Eyepiece

A telescope collects light and forms a small fixed-size image at a point (called the prime focus) that's determined by the focal length of the optical system. You can see this image by aiming your telescope at something bright, such as the Moon, taking out the eyepiece and star diagonal, and holding a piece of paper behind the focuser. Move the paper back and forth. At some point, you will find a small, but sharp, image of the Moon projected onto the paper. This is the prime focus image formed by the telescope. Unfortunately, human eyes typically cannot focus sharply on an image unless it's more than eight inches from the eye. This makes it difficult to see detail in the small prime focus image formed by the telescope if it's examined solely with the unaided eye. An eyepiece is a small microscope that allows you to get closer than eight inches from that small fixed-focus image -- and the closer you can get to an object, the bigger it appears. A 25mm eyepiece, for example, lets you focus on the scope's prime focus image from an effective distance of only 25mm (one inch away from your eye); a 12mm eyepiece puts you half an inch away; etc. The magnification of an eyepiece is found by dividing the telescope focal length by the eyepiece focal length. A 25mm eyepiece used with a 2000mm focal length scope therefore provides 80 power (2000 / 25 = 80x), making objects appear 80 times larger than they do to the bare eye (or 80 times closer, to put it another way).


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