A telescope "telescope" is an instrument for observation of distant objects. The term usually refers to optical telescopes, but there are telescopes for most of the frequencies of electromagnetic radiation and other signal types. An optical telescope is an optical tool that collects 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 light or radiation focus where the image can be seen, studied or photographed. Optical telescopes used for astronomy and in many non astronomical instruments.
The main objective of a telescope is to gather light, i.e. to collect and focus photons are. We can think of a telescope and 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 - Refractor, Newtonian reflectors, and Catadioptrics.
Newtonian Reflector telescopes:
Newtonians usually use a concave parabolic primary mirror to collect and focus incoming light on a flat secondary mirror, which in turn reflects the image of an opening on the side of the main tube and into the eyepiece.
Refractor telescopes:
Lens Binoculars have a lens on the front of the glass. The light comes 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. It also gives an upright image makes them suitable for terrestrial observations. A refractor has several advantages over other designs. The tubes are sealed so that dust and moisture does 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 will typically provide a higher quality images of planets than other telescopes in the same aperture.
Catadioptric system telescopes:
Telescopes using a combination of both mirrors and lenses called catadioptrics. There are many different designs. Examples of these are the Schmidt-Cass grain and Maksutov-Cassegrain. Normally a full-aperture lenses used to correct deviations in a composite mirror telescope. The correction lens also enhances 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 offers a very portable, short physical length telescope with a long focal length.
Advantage:
1. The aperture of a telescope is several times larger than the aperture of your eye, so the objects can not normally be seen by naked eye can see. Light-gathering power of a telescope is proportional to the area of the aperture and thus 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 positioned to absorb light over a long period by means of photographic film or electronic detectors such as photometers or CCD detectors, while the eye has no ability to store light. A long exposure photograph taken through a telescope reveal objects too weak to be seen with the naked 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 distinguish fine details. Small resolution is good. The resolution is directly proportional to the wavelength observed and inversely proportional to the diameter of the telescope.
