Reflecting Telescopes

A reflecting telescope uses a large concave mirror, instead of a lens, as its objective. The arrangement you need to know is the Cassegrain arrangement:• a large concave parabolic primary mirror with a hole in its centre collects the light
• a small convex secondary mirror reflects the light back towards the primary
• the light passes through the hole in the primary and is focused just behind it, where an eyepiece (or camera) is placedFolding the light path back on itself makes the telescope shorter than a refractor of the same focal length.Advantages of reflectors over refractors:
• mirrors reflect all wavelengths in the same way, so there is no chromatic aberration
• a parabolic primary mirror removes spherical aberration
• a mirror can be supported from behind, so very large mirrors are possible; a large lens can only be held at its edges and sags under its own weight
• only one surface needs to be ground and polished, and the glass does not need to be perfectly transparentDisadvantages:
• the secondary mirror and its supports block some incoming light and cause diffraction, slightly reducing image quality
• mirror surfaces can tarnish and need recoating, and large mirrors can distort with temperature changesFor these reasons, all of the largest optical telescopes are reflectors.