The single most common question one hears from beginning telescope users is "How much will this telescope magnify?". What people fail to realize is that magnification is in many ways the least important function of a telescope, especially for astronomy.
The first thing that telescopes do is make dim objects brighter. Your eye's pupil when completely dark adapted might open up to 7mm or so. Only so much light can get through the pupil to hit your retina. A telescope concentrates more light into a small area, so your eye receives more photons and the image seems brighter. As an example, a telescope with an aperature of six inches gathers roughly 460 times as much light as your eye does. This allows you to see much dimmer objects than you could see with your unaided eye.
When one makes the assumption that all stars are equally bright, then seeing stars which are 460x as dim means that you are on average seeing 460x farther than you can with the naked eye. Again, if you assume that all stars are evenly spaced, this means that you can see vastly more stars, roughly ten million times as many stars as you can see with the naked eye.
Telescopes also make details sharper. The resolution of a telescope, or its resolving power is a measure of how sharply defined the details of an image can possibly be. For instance, if two stars are separated by a very small angle, it may appear to be a single point. Even if the magnification of the telescope is increased, the single spot will only look like a bigger fuzzy spot, and will never resolve into two separate spots.
The resolving power of a telescope is dependant on its aperature, which is the size of the main mirror or lens in the telescope. The bigger the aperature, the higher the resolving power of the telescope. We want bigger telescopes not just because they gather more light, but because they allow us to discern small details better. As an example, here are some pictures of Jupiter as might be seen through telescopes of varying aperature.
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Jupiter as seen by the Hubble Space Telescope. This is just about as good as it gets. The Hubble Space Telescope has a very large mirror, and it is positioned above the Earth's atmosphere where turbulence cannot affect the image quality. The rest of the images on this page are generated by a computer program using this picture as the perfect image. |
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Jupiter as seen through a perfect 2 inch telescope. This is about as good as it gets for the average department store telescope, which has a very small aperature. Most department store telescopes are actually much worse than this, because they typically have less than stellar optics. |
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Jupiter as seen through a perfect 6 inch telescope. You can see that this is MUCH improved over the 2 inch telescope. Under good conditions, a telescope with six inches of aperature can show very significant details on Jupiter. This size of telescope was typical for telescope makers, but is now considered a bit on the small side. |
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Jupiter as seen through a perfect 8 inch telescope. This is probably the most popular size for a first telescope for our workshop. The image is indeed a bit sharper than the 6 inch telescope, but the mirror is only slightly more difficult to make, and only marginally more expensive. |
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Jupiter as seen through a perfect 12 inch telescope. Most of the time, atmospheric turbulence will keep the image from appearing this good, but during moments of calm details like this should be visible in a telescope with this aperature. |
Yes, once you get past all this, telescopes also make small things bigger, or magnify. This is a useful function as well. The smallest angular separation that your eye can normally pick out is only about 1 arc minute, or 1/60th of a degree. This means if your telescope doesn't magnify enough to make the smallest angle that it can resolve equal to 1/60th of a degree, then there are details in the image that your eye isn't picking up.
This minimum useful resolution can be calculated. For instance, a six inch telescope has a resolving power of about 3/4 of an arcsecond. We'd like to magnify that so 3/4 of an arc second was roughly equal to 1/60th of a degree. That means the telescope should magnify roughly 80 times (60 arcseconds / 3/4 arcsecond). In theory, all the detail your eye can discern through a six inch telescope is visible at a magnification of 80 times. In practice, telescopes are often used at higher magnifications, because resolving small details can introduce eyestrain, and also at lower magnifications when wide field of view is more important than resolving power.
We've seen above how the maximum resolving power is dependant on the aperature, not the magnification. The normal rule of thumb is that the maximum power you should use with a telescope is 60x per inch of aperature, or roughly 2x per millimeter of aperature. Thus, for a six inch telescope, we might expect to use a maximum of 360x on a night with excellent seeing. Using more magnification than that doesn't resolve any new details. Thus for a typical department store telescope with sixty millimeter aperature, you should not be using magnifications greater than 120x or so. Advertisements of "600x" are optical snake oil, and usually the sign of a poorly made telescope.
All materials on this website are Copyright 2001, Mark T. VandeWettering. Permission is granted to reproduce and distribute these files for non-profit, personal use.
Mark T. VandeWettering <markv@telescopemaking.org>