• A Newtonian telescope has a big drawback: it uses a single reflection from a curved mirror to send light to an eyepiece. That means the length of Newtonian is roughly equal to the focal length, so you get a long and heavy telescope with a big and awkward mount.
• But shortly after Newton developed his reflector, an obscure French Catholic priest named Laurent Cassegrain invented a reflector that used two mirrors to fold a long optical path into a shorter tube. Now nearly all reflectors use a variation of the Cassegrain telescope.
• Not until 1930 did Bernard Schmidt add a new twist. He combined a simple spherical mirror with a specially-figured lens at the front of the tube to correct for spherical aberration. At the focal plane, he placed a piece of film. This layout is a Schmidt camera. It's used for imaging wide-field views of the sky.
• Finally, in 1946, an architect and artist named Roger Hayward placed a convex mirror behind the corrector lens to send light out the back of the tube to an eyepiece or a camera. Celestron built on this design and developed manufacturing techniques to produce SCT's in large quantities that revolutionized amateur astronomy.
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