Reflecting Telescope

A reflecting telescope replaces the refracting telescope's large objective lens with a large concave (parabolic) primary mirror, which collects incoming parallel light from a distant object and brings it to a focus by reflection rather than by refraction; a small secondary mirror is then typically used to redirect this converging beam sideways or back out through a hole in the primary mirror, to a convenient, accessible point outside the main light path where an eyepiece (used exactly as in the refracting telescope, as a simple magnifier on the real image formed by the mirror system) or a modern electronic detector is placed, without the observer's own head or instrument blocking the incoming starlight. The reflecting design offers several real practical advantages over the refracting telescope that are precisely why every major modern research observatory telescope is a reflector rather than a refractor. First, a mirror suffers no chromatic aberration at all, since reflection (unlike refraction through a lens) bends every wavelength of light by exactly the same amount, giving a sharper image with no colour fringing, whereas a large lens inevitably focuses different colours very slightly differently. Second, a large mirror needs to be figured accurately on only one reflecting surface, and (crucially) can be mechanically supported firmly from its entire back surface across its whole area, whereas a large refracting lens can only ever be supported around its thin outer edge (since light must pass freely through its whole face), so a very large lens t …