A reflector telescope uses a primary mirror to collect the light from distant stars and galaxies.
In 1616, just seven years after Galileo built his first refractor, Niccolo Zucchi attempted to build a reflecting telescope using a metal reflector. He found it impractical. Ten years later, Robert Hooke built a workable reflector. It was small and the astronomer’s head blocked much of the incoming light, as you can interpolate from the sketch. The astronomer had to look directly down the telescope.
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Then in 1668, Sir Isaac Newton had one of his many flashes of genius. If he put another smaller flat mirror in the path of the light reflected from the primary mirror, he could redirect the image to a comfortable viewing position. He did, and the Newtonian Reflector, the basis of all astronomical telescopes since, was born. The secondary mirror is called the diagonal.
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But Newton’s reflector and all built after him for centuries suffered from one failing. The primary mirror was spherical. This meant the light rays did not all come to a common focal point. This caused spherical aberration and other problems that interfered with seeing. It was not until the latter part of the 19th century that telescope makers learned to grind parabolic mirrors.
Parabolic mirrors focused the light rays to a single point. Gone was spherical aberration and some of the other problems, but some others remained.
Newtonian reflectors tend to be quite long for a given focal length (fl). The light from the primary must travel some distance to reach the diagonal.
But if the secondary can reflect the light back through a hole in the primary, the telescope’s length can be greatly reduced for a given fl.
