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.