How do Telescopes Work

(and How do I Choose one?)

High Power Myth

Beginners often mistake magnification for quality. Certainly magnification has a place in observing with a telescope, but it is only part of the answer. A general rule of thumb for maximum useable power in telescopes is about 40x to 50x per inch diameter of the objective (or aperture). So a 2.5" telescope (refractor or reflector, it makes no difference), can usefully go to a power of 2.5 x 50 or about 125x.

This rule is only true if it's a very clear night, when stars are not twinkling. When the stars twinkle, it's because winds high above our heads are causing the star light to bend this way and that. When you look in the telescope at high power, this shaky motion gets magnified, and it's much the same as looking at a home movie where the photographer was rapidly moving the camera: you can't see anything well, everything looks blurry. Or like trying to see the bottom of a pool of water when someone drops a rock and waves move by. On bad nights, with twinkling, 30x is more appropriate. Of course you can decide for yourself what magnification you want to use; we're only explaining how to get satisfying images where you see all the detail you can, without blurriness.


Low Power

Another way to think about how telescopes work is to imagine the telescope as a funnel. The wide end of the funnel is the aperture (the mirror at the bottom of a Newtonian telescope), the narrow end of the funnel is the exit pupil at the eyepiece. The magnification is the ratio of the wide end to the narrow end. The idea is to concentrate the light so you can see faint objects that you can't see with your unaided eyes. Few people stop to think that the stars are always out and visible in the daytime, not just at night. The reason we can't see them is that they are not bright enough.

Since your eye is at the narrow end, larger telescopes give more light and more magnification. The pupil of your eye only opens to about 7mm, so larger telescopes require higher magnification to get all of their light into your eye. This is what we all want, more magnification, so we can see more detail.

Resolution is in the Details

The detail in starlight is always there, we just can't see it because our eyes cannot see things when they get too small or too close together. This limitation is based on the number of rods and cones we have in our eye and the area they occupy. We just can't see things seperated by less than 1/10,000th of an inch.

So we use a magnifier, an eyepiece, to bring out the detail. It's just like enlarging a photgraph- you can't get more detail than was in the original slide or negative, but making the photo larger (up to a point) improves what we can see. The telescope mirror, like film, holds much more detail than our eyes can see without magnifying the image. The question then is, how much can we magnify before it no longer does any good? Once again, it depends on the size of the mirror. By now you may be beginning to understand why Astronomers are always making bigger and bigger telescopes!

The air is what limits resolution for many telescopes. This is the major reason why photos from Hubble are so stunning- it does not have to look through miles of air to get it's light. The result is seeing higher resolution when we magnify the images. The air is the reason why 40x-50x per inch is a practical limit for our telescopes on earth. By the way, telescopes do perform much better at high altitudes, due to the reduced amount of air. The Keck Telescope is located atop Mauna Kila in Hawaii (14,000 feet) for this reason.


Here is a simple demonstration of the minimum useful magnification of a telescope. This will make a faint object as bright as possible. A 2.5" (50mm) telescope then has a minimum power of about 7x., which is typically what you find on nice binoculars.

High Power

(not completed )


 Telescope Diameter

Minimum Magnification

Maximum Magnification
1.5  7  75
2  8  100
 2.5 (50mm)  10  125
 3.0  11  150
 4.25  16  210
 6" (150 mm)  22  300
 8"  30  400
 10"  38  500
 12.5"  48  625
 14"  52  700

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Copyright Robert Duvall 1997