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Newtonian Telescope: Size of secondary mirror and exit pupil

A Newtonian telescope uses a curved mirror to focus light from a star. A flat mirror called a secondary mirror intercepts the converging light and redirects the light 90 degrees to the side of the telescope for viewing. See attachment.

Determine the minimum size for the secondary mirror so that it collects all the light from a 200mm diameter mirror operating at f/6 for objects subtending up to 0.5 degrees. What is the focal length of an eyepiece that will give you an exit pupil diameter of 7mm?


Solution Summary

The specifications for a Newtonian telescope are given. The length of the secondary mirror and the focal length of the eyepiece for given exit pupil diameter are calculated.