are each 30 minutes and the smaller tic marks are each 10 minutes. The metal index mark is better positioned here and with a little practice you could split the 10 minute tic marks and get a 5 minute resolution. 5 minutes RA is about 1 degree of arc if you’re on the Celestial Equator ( hit the “I believe” button again.)
OK. Bottom line - If your EQ1 is perfectly polar aligned and you are using an eyepiece with a 1 degree FOV you would be able to get within a couple of FOVs in RA and within 3 or 4 FOVs in DEC by using the setting circles. In other words you’ll need to move around a bit but you will be “close enough” to the object you’re looking for.
If you get the RA/DEC of an object within 2.5 degrees in DEC and within 5 min of RA you are “close enough” to find your object with some panning around.
This is really not bad for a mount you can pick up for a$100 or less on the used market.
I have an Orion StarBlast 4.5 that I use on my EQ1. I get a little over 30 FOV and it doesn’t take much panning to find what I’m looking for.
More expensive mounts can be very accurately aligned, have larger setting circles, and in some cases verniers. Some of them are motor driven and some are pushto or goto. The EQ1 provides the same basic functionality as any of them and is a lot of fun to take to the field… every now and then.
All of this is much easier to do than to explain!
The last thing is the dreaded Meridian Flip… and it should not be dreaded. And again, it’s much easier to do than to explain.
If you’re looking at an object that is close to the zenith and on the East side of the Meridian you may find that the telescope may bump against the tripod legs or the mount itself and prevent you from moving to the object as it moves to the other side of the Meridian. Not a problem. Perform a Meridian Flip. If the RA of your object is 12 Hours or higher you need to subtract 12 and move the scope in RA to that new location. All this does is move the telescope 180 degrees from its original position. Since its polar aligned this means the object will still be parallel to the mount.( hit the “I believe” button till you get a chance to do this) If the RA of the new object is 12 hours of less you’ll need to add 12 hours to the object’s RA and move the scope to that new location. In both cases, you’ll need to swing the scope in DEC to the general direction of the new objects and then adjust the scope to the objects DEC. The DEC setting circles go from 0 degrees to 90 degrees back to 0 degrees back to 90 degrees and then back to the same 0 degrees you started with. When you point the scope in DEC to the general direction of the new object it puts you in the right section of the DEC setting circles. Simply fine tune the DEC to the objects DEC - no math involved.
Again, many DEC setting circles are set at the factory and do not move. It can get bumped and if you’re having a hard time finding things it could be because the 90 degree mark has moved. This will cause you to align the entire mount several degrees away from Polaris - not good. See the separate article on “Is the Mount Square?” that tells you how to check the 900 DEC position on your mount. Another possible problem is called Cone Error. All this means is that your telescope optical tube is not parallel to the mount’s main axis. This will make it point a little high or low and your alignment will be off. This is easy to check and fix and will also be covered in the “Is the Mount Square?” article.
Other things to keep straight with an equatorial mount:
1. You will have to loosen the OTA from time to time and rotate it to put the eyepiece in a better position. This does not hurt the alignment.
2. Be sure to use the correct set of RA numbers. There will be two sets of numbers on the RA setting circle: one for the Northern Hemisphere and one for the Southern Hemisphere. One of the first things you do after getting polar aligned is to stand behind the telescope, face North, move the scope clockwise in RA, and see which set of numbers is increasing as the mount rotates to the East. Those are the setting circle numbers you’ll be using.(You use the other set of numbers in the Southern Hemisphere.) It is REAL easy to use the wrong set of numbers.
3. Be sure the RA setting circles do not move when you move the scope from object to object. The EQ1 uses a thick grease to hold it and as the mount gets older the RA setting circle can slip and lose the RA alignment. Simply focus on a star of known RA and reset the setting circle. Pricier mounts will not have this problem.
4. The slow motion controls on the EQ-1 have limited travel and you’ll need to “center” them up from time to time. Again, this is not an issue with the more expensive mounts.
5. Can you see the Dec setting circles? If they are not on front of the mount facing the direction the OTA is looking then you have put the OTA on backwards -turn it around facing the other way…
6. Get familiar with where the RA and Dec locks are so you can find them in the dark.
Try an equatorial mount and see what you think…
15. Tips On How To See Something Once You Find It
Sooner or later you will utter the words, “ I don’t see anything!” Well, you may be in the wrong spot or your equipment may not be up to the task on that particular night or it could be that the object is in the center of the field of view and you haven’t learned how to see really faint objects. This is a very real skill you develop as you get experience observing.
There are a few age-old tricks of the trade you can try before you move to the next object. They include:
1. Use a red light. We mentioned this earlier. A white light will close up the eye’s pupil and a red light will allow it to open up and see fainter details.
2. Get a comfortable observing chair - this is the single best thing you can do.
3. Take your time - many times people will look into an eyepiece for 5 seconds and move on. Wait a little while longer and let your brain work on the image.
4. Move your eye slowly from side to side and up and down. This uses your “averted vision”. If you look a little to one side your eye is more sensitive to dim objects.
5. Tap the scope - this slight movement can reveal subtle changes.
6. Cover your head with a dark hood - this eliminates any background light around you that may be reflecting off the eyepiece. A dark towel at your feet can help if it’s really bright.
7. Change the magnification - this can darken or brighten the background and may improve contrast.
8. Smoking, alcohol, and altitude above 10K feet all have adverse effects on your eyes.
9. Wait for the object to get higher.
10. Try O3, UHC, or other filters
Get used to trying the above simple techniques and the next thing you say may be, “Ah, there it is.” There are other things as well but the above will get you started.
16. What's In The Night Sky Right Now
At every star party I eventually hear someone say, “What do I look at next?” You can start with a list. These lists will usually include the constellation the object is in and also the object’s polar coordinates in RA and DEC. A star chart will show you the locations for star hopping, and the planetarium apps will give you the current alt and az for the object.
There are two lists that I like in particular. One is provided by the River Bend Astronomy Club in Southern Illinois. The list is actually a combination of several popular lists and can be found here. The other list is the “All Splendours, No Fuzzies” that was put together by the Royal Astronomical Society of Canada. (“splendors is spelled “splendours” in Canada🙂) The Canadian list includes objects in the Southern Hemisphere that will not be seen from our Northern latitudes. The Messier and Caldwell catalogs are very popular. And there are observing programs by different organizations; the most popular being the ones of the Astronomical League. These lists are also not limited to the Northern Hemisphere and you can even get a decorative pin when you complete a program. There are lists of objects you can see with just your eyes if the sky conditions are good. There are lists of things to see with Binoculars. There are lists of things to see with telescopes of all sizes. There’s all kinds of lists that have been put together by professional and amateur astronomers over the years and you’ll probably come up with your own list of favorites.
There are also websites that point out interesting highlights of the current sky. These include Space.com, What’s up in Tonight’s Sky, and This Week’s Sky at a Glance. An internet search for “what’s up tonight” will show you many more.
Get a list of objects before you go out and have a plan.
You’ll see much much more.
17. Is The Mount Square?
Many of the more expensive equatorial mounts have a polar scope that actually fits inside the main RA axis of the mount and greatly facilitates lining up the main axis with the North or South Celestial Pole. The less expensive mounts - like the EQ1 - do not have this and you must use the telescope itself to polar align as we described earlier. The telescope must be parallel to the main axis in altitude and azimuth in order for an equatorial mount to point to where you want it to. More than likely the mount you are using was lined up well enough at the factory that you don’t need to be concerned about this. If not, depending on how out these two alignments are, finding things will be almost impossible.Luckily this is easy to check and doesn’t take long to do so. This is called checking for “cone error”. It’s also called “orthogonality correction” if you want to impress your friends.
First you check to see if the telescope is pointing high or low relative to the main axis. A good explanation of this is found here. And how to check for it and correct it if needed is also found in these videos here and here. This is easy to check and you can save A LOT of time.
It is rare that the DEC setting circle needs calibrating. These are set at the factory and in many cases there is no way to make any adjustments ( as is the EQ1). The more expensive german mounts will have small grub screws that can be loosened. (This is easy to do with a fork mounted scope as seen on p. 28 here.) For the german mounts you can verify that the telescope DEC setting circle 90 degree setting is correct. If this is off then the telescope is pointing a little to the left or right away from the main axis and when you see Polaris in the eyepiece the main axis will not be pointing to the Celestial Pole.
Balance the mount itself first. Then lower the main RA axis until it is level. Next move the scope in DEC until the counterweight arm is pointing straight down according to a level. Check the DEC setting circle setting and it should read 90 degrees. If it doesn’t and your mount’s DEC setting circle is non-adjustable, make a note of the actual setting and use that as your 90 degree point in the future. You’ll need to add or subtract this difference to the DEC readings for objects you are locating. If your mount has an adjustable DEC setting circle you can correct it at this time.
18. All Those Catalog Names!
But what about all those designations? NGC, SAO, Messier, Herchell, IO, Abell, and hundreds more.
Throughout the ages people have compiled catalogs of different classes of objects. They may have been studying these professionally or, in the case of Messier, these were objects that got in the way of what he was really studying ( comets) and he wanted to make a note so he wouldn’t confuse them in the future. We now know what those particular objects were.
You will see these various names, and in some cases the same object will have different names in different catalogs. Go here and here to get a fairly comprehensive list.
There is no easy way to become familiar with all of this! Most folks start with the Messier and Caldwell lists and go from there. Not a bad way to begin. As you gain experience you will become familiar with and find it easy to reference all the other designations.
19. Types of telescopes, eyepieces, reversed images, and finders
This topic has been discussed in the magazines, on the internet forums, and at star parties ever since the invention of the first telescope. Everyone has their idea of the perfect telescope. And they’re all right - for them. There is no perfect telescope for everyone but everyone has a telescope that is perfect for them. And there’s nothing wrong with having more than one… :-)
There are three basic types of telescopes: the refractor, the reflector, and the catadioptric ( a fancy name for using mirrors to “fold” the light path and make a compact instrument). There are two main types of catadioptric telescopes: the Maksutov and the Cassegrain. Two good sites to get familiar with the different types of telescopes are found here and here.
There are a few numbers that describe the telescope: aperture, focal length, f/number.
Aperture is the diameter of the main objective lense in a refractor or the diameter of the primary mirror in a reflector or catadioptric. The larger the aperture, the more faint an object you can see, the greater detail you can see on planets, and the tighter double stars you can resolve.
Focal length is the distance from the objective or primary mirror to where an image is formed. Shorter focal lengths mean larger fields of view and longer focal lengths mean smaller fields of view.
F/number is a combination of the aperture and the focal length. To get it you divide the focal length by the diameter of the objective. The smaller the f/number the larger the field of view, the larger the f/number the smaller the field of view.
Add to this the ability of a telescope to use either 1.5” eyepieces or 2” eyepieces. The 2” eyepiece will show you more field of view.
This is a good place to talk about eyepieces. There are LOTS of eyepieces out there. We mentioned the 1.5” and 2” sizes ( and there are others) and there are two other basic considerations: Apparent Field of View (AFOV) and focal length. The AFOV gives you an idea of how much sky you’re going to see. Typical values are 50 degrees, 68 degrees, 82 degrees, and 100 degrees. The larger the FOV, the more sky you see, the more expensive the eyepiece. A good place to start is 50 degrees. This usually gives a 1 degree FOV of the sky at low power and you can see a lot with a 10 FOV, contrary to what the advertisements would have you think.
Similar to the objective lense or primary mirror, the focal length of an eyepiece is the distance from the eyepiece to where the image is formed, except its looking the other way. ( hit the “I believe” button and look at the sketch below)
The focal length of an eyepiece determines how the object is magnified. The higher the focal length the lower the magnification and the more sky you can see. The lower the focal length, the higher the magnification, and the more detail you can see ( within limitations). A typical low power eyepiece would have a focal length of 25mm. A typical medium power eyepiece would have a focal length of 10mm. A typical high power eyepiece would have a focal length of 5mm. This basic understanding is really all you need to know. But… if you want to know more please see the section on Math.
All of the above would make you think the ideal telescope would have a large objective lense or primary mirror, a low f/number, and a 2” focuser with a 100 degree AFOV eyepiece. This would maximize the amount of sky you could see at one time and also the amount of detail you could see.
Well, maybe…
This is where the other consideration for a telescope come in.
Where am I going to store it.
How am I going to transport it (either to the backyard or to a dark site)
How hard is it to set up and break down.
How hard is it to operate.
Cost
Also remember that 300X is the highest useful magnification no matter what size telescope.
Also remember that no matter how big your telescope is, there is always one a little bit bigger that will show you a little bit more… this is called Aperture Fever. There is also Techno Fever where you get fancier and fancier mounts and other equipment. There is nothing wrong with either of these! Just remember that you don’t have to spend a lot of money to get into and enjoy this hobby. There is great beauty in simplicity as well as complexity.
A telescope with an aperture of 80mm and under is considered a small scope. A telescope with an aperture between 80mm and 300mm is considered a medium aperture scope. Anything over 300mm is considered a large telescope.
The larger the aperture, the larger and heavier the telescope.
The larger and more complex the telescope, the longer it will take to set up and take down.
Think about where you’re going to store the telescope and how big your transport vehicle is. Also think about how heavy the heaviest component is - can you handle it in the dark; can you handle it at all?
Another consideration is “collimation”. This is where you check the alignment of the mirrors in a reflector telescope. This is not an issue with refractors or Maksutov catadioptric telescopes. The other general type of catadioptric telescope is the Cassegrain and it does need to be collimated.
Collimation is not that hard to do - but does take practice at first.
You will see lots of collimation tools advertised to help you. All you may need is a simple plastic cap with a tiny hole in it or an inexpensive laser collimator that fits in the eyepiece holder. There are more expensive laser collimators for telescopes with lower f/numbers. These need the additional accuracy - f/numbers lower that f/6 can benefit from the more expensive laser collimators.
Also look into Bob’s Knobs; these make things a lot easier.
There are some good sites that explain collimation found here and here.
One thing to get familiar with is the way the image will appear in the eyepiece. Some images will be upside down, some will be swapped left to right, and some will be correctly oriented. This can make interpreting a star chart a little challenging… again, at first. A good explanation of what to expect is found here.
Your telescope will have a finder. This is most likely a smaller telescope attached to the larger one and that shows a larger area of the sky. This smaller scope is aligned with the larger scope such that when you line up an object in the small scopes crosshairs, the object will be in the eyepiece of the larger instrument. Another type of finder is the red-dot/reflex finder. These project a red dot or bullseye onto the clear surface of the finder and all you do is point the red dot to where you want to go in the sky. Both types have pros and cons. A good article on these is
found here.
Another consideration for refractor telescopes is apochromatic (APO) or achromatic (ACR). The APO uses more expensive glass and lense configurations and virtually eliminates “fringing” where you see a purple glow around bright objects like the moon or Venus. The view through an APO is really nice but telescopes with these optics are quite pricey. An ACR uses less expensive glass and a simple lense configuration but does have fringing around brighter objects. Some folks like the added color. :-) The images of deep sky objects can be very nice with very little or no fringing. There are also fringing filters that help. There are some very nice ACR telescopes.
We’ve discussed mounts already in previous articles but we need to mention them one more time:
There are two types of mounts: alt/az and equatorial.
Alt/Az stands for altitude and azimuth and is the classic tripod that moves left and right in azimuth and up and down in altitude. These are simple to operate and find things. Some of the more expensive ones are even computer guided “goto” models that will find an object and
track it as it moves (or we move :-) )
There is also the Dobsonian mount. This is another variation of the alt/az mount that sits low on the ground and makes it easy for a large reflector telescope to move around.
The equatorial mount is a little intimidating at first but is actually the easiest to use to find and track things. It has a main axis that is oriented with the North Celestial Pole (the North Star is close enough) and it allows a telescope to rotate the way the stars move. You can use graduated marks called setting circles to find any object in the sky ( again, within the limitations of your equipment, the sky, and you) We discussed the german equatorial mount but another variation of the equatorial mount that functions just like the german mount is the “fork mount” shown below next to the german.
See the articles above on Putting It All Together using an Inclinometer and Putting it all Together with an Equatorial Mount.
There is also go-to and push-to technology. The go-to telescopes have computer linked servos that will go to an object all by themselves by simply entering the object’s name. The push-to telescopes have the same computer locator but you must manually push the scope to the position indicated on a controller handset. These are beyond the scope of this short article but there are many telescope packages that include these features as well as stand alone mounts you can purchase for an existing telescope. Search on go-to or push-to telescopes for A LOT of options.
I need to mention a new type of telescope in the hobby - the fully automated imaging telescope. These are relatively inexpensive and link with your cell phone to show some pretty impressive pictures. They are fully self contained and are gps linked and utilize plate-solving technology to locate things. All of this for around $500 as of this date. We’re not really going to get into imaging here but these new scopes are worth a look. Examples are Vespera, Dwarf, and SeeStar. And more are popping up.
This will give you a working knowledge of some things to keep in mind about telescopes. The best thing to do next is to go to a local “star party” where a club or school will have several telescopes set up to observe what there is to see. Ask them about their telescopes. We love to share
20. Is It Going to Dew Up?
Sooner or later your equipment is going to dew-up and shut you down for the night. The good news is that you can see this coming and be
ready to deal with it.
You don’t need to go all thermodynamic and be able to discuss, at length, convective, conductive, and radiative heat loss. All you need to do is
look at the weather report and find out the evening’s forecast for low temperature, dew point, and wind- UNLESS it’s been raining for a few
days. If it has, then there’s a lot of moisture in the ground and, if it’s not real breezy, be ready for dew.
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Many weather forecast sites will give you an hour to hour projection. Check these and see if there is any time during the night where the the
expected temperature is going to be within 5 degrees of the dewpoint. If so, AND the expected wind is below 5mph, it’s going to dew-up. If
the expected wind is 5mph+ then you’ll be ok. The only exception here is if the temperature is expected to drop to be equal to the dew point.
In this case you need to be ready for dew/frost. Simple as that. You will find all manner of explanations and more precise ways to determine the
exact point it’s going to dew-up but the bottom line is to be ready for it. Why the 5 degrees? Any horizontal surface exposed to the night sky
will be cooler than the ambient air. Hit the “I believe” button and see here for an explanation. This is not really too much a simplification of,
what can be, a very complex analysis. These guidelines work 95% of the time. If you have your dew prevention equipment they work 100% of
the time.
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An example of planning for a night of observing is shown below. It is a snapshot from the Astrospheric weather website. This is one I like. Look
at 10:00PM on Friday.There is a highlighted vertical column at that time and the values for that hour are shown at the bottom of the image. We
see that the forecast temperature is 61 degrees, the dewpoint is 45 degrees, and the wind is 1 mph. The temperature and dewpoint are 16
degrees apart and, although the wind is slight, there will be no dewing problems tonight. The color of the wind icon in this app also changes
color to give you an indication of the strength. The legend is shown in the second image below.
In the example above the night was going to be dew free. What about when it’s definitely going to dew-up?
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A dew shield can be effective. This is a tube you put on the end of your refractor or cassegrain telescope that extends past the end of your
scope. This limits the amount of open sky the optics are exposed to. It limits it significantly and unless heavy dew is expected (high dew point
or recent heavy rain) this may be all you need. The accepted rule of thumb for the length of the tube is 1.5X the diameter of the objective or
corrector plate. This has the added benefit of preventing stray light from entering the telescope. Nice.
The primary mirror of a reflector telescope has the tube itself acting like a long dew shield. The secondary mirror at the top of the tube and the
eyepiece and finder will dew up and need heaters as explained below.
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A 12V low temperature heat gun ( hairdryer…) can also be used to temporarily remove dew. Although, it’s going to dew back up after a few
minutes.
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Battery powered heaters are very effective in combating dew ( and frost). These are resistive heater strips that can be wrapped around the
objective or corrector plate and powered by a battery. You will also need separate strips for your eyepieces and finders and the secondary
mirrors in reflectors. Some folks have them for their handsets in the case of goto equipment. The heater controllers can provide dc power
continuously, intermittently, or be connected to a temperature sensor that uses only the dc power needed to keep the optics a few degrees
above the dewpoint. You can also get heated dew shields for the super prepared.
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It’s best to use a stand alone battery to power the heaters. You can use your car battery but you’re running the risk of a dead battery.
If you don’t have any heaters or heat guns or dew shields you can simply point the scope toward the ground for a few minutes as a temporary
solution. The ground is a source of optics heat gain/ a clear sky is a source of optics heat loss. ( please, no hate mail from the radiative heat-loss
denyers :-) )
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Also keep things like maps, tablets, binoculars, and phones covered. All cases should be closed. It’s also a good idea to keep your vehicle’s
windows up and if you’re not sitting in a chair try to lean it up against something. A couple of hand towels will come in handy.
A fan blowing on the primary of a newtonian helps to get the mirror to the right temperature and I guess it could help with dew… honestly I’ve
never had a problem with a primary dewing up
It’s important to note that the image in the eyepiece can also be affected by your optics not being at the ambient temperature. Always give
your optics time to “cool down” or “warm up” depending on the season. With some scopes this can take up to 45 minutes!
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Brightness is a measure of how much light pollution there is. This can also change from night to night or from hour to hour. If there is more haze
or water vapor in the air then this reflects the local lights and the sky is brighter. If the air is dry with little dust and humidity then there is less
for the local lights to reflect off of and the night is darker. In most areas it gets “darker” after midnight when many businesses turn their lights
off or turn them down. Local sports events are usually over by then also.
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There are three good measures of how bright the sky is: the Bortle Scale, Sky Quality Meter, and faintest star. The Bortle Scale is a pretty good
qualitative measure. It is a simple scale from 1 to 9, with a 1 being a perfectly dark sky far from any lights, and a 9 being as bright a night sky as
it gets from the middle of a large city. Check here for a more detailed explanation. The scale is illustrated below:
The Sky Quality Meter is a physical device you can purchase for about $175 US at the time of this writing. It gives you a quantitative measure of
the sky’s luminance in Mpsas (Magnitudes per square arcsecond2) Hit the “I believe” button and go here for a more detailed explanation. The
Sky Quality Meter gives you a direct readout of how bright the sky is. A measure of 22 is as good as it gets in the desert SW of the USA or the
Atacama Desert in Chile. Typical urban readings are in the 15 range, and a good average reading for rural areas here in the USA would be 20 to
21.
There is also a cell phone app that gives reasonable readings. See here for more info.
Faintest Star is a measure of exactly that, the faintest star you can see. This is very closely related to measuring Transparency above except you
use the faintest star’s brightness to describe the sky. For example, if the faintest star you can see in the Little Dipper is Pherkad, you would
reference the image below and say that tonight’s sky was a Mag 3sky. If you could see all the stars in the Little Dipper then it would be a Mag 6
sky. This is closely related to the description of transparency above.
There are two good internet sites I like that forecast sky conditions. These are well worth your time in getting familiar with. The first is the
Clear Sky Chart and the other is Astrospherics.
Another good site for predicting sky cover, temperature, humidity, and dew point is the National Weather Service.
22. Three Twilights and When Night Begins
You know that there’s a time in the evening after sunset that it’s still too bright in the West to see faint objects. And you know that there is a time in the morning before sunrise when the sky begins to get bright in the East and it’s no longer dark enough to see faint objects; these periods of time are called twilight - so when does night begin and when is it dark enough for the faint stuff?
Generally you will begin to see the brighter stars and planets about 30 minutes after sunset and it’s not really dark enough for really faint objects until an hour and a half later. This is all defined, however, and there are more accurate references you can use to help you plan when to start observing.
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Sunset and Sunrise are defined by the specific times that the center of the Sun crosses the horizon.
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In the evenings the period of time between sunset and when the sun has dropped to 6 degrees below the horizon is called Civil Twilight. This is the time when it’s still bright enough to do most things and you don’t really need to turn outside lights on. Usually the streetlights haven’t come on yet. The exact time when the Sun is 6 degrees below the horizon is called Civil Dusk. In the evenings the period of time after Civil Dusk and when the Sun reaches 12 degrees below the horizon is called Nautical Twilight. This is the time that sailors can still see the horizon well enough to take readings with a sextant. The specific time when the Sun reaches 12 degrees below the horizon is called Nautical Dusk. If you’re using goto or push-to equipment you can usually see alignment stars after Nautical Dusk.
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In the evenings the period of time after Nautical Dusk and when the Sun reaches 18 degrees below the horizon is called Astronomical twilight. During this time the sky is just bright enough to make it hard to see the most faint objects in the sky. The exact time that the Sun reaches 18 degrees below the horizon is called Astronomical Dusk.
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After Astronomical Dusk it is as dark as it’s going to get and night officially begins.
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Night officially ends in the morning when the Sun rises to 18 degrees below the horizon and Astronomical Twilight begins again.
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In the morning the specific times the Sun reaches 18 degrees, 12 degrees, and 6 degrees are called Astronomical Dawn, Nautical Dawn, and Civil Dawn respectively.
This is shown in the illustrations below.
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Corresponding to dusk at the end of the day is dawn at the beginning of the day.
The rise and set times for the sun and the associated daily times for twilight can be found here:
https://www.timeanddate.com/astronomy/different-typ es-twilight.html
https://science.howstuffworks.com/nature/climate-wea ther/atmospheric/twilight-dusk.htm
23. When Does the Moon Rise and Set
For deep sky there should be no interference from the bright glow of the moon. The absolute best time for this is during the “New Moon”. This occurs usually once a month* and is marked on many calendars. During this time the Moon is almost directly between the Earth and the Sun and the side facing us is low on the horizon, not illuminated, and is not seen all night. See the illustration below from the NASA Science website:
Each night after a New Moon you’ll notice the Moon has returned and is a little higher in the sky in the West after sunset. Approximately 15 degrees higher - meaning it sets about an hour later. Each night what started out as a thin crescent the day after the New moon is slowly becoming a widening waxing crescent, then a first quarter, then a waxing gibbous until approximately two weeks have gone by and a full moon rises in the East at sundown. This full moon is in the sky all night. The next night it rises an hour later and over the next two weeks rises an hour later each night becoming a waning gibbous, last quarter, waning crescent, back to New Moon and everything starts over again.
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If you know the date of the New Moon you can pretty much plan for the whole month. The First Quarter Moon occurs about one week after the New Moon. This sets at Midnight. During the nights between the New Moon and the First Quarter Moon the Moon sets early enough that you can get a couple of hours of Moon-free observing before midnight. And if you’re willing to stay up past midnight then you have a Moon-free sky till the next morning.
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For about a week after the First Quarter moon there is a waxing gibbous Moon in the sky most of the night. At the end of that week is the Full Moon and there is a moon in the sky all night.
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For about a week after the Full Moon there is a waning gibbous moon in the sky most of the night. At the end of that week is the Last Quarter Moon which doesn’t rise until Midnight. For the next week after the Last Quarter Moon the Moon rises later and later each night and you can observe until midnight or later each night.
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And then you’re back again to the New Moon.
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And if you want to view the Moon? Choose a time when your objects will be in the shadow near the terminator. The Terminator is the edge of the shadow that divides the sunlit side of the Moon from the shadow side of the moon. Objects near the terminator will be best contrasted by the shadows that are cast. Check out the Lunar V, X, and L. and other “claire obscur”. Here’s a YouTube on this.
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The bottom line is that there are at least two weeks during the month that you can view a moonless sky - you may have to get up a little early or stay up a little late, but it can be done.
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The week before the new moon the Moon does not rise until after midnight or later so you have from Astronomical twilight to midnight or later to observe. The week after the new moon the Moon sets early enough to still get some observing in before it gets too late.
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There are several sources of specific rise and set times for the Moon. Here is a good one.
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Remember: There are plenty of objects to observe that are not affected by a bright moon. These include The Moon itself as mentioned above, Carbon Stars, Multiple stars, Planets, and some Open Clusters.
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* rarely, it can happen twice a month and is called a Blue Moon. See this article from Space.com on the definition of “Blue Moon”
24. A Close-Enough Sidereal Time Estimate
OK. You’ve got a clear enough night and you’ve chosen a list of targets you’d like to work on. Now to put the targets in some kind of sequence. Usually this means by RA or by constellation.
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You can make this part easy if you know the Sidereal time for your observing. We explained Sidereal time in an earlier article. Now we're going to show you an easy way to estimate this close enough for planning a night out.
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Remember that the Earth makes a 12 month rotation around the Sun every year and that this rotation is divided up into 24 Hours of Right Ascension. This means that the Earth moves 2 Hrs of Right Ascension in its orbit every month. Here’s an article from Sky and Telescope describing Right Ascension.
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Now. Remember one date: September 22nd. This is the date that the Earth passes 0 degrees Right Ascension.
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Simply count the number of months from the current month until September 22nd or the number of months from the current month since September 22nd. Multiply that times 2 and either add or subtract that number from the local time when you’ll be observing. Don’t worry about fractions of a month - this will get you within an hour of the actual local sidereal time. And that’s good enough for planning purposes.
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Let’s look at examples before and after September 22.
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To estimate the sidereal time for 9PM on November 15th: November is 2 months after September, 2 months X 2hoursRA/month = 4 hoursRA . In September the Sidereal Time is 00:00. Since November comes after September we will add this to 00:00 to come up with the offset to be used with the local time. In this example 00:00 + 04:00 = 04:00 as the offset. Using a 24 hour clock, 9PM is 21:00 hours. 21 + 4 is 25. 25 is more than 24 so we subtract 24. Your estimate of the Sidereal Time for 9PM on November 15th is 25 - 24 = 01:00hoursRA. You check a planetarium program and find the exact Sideral Time would be 00:45hoursRA… plenty close enough to check a list of objects sorted by RA to see which object is on your meridian at that time.
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To estimate the sidereal time for June 10th: June is three months before September. 3 months X 2 hoursRA/month = 6 hours. In September the RA is 00:00hours. On a 24 hour clock this is the same as 24:00 hours. June is before September so we subtract, 24 - 6 = 18:00 hours offset. (You will always add the offset to the local time regardless of whether the date is before or after September 22nd) 9PM is 21:00 on a 24 hour clock. 21 + 18 is 39 which is more than 24 so we subtract 24. 39 - 24 = 15 hoursRA as your estimate. We check a planetarium app and the exact Sidereal Time is 14hourRA. Again, plenty close enough to see what objects are on your meridian.
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Remember - Right Ascension increases to the East and Decreases to the West. The constellations and objects with a RA greater than your Sidereal Time will be to your East and objects with a RA less than your Sidereal Time will be to your West. Begin with the ones in the West first since they will be setting first.
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25. How Far South/North Does Your Horizon Go?
If you’re in the Northern Hemisphere you’ll need to know how “low” your Southern horizon is. This same logic goes for your Northern Horizon if you’re in the Southern Hemisphere. Any object on your list with a Declination outside of these ranges will always be below your horizon and unobservable.
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e.g., here in Birmingham the North Celestial Pole is 33.3 degrees above the northern horizon. This means that the South Celestial Pole is 33.3 degrees below the southern horizon. The Declination is zero at the Celestial Equator and goes to +90 degrees to the North and to -90 degrees to the South. 33.3 degrees from the South Celestial Pole gives you a Declination of -56.7 degrees for our Southern Horizon here in Birmingham. Any object with a Declination any further South than that is never above our southern horizon and can’t be seen.
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Hit the “I believe” button.
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Similarly, if you are in Adelaide, Australia where the Southern Celestial Pole is 34.9 degrees above their Southern horizon, their northern horizon would be at +55.1 degrees and any object with a declination any further north than that is never above their northern horizon and can’t be seen.
26. Putting It All Together to Plan a Night of Observing
Basic Plan:
Observe when you can, from where you are, to see what you can, with what you have.
1. Do not wait for the perfect night.
2. Do not wait till you get the perfect equipment.
3. Do not wait to go to the perfect location,
a. Every place has a darkest spot.
b. Most places have some sky.
4. Have a list
5. Share the views.
6. Be safe.
7. Take notes.
8. Always spend some time just taking it all in.
There are two ways to plan an observation session:
1. Plan for the perfect night that’s coming up.
2. Plan for the not-so-perfect night you’ve got coming now.
A lot of folks I know are Plan 1 observers… they only go out on crystal clear New Moon nights… I do that as well, but honestly I’m a Plan 2 observer. If it’s clear outside - or close enough - I’m going to look at something. :)
By “close-enough” I’m referring to “sucker holes”. All of us have done this, or will do this - scattered clouds in front of a really transparent sky and you get your scope in position and wait for the clouds to pass for a while and the object to come into view. You are chasing sucker-holes; this can be a lot of fun or incredibly frustrating. You’ll learn to judge the size of the sucker-holes and the speed they’re moving and what’s good for you. Get out there because you may be wasting some good observing time.
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There are lots of ways to plan a night of observing. These are the steps I use to plan a session, and like many things in this hobby, most of these steps are done by a brief glance at a weather forecast.
1. Is it clear enough?
2. When is sunset and when is astronomical dusk?
3. When does the moon rise/set?
4. Transparency
5. Seeing
6. Smoke
7. Light pollution
8. Low temp
9. Dewpoint
10. Wind
11. Equipment
12. List
13. Meteor showers/occultations/shadow transits/etc.
Example: You’re going to be able to step out in the back yard for an hour or so around 8PM on Monday evening, Dec. 2nd. You bring up Astrospheric on your laptop and see the following. (Bluer is better)
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Don’t worry - the more times you use a chart like this one, the easier it is to pick out the info.
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The following shows you are going to have a fair night of observing. No moon but the light pollution will make faint deep-sky objects difficult, not the best night for detail on planets, no dew expected, no clouds for a couple of hours.
1 date
2 8PM
3 sunset at 4:38PM (Notice the little dashed line under the sunrise/moonrise horizon line? That is the point where the Sun is 180 below the horizon and Astronomical Dusk occurred at around 5:45PM. After then it’s technically night. - it will be nice and dark by 8PM)
4 moonset at 5:39PM - no moon
5 cloud cover is 0% - a clear night at 8PM
6 transparency is a little below average - faint stars hard to see and faint deep sky harder to see
7 wind is light at 2mph
8 seeing is below average so close doubles and planetary may be tough
9 temp is 32 degrees - a little chilly but the wind is light
10 dew point is 23 degrees - well below the ambient, no dew for the next hour or two.
11 You note that the clouds start moving in between 10PM and 11PM so you should have a couple of hours of good observing till then.
12 Also note that the temperature is getting close to the dew point around that same time. The light wind will help prevent the dew but it's probably going to be an issue around the time the clouds move in.
Make a note that the light pollution from your location is in the Bortle 5 range. This info can also be found on the Astrospherics display but you’ll have to change the overlay to the “light bulb” which is shown darkened on the right in the screen capture below
Shown below is a better description of the information you will find on the Astrospheric’s site. It’s a lot easier to learn the info on the site by spending a few minutes there. There are other sites that show similar info but I like to use this one.
The Kp index refers to solar activity activity. This is useful if you’re far enough North to see aurora. More info on this can be found here.
As mentioned before, this takes a lot of time to explain but just a few minutes to look over and decide if it’s a good night to observe.
OK. You’re happy that you’re going to have a couple of hours of observing time. Now look at your list. For this example we’re going to see what Messier Objects are viewable for the hours of 8PM to 10PM on Monday, December 2, 2024.
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First estimate the Sidereal Time: December is three months from September ( again, don’t worry about fractions of a month - this will be close enough). Three months X 2 hours of RA per month gives you 6 hours. You’ll be observing at 8PM. Always use a 24 hour clock for these calculations - 8PM is 20:00 on a 24 hour clock. 20 plus 6 is 26 which is 2 hours greater so you subtract 24. Your estimate of the Sidereal time at 8PM will be 02:00hoursRA. Using a planetarium program to give you the exact Sidereal Time gives 01:00hoursRA. Plenty close enough to look at a list of Messier Objects sorted by RA to see what objects are on the meridian at that time. Any object with a RA greater than 2 ( using your calculated number) will be to your East and any object with a RA lower than that will be to your West. Any object with a more southerly DEC than -56.7 degrees will always be below your Southern Horizon and can’t be seen.
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This is the way I plan for a night of observing. There are many variations depending on who you talk to but everyone looks at this same basic information. You will come up with your own way of planning a night but this will give you a good start.
Now get out there… :-)
27. Astro-math and a Few More Terms Explained
As we’ve talked about before, you don’t really need that much math - none really - but having a working knowledge of a few of the basics can really help.
First a review of some definitions:
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Focal length: the distance from a telescope’s primary mirror or objective lense to where an image is formed. For a generic 6” dobsonian this is usually 1200mm or approx. 48”. This is why the tube is almost 4’ long. Eyepieces have a focal length also except its the distance from the eyepiece to the image formed by the lense. Typical eyepiece focal lengths are 30mm, 26mm, 12mm, 6mm, etc..
Focal Ratio: the ratio of the focal length of a telescope’s primary mirror or lense to its diameter . The generic 6” (150mm) dobsonian is 1200mm/150mm = 8, shown as F/8. Eyepieces don't use Focal Ratios. The lower a telescope’s F number(e.g., F/5) the “faster” the telescope. The higher the F number(e.g., F/10) the “slower” the telescope. Fast telescopes typically show a wider “field of view” but require expensive eyepieces to bring out the best views. Field of View (FOV) is how much sky you see when looking through an eyepiece. This is explained in more detail below. Slow telescopes typically show a narrow field of view but are more forgiving of less expensive eyepieces.
Field Stop:
The field stop is an important part of the eyepiece. Its purpose is to “clean up” the outer periphery of the image. It is what determines “how much” sky - the “field of view” - an eyepiece will show. If you didn’t have a field stop you would be able to see the sides of the eyepiece. The sides are not located at the point where the image comes to focus and would yield a blurry edge instead of the sharp edge that a field stop provides. There are other optical issues (aberrations) with all eyepieces that are somewhat “cleaned up” by having a field stop. This is not too much of an oversimplification. The field stop is almost always measured in millimeters and this number can be used in calculating the field of view.
Field of View: This is “how much” sky you can see with an eyepiece. It is measured by the diameter of the circle of sky you see when you look through an eyepiece. That diameter is measured in “degrees of arc”. There are 180 “degrees of arc” from one horizon straight overhead to the other horizon behind you. A degree of arc is a measure of something’s “apparent” size at a distance. If you’ve looked at a full moon rising over a mountain in the East, you know it can “appear” to be the same size as a tree or tower on that mountain. Through an eyepiece both the tree and the moon would be the same “apparent” size - in degrees of arc.
180 degrees from horizon to horizon
60 arcminutes in one degree
60 arcseconds in one arcminute
A typical low power eyepiece in a 6” F/8 reflector would show you a 1 degree ( 60 arcminutes) field of view.
The moon is approximately ½ degree or 30 arcminutes wide.
The Orion Nebula ( M42) is 65 X 60 arcminutes
Jupiter is approximately 40 arcseconds in diameter
The double star Alberio has a 34 arcsecond separation
The double star Castor has a 6 arcsecond separation
The Dumbbell Nebula is 8 arcminutes X 6 arcminutes
To calculate the Field of View - ((Field Stop Diameter in mm)/(focal length of telescope in mm)) X 57
Example: What would be the field of view through a Meade 26mm Plossl eyepiece in an Orion 6” dobsonian, F/8 telescope?
The Meade 26mm Plossl has a field stop opening diameter of 23.9mm
The Orion dobsonian has a focal length of 1200mm
(23.9/1200) X 57 = 1.14 degrees or 68 arcminutes. This tells you that a full moon would take up about half of this eyepiece/telescope combination’s field of view.
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You can see how knowing an object’s size and your field of view would come in handy.
Apparent Field of View: I’m a math-geek so I like to use the more exact formula above to calculate the FOV. But there’s another formula using the Apparent Field of View of an eyepiece that is really close enough and most folks find it easier.
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All eyepieces have an “apparent” Field of View (AFOV) as well as a focal length. The apparent FOV is a measure of how much sky you could see if you were just looking through the eyepiece by itself. The Apparent Fov gives you an idea of how an eyepiece is going to perform in your telescope. Typical values are 40 degrees, 50 degrees, 68 degrees, 82 degrees, 100 degrees, and 110 degrees. If two eyepieces of the same focal length have different AFOVs, both eyepieces will give you the same magnification but the one with the higher AFOV will show more of the sky. The formula for this is: FOV = AFOV/magnification.
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This is helpful because the AFOV of an eyepiece can always be found but the field stop information described above is sometimes hard to find.
Here’s an example comparing the FOV calculated using the AFOV to the FOV calculated using the field stop formula above:
For a 26mm Meade Super Plossl eyepiece in the same 6”, F/8 dobsonian telescope we used in a previous example, the magnification would be 1200/26 = 46X. This eyepiece has an AFOV of 50 degrees. FOV = 50/46 = 1.090 or 65 arcminutes. This compares favorably to the field stop calculation result above of 68 arcminutes for the same eyepiece.
Exit Pupil: Exit Pupil tells you the size of the image that is actually reaching your eye. This is important because if this image is larger than your eye’s fully dilated pupil then the image will still be seen but some of the light is “wasted” - it’s like you had a smaller primary objective or mirror.
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I wear eyeglasses and during my yearly check up I ask the Optometrist to measure my fully dilated pupil. That number - again, for me - is 5.5mm. I am in my 7th decade as I write this and that’s a pretty decent number for someone my age.
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Exit pupil is defined as (eyepiece focal length)/telescope Focal ratio)
E.g., if I am using a 30mm eyepiece with an F/8 telescope then the exit pupil is 30/8 = 3.8mm
No wasted light.
The same eyepiece with an F/5 telescope would give you 30/5 = 6mm exit pupil - wasted light for my eyes.
Note that a younger person’s fully dilated pupil is typically 7mm.
Once again, there’s nothing wrong with having an exit pupil larger than your fully dilated pupil but in extreme cases you might actually see the shadow of the secondary mirror in an SCT or Newtonian telescope.
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If the exit pupil image is too small it may be interfered with by “floaters” in your eye itself. A commonly accepted “smallest” exit pupil is 0.5 mm.
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E.g., we have discussed previously how 300X is the highest usable magnification for any telescope due to turbulence in the atmosphere. To get 300X with a 1200mm focal length, 6”, F/8 telescope you would need an eyepiece with a 4mm focal length ( 1200/4 = 300X). The exit pupil with a 4mm eyepiece is 4/8 = 0.5mm, the smallest typical exit pupil for no image problems.
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Exit Pupil is illustrated in the image below.
A good explanation of all this is found here.
Eye Relief: We need to mention eye relief. This is the distance from the eyepiece to the point where you can see the entire FOV. I.e., it tells you how close to the eyepiece you have to be. Any closer or farther away and the image is distorted. This is shown in the exit pupil image above. This is important because many folks wear eyeglasses when they look through an eyepiece. If the eye relief is too short then the eyeglasses will hit the eyepiece. A good eye relief for glasses wearers is 15mm or greater.
Resolving power (Dawe’s Limit):
This is a measure of how small a detail you can see with excellent sky conditions. In general the larger the aperture of the telescope, the smaller the detail you can see. Resolving power determines how small a crater you can see on the moon, how tight a separation you can detect between the components of a double star, and how much detail you can see on planets.
There is a formula for this (Dawes Limit):
ɸ = 116/D, where D is measured in mm. Also, ɸ = 4.56/D if D is measured in inches. ɸ in both formulas is measured in arcseconds.
Note that when this formula is applied to double stars that it is valid for two stars of equal magnitude. If the primary star in a binary is significantly brighter than the secondary then all bets are off.
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Using this formula for the 6” dob we’ve been using so far, ɸ = 4.56/6 = 0.76 arcseconds of separation. Again, this would be for two stars of equal brightness, a sky that was perfectly still, and a telescope/eyepiece combination of good quality.
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This number is usually shown in the telescope’s specifications. Airy disc and diffraction rings
When you focus a star at high power (200X or higher) you will see a point of light surrounded by concentric rings of light. The point of light is called the Airy Disc and the concentric rings of light are called diffraction rings. The diffraction rings will tell you a lot about the stability of the atmosphere for the night ( see Pickering Scale in the section on “How to rate the night”). The diffraction rings will also show you if your telescope is properly aligned. See here for a discussion of collimation.
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Limiting Magnitude: This number tells you the faintest star that the telescope will show. It also assumes you have a perfectly still night, good optics, and no light pollution… not realistic but it gives you a benchmark as to what you can realistically expect to see.​​​
There’s no need to really remember this formula. The limiting magnitude is one of the numbers listed in a telescope’s specifications.
A good discussion of this is found here.
All of these articles cover a lot of topics and will get you off to a good start… but are by no means all there is.
Below we’ve listed a few more things to enter into your favorite search engine on a cloudy night:
1. Join a local club
2. read an astronomy book (search on popular amateur astronomy books)
3. go to a star party
4. Occultations
5. libration of the Moon
6. retrograde of the planets
7. trace the precession of the equinox and the path the NCP will take over the next 26,000 years
8. sundials (one of my favorites)
9. Moondials, clock time vs sidereal time
10. the constellations mentioned in the Bible
11. parallax
12. red shift
13. how stars evolve ( or at least how we think they evolve)
14. classification of galaxies by shape, relative distances between things in our own Milky Way Galaxy
15. How the latest space telescopes prove/disprove the accepted theories
16. Today’s cosmology
17. black holes 18. celestial navigation
19. celestial mechanics
20. read astronomy blogs
21. read online astronomy magazines
22. LOTS of stuff on imaging
23. Relativity
24. biographies of famous astronomers
25. history of astronomy’s development
26. amateur telescope making
27. get involved in Dark Sky International
28. visit a planetarium
29. take an online course
30. take a local college course
31. find out when the next lunar or solar eclipse is going to be (partial or total)
32. when is the next meteor shower ( or sprinkle)?
33. make a pinhole projector and look at some sunspots(never look at the sun directly!)
34. watch a variable star grow dim and then brighten again
35. find out when the ISS will pass over and observe it,
36. watch aurora
37. measure how dark it is (NELM or SQM)
38. join Citizen Science groups
39. Make a seasonal calendar of objects to see that are examples of types of objects mentioned?
40. Note the location of the sun set and rising during the year
41. See a planet back-up
42. Phases of Venus
43. See how the moon moves N and S of the ecliptic ( see ascension and descending below)
44. APOD astronomy pictures 45. Interstellar zoo
46. what is albedo?
47. Look for land features and dust storms on Mars
48. Note the change in size of Mars in your eyepiece as it goes through its two year cycle
49. Observe the Milky Way
50. Observe the galactic bulge
51. Observe the gegenschein
52. Observe the zodiacal light
53. Watch a meteor shower (or sprinkle)
54. Read forum posts and comment yourself
55. Learn the Greek alphabet
56. Look at a picture of Earth’s location in the Milky Way
57. Look at a picture of the Milky Way’s location in the Universe.
58. Learn about equatorial platforms
59. Read up on how to build a radio telescope
60. Observe a supernova
61. View a comet
62. Read up on orreries.
63. Read up on astrolabes
64. Read about ancient structure orientation with the stars
65. Listen to a podcast
66. Buy some Astro stuff: shirt, hat, bumper sticker.
67. Observe a planetary alignment
68. Buy a meteor
69. The NASA website
70. Observe a ring around the moon
71. Observe sun dogs
72. Read up on and/or build an analema
73. Learn some basic geometric shapes to help describe Star groups( diamond, rhombus, rectangle, acute triangle, right triangle, etc..)
74. Learn about Supermoons and Micro Moons
75. Learn about orbital nodes, geocentric, heliocentric, galactic?. Ascending and descending
76. Take a look through a starlight scope or light amplified eyepiece
77. Use a tape measure and a yardstick to measure your fov. and LOTS more…
Now get out there.