A telescope "telescope" is an instrument designed for the observation of remote objects. The term usually refers to optical telescopes, but there are telescopes for most of the spectrum of electromagnetic radiation and for other signal types. An optical telescope is an optical tool that gathers and focuses electromagnetic radiation. Telescopes increase the apparent angular size of distant objects, as well as their apparent brightness. Telescopes work by employing one or more curved optical elements - lenses or mirrors - to gather light or other electromagnetic radiation and bring that light or radiation to a focus, where the image can be observed, photographed or studied. Optical telescopes are used for astronomy and in many non-astronomical instruments.
The main purpose of a telescope is to gather light, i.e. to collect and focus photons. We can think of a telescope then as a "light bucket" - the bigger the bucket, the more photons a telescope can collect.

Types of telescopes:
There are three basic types of telescopes -- Refractors, Newtonian reflectors, and Catadioptrics.

Newtonian Reflector Telescopes:
Newtonians usually use a concave parabolic primary mirror to collect and focus incoming light onto a flat secondary mirror that in turn reflects the image out of an opening at the side of the main tube and into the eyepiece.

Refractor Telescopes:
Refracting telescopes have an objective lens at the front of the tube. The light exits out through the back of the tube to the eyepiece. Since many observations are made high in the sky, a right-angle diagonal is used to avoid neck strain. This also provides an upright image making them suitable for terrestrial observations. A refractor has several advantages over other designs. The tubes are enclosed so that dust and moisture do not enter the tube, they have fixed optics that do not normally require collimation, and they do not have a central obstruction which reduces the light entering the tube. A refractor typically will give higher quality images of planets than other telescopes of similar aperture.

Catadioptric Telescopes:
Telescopes using a combination of both mirrors and lenses are called catadioptrics. There are many different designs. Examples of these are the Schmidt-Cass grain and the Maksutov-Cassegrain. Usually a full aperture lens is used to correct aberrations in a compound reflecting telescope. The corrector lens also increases the performance of the instrument as air currents are eliminated. The main advantage of the design is that, because the light path is folded back on itself, it provides a very portable, short physical length telescope with a long focal length.

Benefit:
1. The aperture of a telescope is several times larger than the aperture of human eye so that the objects that can not be normally seen by unaided eye can be seen. Light- gathering power of a telescope is proportional to the area of its aperture and hence depends on the square of the radius of the mirror. Therefore a 20 cm diameter telescope collects four times more photons than a 10 cm diameter telescope.

2. A telescope can be equipped to record light over a long period of time, by using photographic film or electronic detectors such as photometers or CCD detectors while the eye has no capability to store light. A long-exposure photograph taken through a telescope reveals objects too faint to be seen with the eye, even by looking through the same telescope.

3. A third major advantage of large telescopes is that they have superior resolution, the ability to discern fine detail. Small resolution is good. The resolution is directly proportional to the wavelength being observed and inversely proportional to the diameter of the telescope.