How Astronomy Works
A lot of people think that telescopes are for magnification.
While this certainly seems right, the use of astronomical telescopes is a little different.
Would it shock you to learn that most astronomers enjoy viewing the most with the lowest magnification eyepieces?
It shouldn’t. Here’s how astronomy really works, and what telescopes are for.
The Light Funnel
Early scopes weren’t much different than the stuff you see on the shelf at the local department store around Christmas time. Galileo’s scopes were terrifically humble by today’s standards, ranging from 8x to 20x magnification, but it was with such simple devices that he contributed to a change in our understanding of the universe and our place in it.
With simple, low magnification scopes, many things become clear. More stars become visible. Not just bigger, they actually seem to appear where there were no stars before. The reason is because telescopes not only magnify, but they act as light funnels.
If you look at the human eye, you will see a tiny black pupil near the center. It works like the f-stop on a camera, opening to let more light in when it is dark, and shrinking to reduce bright light and increase sharpness.
The problem is that eyes are only so big, and generally a human pupil won’t grow to more than about 7mm across. This limits the amount of light that can be collected, and stuff that is really faint can’t squeeze enough photons through to make an image in our brains.
A telescope or binocular changes this. If the front lens is, say, 70mm, the scope collects light from that big lens and stuffs it down into a light cone that can enter your pupil. Think of it as an amplifier for night vision, allowing you to get a better look at things that were otherwise too dim to see before.
Of course this isn’t the whole story, but it is a terrific way for the newcomer to look at scopes. If you want a number to focus on, forget about the magnification claim on the box and check out the objective diameter. More objective means more light.
What is an Objective
Objective is a fancy word for the primary and usually the biggest element in a telescope. In a telescope made from lenses, this is the big glass piece at the end that points to the sky. In a reflector, it’s the large mirror at the back of the scope.
The surfaces on these elements are curved, which bend light rays toward a single point, known as a focal point. This is the spot that parallel light rays will meet after bouncing off the mirror or being redirected by the lens.
On the other end of the light’s journey through the tube is the eyepiece. For the sake of simplicity we can treat it as a single lens even though modern eyepieces have 3 or more lenses at a minimum.
After the focal point, the light begins to expand again, and by placing an eyepiece at the proper distance from the focal point, an image can be transferred through the glass and into your eye.
As you probably know, the image has been magnified at this point, but the bigger that objective is, the more you will be able to see. A bigger objective for the same magnification will make the cone of light leaving the eyepiece physically bigger.
This light cone is known as exit pupil, and if you can get it as big as your pupil then you will see as much as you can for that magnification. Remember, your pupil is the final f-stop, so going bigger doesn’t get you much.
On bright objects, this won’t matter so much. The moon and brighter planets are big enough to resolve details without collecting as much light, and allow the use of filters to add some contrast.
On dimmer objects, such as galaxies, nebulae, and very dim stars, the more light you can collect, the better. This is why Dobsonian telescopes, known as “light buckets,” are so popular for beginners. They give a good sized aperture at a great price compared to other types of scopes.
Calculations
To really know your stuff when it comes to telescopes, some basic calculations will help you sort out what you can see with different scopes.
The first thing to note is the size and focal length of the primary optic. We talked about this for a moment, but knowing the size and focal length in millimeters is incredibly helpful to figuring out magnification and exit pupil.
Focal length just means the distance from where light enters the lens or lens group until it focuses to a single point. Both your scope and your eyepiece will have a focal length.
Magnification is quite easy to calculate. The focal length of the scope divided by the focal length of the eyepiece will give you the magnification. A 200mm focal length objective lens paired with a 20mm eyepiece will give you 10x magnification.
There is a catch here, where the useful magnification has a maximum limit defined by physics. For simplicity it is twice the diameter (in mm) of the main objective. So, that 50mm scope at the store that claims it will magnify 525x? Yeah, the lenses are there for the calculation we just ran through, but past 100x you will see nothing of value, assuming the glass is good enough even for that.
To figure out exit pupil, you divided the objective size (the diameter of the big lens in mm) by the magnification. A 7x50 binocular has an exit pupil of about 7.1mm. A big eight-inch mirror set to 100x will have an exit pupil of 2mm.
I know I said bigger is better on exit pupil, but don’t freak out about that 2mm just yet.
Optimal Exit Pupil Size
Often overlooked by new astronomers, exit pupil is kind of a big deal. It will determine what you are able to see. At less than half a millimeter, you probably won’t see anything of value. That corresponds to the two times the diameter magnification rule in the previous section as well. The maximum usable magnification will give an exit pupil of 0.5mm.
Assuming a good quality scope, at least.
What is this magnification useful for? Basically planets, the moon, and other bright objects that you want to blow up nice and large. If the skies will allow it. Once again there is a limit, and this time it’s the sky itself, which can wash out details at magnifications over 200x.
Either way, maximum magnification is for the big and bright. If you are splitting double stars, you can get away with a tiny exit pupil in some cases.
For deep sky objects, you will want more light and less magnification. You will find out through practice which eyepieces work best for you, but for galaxies, it’s good to keep in the 2-4mm exit pupil range.
If you want to pan the sky and see everything as well as you can, then forget magnification and focus on exit pupil. Exit pupils over 5 are recommended for really dim objects that are especially large and spread out, such as the Andromeda and Triangulum galaxies.
In fact, a 7x50 binocular can sometimes be better than a scope for these objects, allowing you to use both eyes to take in the view.
Very faint nebulae will require not only a big exit pupil but a big scope. Anything less than 8 inches will probably not let you see the Horsehead nebula, even with the proper filters and really dark skies.