General Astronomy/Telescopes/Optical Telescopes
Behind this lens could be a camera device or else an eyepiece for an observer.
Optical telescopes increase the apparent brightness and magnification of an object. Infrared telescopes look for heat signatures. There are also huge radio telescope dishes, as well as ultraviolet and x-ray telescopes that have been launched into orbit.
There are two main types of optical telescope - reflectors and refractors. Reflectors use a mirror to collect the light, whilst refractors use a lens. Generally, reflectors are better for deep sky objects whilst refractors are useful for planetary observations. Some telescopes use a combination of lenses and mirrors to use best the advantages of each.
Refractor telescopes require a large lens at the front to direct the incoming light to the lens. This lens can be made of more than one piece, and the differing refractive properties of the components serve to reduce aberration at the edge of the image. The lenses can also be coated to reduce internal reflections.
Catadioptrics are short, wide telescopes that use both mirrors and lenses. The price range is between reflectors and refractors.
Large lenses suffer from the disadvantage of sagging under their own weight. This causes the shape to become distorted and so the image will be irregular. For this reason the largest refracting telescope ever built only has a diameter of 41". All larger telescopes instead use a mirror are the primary collecting surface.
In the optical spectrum, great precision is required in crafting the lenses and mirrors used in a telescope. The standard of precision for a telescope mirror is accuracy to 1/4 the wavelength of light, an exceedingly small quantity. But even greater accuracy is generally preferred, especially for space-based telescopes. Special tools have been developed for measuring the accuracy of a telescope mirror and lens, and the procedure for finishing the surface is very exacting.