Optical telescopes increase the apparent brightness and magnification of an object. Other telescopes are capable of "seeing" in the electromagnetic spectrum beyond the abilities of our eyes and transforming those images to those we can use. Infrared telescopes are capable of detecting heat signatures. There are also huge radio telescope dishes, as well as ultraviolet and x-ray telescopes that have been launched into orbit beyond the our Earth's obscuring atmosphere.
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 the advantages of each.
Refractor telescopes require a large lens at the front to direct the incoming light to the lens. This lens may 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 may also be coated to reduce internal reflections.
Catadioptrics are short, wide telescopes that use both mirrors and lenses.
Large lenses suffer sag under their own weight. This causes their 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 as 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.