The first known practical telescopes were invented in the Netherlands at the beginning of the 17th century, using glass lenses. They found use in terrestrial applications and astronomy. Within a few decades, the reflecting telescope was invented, which used mirrors. In the 20th century many new types of 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 detecting different regions of the electromagnetic spectrum, and in some cases other types of detectors.
1. The Refracting Telescope or Refractor
Refracting telescopes are the most common form of the telescope – a long, thin tube where light passes in a straight line from the front objective lens directly to the eyepiece at the opposite end of the tube.
* Easy to use and consistent due to the simplicity of design.
* Good for distant terrestrial viewing
* Excellent for lunar, planetary and binary stargazing especially with larger apertures
* Sealed tube protects optics and reduces image degrading air currents
* Rugged, need little or no maintenance
* Generally have small apertures, typically 3 to 5 inches
* Less suited for viewing small and faint deep sky objects such as distant galaxies and nebulae
* Heavier, longer and bulkier than equivalent aperture reflectors and catadioptrics
* Limited practical usefulness
* Good-quality refractors cost more per inch of aperture than any other kind of telescope.
2. The Reflecting Telescope or Reflector
Reflecting telescopes use a huge concave parabolic mirror instead of a lens to gather and focus the light to a flat secondary mirror that in turn reflects the image out of an opening at the side of the main tube. You look through an eyepiece on the side of the tube up near the top.
* Easy to use and even construct
* Excellent for faint deep sky objects such as remote galaxies, nebulae and star clusters because of their larger apertures for light gathering.
* Low in optical irregularities and deliver very bright images
* Reasonably compact and portable
* A reflector costs the least per inch of aperture compared to refractors and catadioptrics since mirrors can be produced at less cost than lenses
* Generally, not suited for terrestrial applications
* Slight light loss due to secondary obstruction when compared with refractors
* The tube is open to the air, which means dust on the optics even if the tube is kept under wraps
* Reflectors may require a little more care and maintenance
3. Catadioptric Telescope
Catadioptric telescopes use a combination of mirrors and lenses to fold the optics and form an image. Catadioptrics are the most popular type of instrument, with the most modern design, marketed throughout the world in 3 ” and larger apertures. There are two popular designs, the Schmidt-Cassegrain and the Maksutov-Cassegrain. In the Schmidt-Cassegrain, light enters through a thin aspheric Schmidt correcting lens, then strikes the spherical primary mirror and is reflected back up the tube to be intercepted by a small secondary mirror. The mirror then reflects the light out an opening in the rear of the instrument where the image is formed at the eyepiece.
* Most versatile type of telescope
* Best near focus capability of any type telescope
* First-rate for deep sky observing or astrophotography with fast films or CCD’s
* Excellent for lunar, planetary and binary star observing plus terrestrial viewing and photography
* Closed tube design reduces image degrading air currents
* Compact and durable
* More expensive than reflectors of equal aperture
* Its appearance may not be suited to everybody’s taste
* Slight light loss due to secondary mirror obstruction compared to refractors
The Maksutov-Cassegrain telescope design has basically the same advantages and disadvantages as the Schmidt. It uses a thick meniscus-correcting lens with a strong curvature and a secondary mirror that is usually an aluminized spot on the corrector. The Maksutov secondary mirror is typically smaller than the Schmidt’s giving it slightly better resolution for planetary observing. However, the Maksutov is heavier than the Schmidt and because of the thick correcting lens, it takes a long time to reach thermal stability at night in larger apertures. The Maksutov optical design typically is easier to make but requires more material for the corrector lens than the Schmidt Cassegrain.
Radio telescopes are directional radio antennas used for radio astronomy. The dishes are sometimes constructed of a conductive wire mesh whose openings are smaller than the wavelength being observed. Multi-element Radio telescopes are constructed from pairs or larger groups of these dishes to synthesize large ‘virtual’ apertures that are similar in size to the separation between the telescopes; this process is known as aperture synthesis. As of 2005, the current record array size is many times the width 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 now 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 is used to collect radiation when any visible light is obstructed or faint, such as from quasars. Some radio telescopes are used by programs such as SETI and the Arecibo Observatory to search for extraterrestrial life.
X-ray telescopes can use X-ray optics, such as a Wolter telescopes composed of ring-shaped ‘glancing’ mirrors made of heavy metals that are able to 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. Examples of an observatory using this type of telescope are the Einstein Observatory, ROSAT, and the Chandra X-Ray Observatory. By 2010, Wolter focusing X-ray telescopes are possible up to 79 keV.
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 on Earth-orbiting satellites or high-flying balloons since the Earth’s atmosphere is opaque to this part of the electromagnetic spectrum. However, high energy X-rays and gamma-rays do not form an image in the same way as telescopes at visible wavelengths. An example of this type of telescope is the Fermi Gamma-ray Space Telescope. The detection of very high energy gamma rays, with shorter wavelength and higher frequency than regular gamma rays, requires further specialization. An example of this type of observatory is VERITAS. Very high energy gamma-rays are still photons, like visible light, whereas cosmic rays includes particles like electrons, protons, and heavier nuclei. A discovery in 2012 may allow focusing gamma-ray telescopes. At photon energies greater than 700 keV, the index of refraction starts to increase again.
High-energy particle telescopes:
High-energy astronomy requires specialized telescopes to make observations since most of these particles go through most metals and glasses. In other types of high energy particle telescopes there is no image-forming optical system. Cosmic-ray telescopes usually consist of an array of different detector types spread out over a large area. A Neutrino telescope consists of a large mass of water or ice, surrounded by an array of sensitive light detectors known as photomultiplier tubes. Energetic neutral atom observatories like Interstellar Boundary Explorer detect particles traveling at certain energies.
Other types of telescopes:
Astronomy is not limited to using electromagnetic radiation. Additional information can be obtained using other media. The detectors used to observe the Universe are analogous to telescopes, these are:
-Gravitational-wave detector, the equivalent of a gravitational wave telescope, used for gravitational-wave astronomy.
-Neutrino detector, the equivalent of a neutrino telescope, used for neutrino astronomy.