Reflecting Telescopes

Abstract

The severe color aberrations of the early single-lens refractors soon led to the invention of the Newtonian reflector by Sir Isaac Newton. This form uses a concave parabolic (or spherical) primary mirror to collect light and bring it to a focus. Since the light never passes through the glass mirror but only bounces off its reflecting surface, the image has no spurious color. The converging light-cone reflected from the primary mirror at the bottom of the telescope tube is turned 90 degrees by a small optical flat (or diagonal mirror) before it exits the top and is reflected through the side, where it comes to a focus. All the world’s great observatory telescopes today are reflectors of one form or another, including the legendary 200-inch Hale reflector at Palomar and the twin Keck 400-inch reflectors in Hawaii (and the famed Hubble Space Telescope). This is partly because their huge mirrors can be supported from behind (instead of around the edge, as with refractors). It’s also due to the fact that the glass itself does not need to be of “optical” quality, since the light merely reflects off its polished and coated surface rather than passing through the glass itself (again, as is the case with refractors).