Telescopes

The Nordic Optical Telescope (NOT) telescope at Roque de los Muchachos Observatory in June 2001. The height of the observatory above the Atlantic Ocean ensures that it is almost always above the clouds. Image courtesy of Wikipedia.
The Nordic Optical Telescope (NOT) telescope at Roque de los Muchachos Observatory in June 2001. The height of the observatory above the Atlantic Ocean ensures that it is almost always above the clouds. Image courtesy of Wikipedia.

What is a Telescope?

A telescope is an instrument that aids in the observation of remote objects by collecting electromagnetic radiation (such as visible light). 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.


Types of Telescopes I

Refracting Telescope:

A refracting or refractor telescope is a type of optical telescope that uses a lens as its objective to form an image (also referred to a dioptric telescope). The refracting telescope design was originally used in spy glasses and astronomical telescopes but is also used for long focus camera lenses. Although large refracting telescopes were very popular in the second half of the 19th century, for most research purposes the refracting telescope has been superseded by the reflecting telescope.

8-inch (20 cm) refractor at the Observatories at Chabot Space & Science Center in Oakland, California. Credit: Wikipedia
8-inch (20 cm) refractor at the Observatories at Chabot Space & Science Center in Oakland, California. Credit: Wikipedia

Reflecting Telescope

A reflecting telescope (also called a reflector) is an optical telescope which uses a single or combination of curved mirrors that reflect light and form an image. The reflecting telescope was invented in the 17th century as an alternative to the refracting telescope which, at that time, was a design that suffered from severe chromatic aberration. Although reflecting telescopes produce other types of optical aberrations, it is a design that allows for very large diameter objectives. Almost all of the major telescopes used in astronomy research are reflectors. Reflecting telescopes come in many design variations and may employ extra optical elements to improve image quality or place the image in a mechanically advantageous position. Since reflecting telescopes use mirrors, the design is sometimes referred to as a “catoptric” telescope. Reflecting telescopes range is size – from a backyard telescope to the Hubble telescope in earth orbit.

Meade 114EQ-AR Equatorial Reflector Telescope. Credit: Meade
Meade 114EQ-AR Equatorial Reflector Telescope. Credit: Meade
he Hubble Space Telescope as seen from the departing Space Shuttle Atlantis, flying Servicing Mission 4 (STS-125), the fifth and latest human spaceflight to it. Image from NASA.
The Hubble Space Telescope as seen from the departing Space Shuttle Atlantis, flying Servicing Mission 4 (STS-125), the fifth and latest human spaceflight to it. Image from NASA.

Types of Telescope II

The name “telescope” covers a wide range of instruments. Most detect electromagnetic radiation, but there are major differences in how astronomers must go about collecting light (electromagnetic radiation) in different frequency bands. Telescopes may be classified by the wavelengths of light they detect:

Optical Telescope:

An optical telescope gathers and focuses light mainly from the visible part of the electromagnetic spectrum (although some work in the infrared and ultraviolet). Optical telescopes increase the apparent angular size of distant objects as well as their apparent brightness. In order 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:

1. The refracting telescope which uses lenses to form an image.
2. The reflecting telescope which uses an arrangement of mirrors to form an image.
3. The catadioptric telescope which uses mirrors combined with lenses to form an image.

Beyond these basic optical types there are many sub-types of varying optical design classified by the task they perform such as Astrographs, Comet seekers, Solar telescope, etc.

Typical optical refractor telescope image by Jim Mills from Fotolia.com
Typical optical refractor telescope image by Jim Mills from Fotolia.com

Radio Telescopes:

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.

The Arecibo Radio Telescope in Puerto Rico. Image courtesy of SETI.
The Arecibo Radio Telescope in Puerto Rico. Image courtesy of SETI.

X-Ray Telescopes:

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.

Chandra X-Ray Observatory in Space. NASA Image.
Chandra X-Ray Observatory in Space. NASA Image.

Gamma-ray Telescopes:

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 energizes greater than 700 keV, the index of refraction starts to increase again.

Very Energetic Radiation Imaging Gamma Ray Telescope Array System . Image courtesy of the Fred Lawrence Whipple Observatory
Very Energetic Radiation Imaging Gamma Ray Telescope Array System . Image courtesy of the Fred Lawrence Whipple Observatory

Using Multiple Images from Multiple Telescopes:

Different types of telescope, operating in different wavelength bands, provide different information about the same object. Together they provide a more comprehensive understanding.

A 6′ wide view of the Crab nebula supernova remnant, viewed at different wavelengths of light by various telescopes. Credit: NASA
A 6′ wide view of the Crab nebula supernova remnant, viewed at different wavelengths of light by various telescopes. Credit: NASA