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 of its reflecting surface, the image has no spurious color. The mirror is located at the bottom of the telescope tube rather than at the top, so the converging light cone is turned 90° by a small flat mirror (or diagonal) before it exits the tube and reflected through the side where it comes to a focus. All of the world’s great observatory telescopes are reflectors of one form or another, including the famed 200-in. Hale reflector at Palomar and the twin Keck 400-in. reflectors in Hawaii. So too is the 94-in. orbiting Hubble Space Telescope. (And the two “monster telescopes” now on the drawing boards having apertures of 1,200 and 1,600 in. will also be reflectors!) 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) (Fig. 5.1).