Newtonian telescope

A Newtonian telescope is one of the simplest kinds of reflecting telescope, popular with amateurs because of its effectiveness and relatively low cost. It uses a parabolic primary mirror to focus an image onto a flat diagonal, which, in turn, reflects the image to a side-mounted eyepiece near the top of the tube. The Newtonian overcame the problem of chromatic aberration that blighted early refractors but can itself be affected by coma at focal ratios of less than f/3.

Newtonian telescope, cutaway view