Some Telescope Designs:

Mainly there are two categories of telescopes -

Refractor Telescopes and Reflector Telescopes -

In Refractors, in front of the tube there may be various variety of lenses from plano convex to complex achromats and apochromats. Simple single

lenses gives lot of defects like colour (chromatic aberration), spherical aberration, coma, astigamatism etc. As after one inch of aperture these defects are too much, so to remove these defects special glasses and double or multiple lenses are a necessity in Refractors. Over these lenses applying anti-reflection coating is also a necessity so that light losses are minimum This types of telescopes having good achromats and apochromats using high optical quality glasses though give a very good performance according to quality but are extremely costly and difficult to make..

Besides, there is a limit of making big size lenses. After 6 to 8 inches it becomes very difficult and costly. Till now possibly the biggest lens telescope is of 40" aperture. While for Mirrors this limit is very big and the biggest telescope is possibly of 10meters' aperture.

REFLECTORS: In reflectors instead of lenses Mirrors are used. At the end of a tube Mirror condenses the light which is reflected by another

mirror and comes to your eye. Even the simplest spherical concave mirrors can be used. The image is completely colour free. These are easy to make and cheap.

Though in per unit of aperture performance of Achromat/Apochromat Refractors is great. But you can have double/triple-big aperture mirror-telescope in the same cost in which you get a simple achromat lens telescope. So, performance-wise this double-aperture mirror telescope will beat the costly lens telescope in every sphere and show you great images.

Moreover, very big aperture Mirrors telescopes can be made even by amateurs while lenses are limited by size and after 6 to 8" these become prohibitively costly.

Hence, all over the world Reflector or Mirror Telescopes are more popular. These are made  in high quality also, even then are cheaper from Refracting or Lens Telescopes.

There are a lot of varieties among two main Reflecting type of telescopes -
Newtonian and Cassegrain (Classical Cassegrain, Schmidt Cassegrain, Maksutov, Gregorian, Ritchey Chretien, Houghton, Rumak, Simak etc.).

Amongst these Newtonian (with Dobsonian mount) is popular amongst amateur telespope makers for its simple construction and cheap cost; and Cassegrain in general for comfortable use, portability and photography purposes.

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