A telescope is an optical instrument using lenses, curved mirrors, or a combination of both to observe distant objects or various devices used to observe distant objects by their emission, absorption, or reflection of electromagnetic radiation.[1] The first known practical telescopes were refracting telescopes invented in the Netherlands at the beginning of the 17th century using glass lenses. They were used for both terrestrial applications and astronomy.
The reflecting telescope, which uses mirrors to collect and focus light, was invented within a few decades of the first refracting telescope. In the 20th century, many new telescopes were invented, including radio telescopes in the 1930s and infrared telescopes in the 1960s. The word telescope now refers to a wide range of instruments capable of detecting different regions of the electromagnetic spectrum and, in some cases, other types of detectors.
Modern-day telescopes bring with them a wide array of accessories. With the aid of these accessories, it becomes much easier to put the telescopes into proper use successfully. Let us see what the important accessories that go well and are a “must-have” for most telescope users: are -
Filters are essential for telescope users because they reduce the glare on the eyepiece and scatter the light, making it less strenuous for us to view distant objects.
Eyepiece- this is a critical piece of equipment and its absence can render a telescope useless. Eyepieces come in a variety of sizes, and if you add more than one eyepiece to your telescopes, then you can have the pleasure of viewing even the tiniest and distant objects with clarity.
Mounts- Mounts are the stands for telescopes. It is the amount on which the telescope rests. Since a telescope won't function favorably in vibrations, therefore, you should ensure that it is placed on a stable mount.
Barlow lens- A Barlow lens helps in the magnification of telescopes. These lenses are available in different sizes, and we must be careful in choosing the size that best fits the eyepiece.
There are two main telescopes: the refractor (Galileo) telescopes and the reflector (Newton) telescopes. The refractor telescope uses an objective lens that bends the light towards the eyepiece. At the same time, the reflector telescope uses a mirror, which collects the light and then directs it towards the eyepiece.
Though both the refractors and the reflector telescopes are easily available in the market, we should consider the following points before buying a telescope.
Do not get impressed by the magnification of the telescopes. This is because magnification alone is of no use if it just produces large but hazy images.
Aperture- This is the most important factor while deciding on a telescope. Aperture is the opening that collects light, so it's important to choose a telescope with a larger aperture to ensure clearer and more detailed image quality.
Resolution- This enables the telescopes to produce a more detailed image. So always opt for a telescope with a higher resolution; moreover, since the aperture determines the resolution, the larger the aperture, the better the resolution.
Focal length- The distance between the lens or the mirror's optical center and the eyepiece's optical center is known as the focal length. The focal length is what determines the magnification in telescopes. Larger focal lengths imply more magnification.
If we are careful about remembering the above points, it won't take us long to choose a telescope the next time we buy one for ourselves.
Telescopes may be classified by the wavelengths of light they detect:
- X-ray telescopes, using shorter wavelengths than ultraviolet light
- Ultraviolet telescopes, using shorter wavelengths than visible light
- Optical telescopes, using visible light
- Infrared telescopes, using longer wavelengths than visible light
- Submillimetre telescopes, using microwave wavelengths that are longer than those of infrared light
- Radio telescopes that use even longer wavelengths
Optical telescopes :
An optical telescope gathers and focuses light mainly from the visible part of the electromagnetic spectrum (although some work in the infrared and ultraviolet).[14] Optical telescopes increase the apparent angular size of distant objects as well as their apparent brightness. For the image to be observed, photographed, studied, and sent to a computer, telescopes work by employing one or more curved optical elements, usually made from glass lenses and/or mirrors, to gather light and other electromagnetic radiation to bring that light or radiation to a focal point. Optical telescopes are used for astronomy and in many non-astronomical instruments, including theodolites (including transits), spotting scopes, monoculars, binoculars, camera lenses, and spyglasses.
There are three main optical types:
- The refracting telescope uses lenses to form an image.
- The reflecting telescope uses an arrangement of mirrors to form an image.
- The catadioptric telescope uses mirrors combined with lenses to form an image.
Radio telescopes :
Radio telescopes are directional radio antennas that typically employ a large dish to collect radio waves. The dishes are sometimes constructed of a conductive wire mesh whose openings are smaller than the observed wavelength.
Unlike an optical telescope, which produces a magnified image of the patch of the sky being observed, a traditional radio telescope dish contains a single receiver and records a single time-varying signal characteristic of the observed region; this signal may be sampled at various frequencies. In some newer radio telescope designs, a single dish contains an array of several receivers; this is known as a focal-plane array.
By collecting and correlating signals simultaneously received by several dishes, high-resolution images can be computed. Such multi-dish arrays are known as astronomical interferometers, and the technique is called aperture synthesis. The 'virtual' apertures of these arrays are similar in size to the distance between the telescopes. As of 2005, the record array size is many times the diameter of the Earth — utilizing space-based Very Long Baseline Interferometry (VLBI) telescopes such as the Japanese HALCA (Highly Advanced Laboratory for Communications and Astronomy) VSOP (VLBI Space Observatory Program) satellite.
Aperture synthesis is also being applied to optical telescopes using optical interferometers (arrays of optical telescopes) and aperture masking interferometry at single reflecting telescopes.
Radio telescopes are also used to collect microwave radiation, which has the advantage of being able to pass through the atmosphere and interstellar gas and dust clouds.
Some radio telescopes are used by SETI and the Arecibo Observatory to search for extraterrestrial life.
X-ray telescopes :
X-rays are much harder to collect and focus than electromagnetic radiation of longer wavelengths. X-ray telescopes can use X-ray optics, such as Wolter telescopes composed of ring-shaped 'glancing' mirrors made of heavy metals that can reflect the rays just a few degrees. The mirrors are usually a section of a rotated parabola and a hyperbola, or ellipse. In 1952, Hans Wolter outlined 3 ways a telescope could be built using only this kind of mirror.[16][17] Examples of observatories using this telescope are the Einstein Observatory, ROSAT, and the Chandra X-Ray Observatory. By 2010, Wolter focusing X-ray telescopes are possible up to photon energies of 79 keV.[15]
Gamma-ray telescopes :
Higher energy X-ray and Gamma-ray telescopes refrain from focusing completely and use coded aperture masks: the patterns of the shadow the mask creates can be reconstructed to form an image.
X-ray and Gamma-ray telescopes are usually installed on Earth-orbiting satellites, or high-flying balloons since the Earth's atmosphere is opaque to this part of the electromagnetic spectrum. An example of this type of telescope is the Fermi Gamma-ray Space Telescope.
The detection of very-high-energy gamma rays, with shorter wavelengths and higher frequency than regular gamma rays, requires further specialization. An example of this type of observatory is VERITAS.
A discovery in 2012 may allow focusing gamma-ray telescopes.[18] At photon energies greater than 700 keV, the index of refraction starts to increase again.[18]
Other types of telescopes :
Astronomy is not limited to using electromagnetic radiation. Additional information can be obtained by detecting other signals, with detectors analogous to telescopes. These are:
- Cosmic-ray telescopes detect cosmic rays and usually consist of an array of different detector types spread out over a large area.
- Energetic neutral atom instruments study the magnetosphere of various bodies by detecting fast-moving electrically neutral atoms created by the solar wind.
- Neutrino detectors, the equivalent of neutrino telescopes, are used for neutrino astronomy. They consist of a large mass of water and ice, surrounded by an array of sensitive light detectors known as photomultiplier tubes. The originating direction of the neutrinos is determined by reconstructing the path of secondary particles scattered by neutrino impacts from their interaction with multiple detectors.
- Gravitational-wave detectors, the equivalent of gravitational wave telescopes, are used for gravitational wave astronomy. Gravitational waves, caused by violent collisions in space, are detected by exact measurements of the change in length of large earth-bound structures.
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