Astronomy
Telescope Types

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There are 4 major types of telescope: Refractors, Newtonian, Schmidt Cassegrain Catadioptrics, and Maksutov Cassegrain Catadioptrics.   All the telescopes used by Gallileo, Hugens, Cassini, etc were of the Refractor type. Newton designed the first Newtonian telescope using a parabolic reflector for the objective, instead of a lens.

Newtonian Telescopes feature the widest apertures as evidenced by the giant Telescopes at Mount Palomar, and Keck Peak.  They are the easiest to make but the hardest to use. The eyepiece is located near the open end, requiring a large stool or ladder to use the bigger ones. They are also the most fragile of all the designs, needing to be collimated frequently. Their exposed mirrors are subject to damage from the elements.