A telescope is an optical 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,
by using glass lenses. They found use in both terrestrial applications
The reflecting telescope, which uses mirrors to collect and focus
light, was invented within a few decades of the first telescopes being
made. 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
capable of detecting different regions of the electromagnetic
spectrum, and in some cases other types of detectors.
The word telescope (from the
Ancient Greek τῆλε, tele "far" and
σκοπεῖν, skopein "to look or see"; τηλεσκόπος,
teleskopos "far-seeing") was coined in 1611 by the Greek mathematician
Giovanni Demisiani for one of Galileo Galilei's instruments presented
at a banquet at the Accademia dei Lincei. In the Starry
Messenger, Galileo had used the term perspicillum.
2.1 Optical telescopes
2.5 High-energy particle telescopes
2.6 Other types of telescopes
3 Types of mount
4 Atmospheric electromagnetic opacity
5 Telescopic image from different telescope types
6 By spectrum
7 Lists of telescopes
8 See also
11 External links
Main article: History of the telescope
The "onion" dome at the
Royal Observatory, Greenwich
Royal Observatory, Greenwich housing a 28-inch
refracting telescope with a remaining segment of William Herschel's
120-centimetre (47 in) diameter reflecting telescope (called the
"40-foot telescope" due to its focal length) in the foreground.
The earliest existing record of a telescope was a 1608 patent
submitted to the government in the
Netherlands by Middelburg spectacle
Hans Lippershey for a refracting telescope. The actual
inventor is unknown but word of it spread through Europe. Galileo
heard about it and, in 1609, built his own version, and made his
telescopic observations of celestial objects.
The idea that the objective, or light-gathering element, could be a
mirror instead of a lens was being investigated soon after the
invention of the refracting telescope. The potential advantages of
using parabolic mirrors—reduction of spherical aberration and no
chromatic aberration—led to many proposed designs and several
attempts to build reflecting telescopes. In 1668, Isaac Newton
built the first practical reflecting telescope, of a design which now
bears his name, the Newtonian reflector.
The invention of the achromatic lens in 1733 partially corrected color
aberrations present in the simple lens and enabled the construction of
shorter, more functional refracting telescopes. Reflecting telescopes,
though not limited by the color problems seen in refractors, were
hampered by the use of fast tarnishing speculum metal mirrors employed
during the 18th and early 19th century—a problem alleviated by the
introduction of silver coated glass mirrors in 1857, and aluminized
mirrors in 1932. The maximum physical size limit for refracting
telescopes is about 1 meter (40 inches), dictating that the vast
majority of large optical researching telescopes built since the turn
20th century have been reflectors. The largest reflecting
telescopes currently have objectives larger than 10 m
(33 feet), and work is underway on several 30-40m designs.
20th century also saw the development of telescopes that worked in
a wide range of wavelengths from radio to gamma-rays. The first
purpose built radio telescope went into operation in 1937. Since then,
a large variety of complex astronomical instruments have been
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The primary mirror assembly of
James Webb Space Telescope
James Webb Space Telescope under
construction. This is a segmented mirror and its coated with
reflect (orange-red) visible light, through near-infrared to the
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:
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 longer wavelengths than infrared light
Fresnel Imager, an optical lens technology
X-ray optics, optics for certain
As wavelengths become longer, it becomes easier to use antenna
technology to interact with electromagnetic radiation (although it is
possible to make very tiny antenna). The near-infrared can be
collected much like visible light, however in the far-infrared and
submillimetre range, telescopes can operate more like a radio
telescope. For example, the
James Clerk Maxwell Telescope
James Clerk Maxwell Telescope observes
from wavelengths from 3 μm (0.003 mm) to 2000 μm
(2 mm), but uses a parabolic aluminum antenna. On the other
hand, the Spitzer Space Telescope, observing from about 3 μm
(0.003 mm) to 180 μm (0.18 mm) uses a mirror
(reflecting optics). Also using reflecting optics, the Hubble Space
Wide Field Camera 3
Wide Field Camera 3 can observe in the frequency range
from about 0.2 μm (0.0002 mm) to 1.7 μm
(0.0017 mm) (from ultra-violet to infrared light).
With photons of the shorter wavelengths, with the higher frequencies,
glancing-incident optics, rather than fully reflecting optics are
used. Telescopes such as
TRACE and SOHO use special mirrors to reflect
Extreme ultraviolet, producing higher resolution and brighter images
than are otherwise possible. A larger aperture does not just mean that
more light is collected, it also enables a finer angular resolution.
Telescopes may also be classified by location: ground telescope, space
telescope, or flying telescope. They may also be classified by whether
they are operated by professional astronomers or amateur astronomers.
A vehicle or permanent campus containing one or more telescopes or
other instruments is called an observatory.
Modern telescopes typically use CCDs instead of film for recording
images. This is the sensor array in the Kepler spacecraft.
Photon Energy (eV)
less than 0.01 nm
more than 10 EHZ
100 keV – 300+ GeV
0.01 to 10 nm
30 EHz – 30 PHZ
120 eV to 120 keV
10 nm – 400 nm
30 PHZ – 790 THz
3 eV to 124 eV
390 nm – 750 nm
790 THz – 405 THz
1.7 eV – 3.3 eV
750 nm – 1 mm
405 THz – 300 GHz
1.24 meV – 1.7 eV
1 mm – 1 meter
300 GHz – 300 MHz
1.24 meV – 1.24 µeV
1 mm – km
300 GHz – 3 Hz
1.24 meV – 12.4 feV
50 cm refracting telescope at Nice Observatory.
Main article: 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
The refracting telescope which uses lenses to form an image.
The reflecting telescope which uses an arrangement of mirrors to form
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 and solar telescopes.
Very Large Array
Very Large Array at Socorro, New Mexico, United States.
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.
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
Very Long Baseline Interferometry
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
Observatory was a space-based focusing optical X-ray
telescope from 1978.
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.
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
X-Ray Observatory. By 2010, Wolter focusing
are possible up to 79 keV.
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.
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
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
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. Originating direction of the neutrinos is
determined by reconstructing the path of secondary particles scattered
by neutrino impacts, from their interaction with multiple detectors.
Energetic neutral atom
Energetic neutral atom observatories like Interstellar Boundary
Explorer detect particles traveling at certain energies.
Other types of telescopes
Equatorial-mounted Keplerian telescope
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.
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
Types of mount
A telescope mount is a mechanical structure which supports a
Telescope mounts are designed to support the mass of the
telescope and allow for accurate pointing of the instrument. Many
sorts of mounts have been developed over the years, with the majority
of effort being put into systems that can track the motion of the
stars as the
Earth rotates. The two main types of tracking mount are:
Atmospheric electromagnetic opacity
See also: Airmass
Since the atmosphere is opaque for most of the electromagnetic
spectrum, only a few bands can be observed from the Earth's surface.
These bands are visible – near-infrared and a portion of the
radio-wave part of the spectrum. For this reason there are no
far-infrared ground-based telescopes as these have to be observed from
orbit. Even if a wavelength is observable from the ground, it might
still be advantageous to place a telescope on a satellite due to
A diagram of the electromagnetic spectrum with the Earth's atmospheric
transmittance (or opacity) and the types of telescopes used to image
parts of the spectrum.
Telescopic image from different telescope types
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
Telescopes that operate in the electromagnetic spectrum:
more than 1 mm
0.1 mm – 1 mm
30 µm – 450 µm
700 nm – 1 mm
Visible spectrum telescopes
400 nm – 700 nm
10 nm – 400 nm
0.01 nm – 10 nm
less than 0.01 nm
*Links to categories.
Lists of telescopes
List of optical telescopes
List of largest optical reflecting telescopes
List of largest optical refracting telescopes
List of largest optical telescopes historically
List of radio telescopes
List of solar telescopes
List of space observatories
List of telescope parts and construction
List of telescope types
Gravitational wave telescopes
Category:High energy particle telescopes
Amateur telescope making
ASCOM open standards for computer control of telescopes
Telescope Markup Language
Timeline of telescope technology
Timeline of telescopes, observatories, and observing technology
^ Sobel (2000, p.43), Drake (1978, p.196)
^ Rosen, Edward, The Naming of the
^ galileo.rice.edu The Galileo Project > Science > The Telescope
by Al Van Helden: The Hague discussed the patent applications first of
Hans Lipperhey of Middelburg, and then of
Jacob Metius of Alkmaar...
another citizen of Middelburg,
Zacharias Janssen is sometimes
associated with the invention
^ "NASA -
Telescope History". www.nasa.gov.
^ Loker, Aleck (20 November 2017). "Profiles in Colonial History".
Aleck Loker – via Google Books.
^ Watson, Fred (20 November 2017). "Stargazer: The Life and Times of
the Telescope". Allen & Unwin – via Google Books.
^ Attempts by
Niccolò Zucchi and James Gregory and theoretical
designs by Bonaventura Cavalieri, Marin Mersenne, and Gregory among
^ "Jean-Bernard-Léon Foucault Biography (1819-1868)".
^ "Home" (PDF). Cambridge University Press.
^ ASTROLab du parc national du Mont-Mégantic. "The
James-Clerk-Maxwell Observatory". Canada under the stars. Retrieved
^ "Hubble's Instruments: WFC3 - Wide Field Camera 3".
www.spacetelescope.org. Retrieved 2017-04-16.
^ Jones, Barrie W. (2 September 2008). The Search for Life Continued:
Planets Around Other Stars. Springer Science & Business Media.
^ a b "NuStar: Instrumentation: Optics". Archived from the original on
^ Wolter, H. (1952), "Glancing Incidence
Mirror Systems as Imaging
Optics for X-rays", Annalen der Physik, 10: 94,
^ Wolter, H. (1952), "Verallgemeinerte Schwarzschildsche
Spiegelsysteme streifender Reflexion als Optiken für
Röntgenstrahlen", Annalen der Physik, 10 (4–5): 286,
^ a b Tim Wogan – Silicon 'prism' bends gamma rays (May 2012) –
Astronomy – Second Edition, Jay M. Pasachoff, Saunders
Colleges Publishing – 1981, ISBN 0-03-057861-2
Elliott, Robert S. (1966), Electromagnetics, McGraw-Hill
Rashed, Roshdi; Morelon, Régis (1996), Encyclopedia of the History of
Arabic Science, 1 & 3, Routledge, ISBN 0-415-12410-7
Wade, Nicholas J.; Finger, Stanley (2001), "The eye as an optical
instrument: from camera obscura to Helmholtz's perspective",
Perception, 30 (10): 1157–1177, doi:10.1068/p3210,
Sabra, A. I.; Hogendijk, J. P. (2003). The Enterprise of Science in
Islam: New Perspectives. MIT Press. pp. 85–118.
Wikiquote has quotations related to: Telescope
Wikimedia Commons has media related to Telescope.
Galileo to Gamma Cephei – The History of the Telescope
The Galileo Project – The
Telescope by Al Van Helden
"The First Telescopes". Part of an exhibit from Cosmic Journey: A
History of Scientific Cosmology by the American Institute of Physics
Taylor, Harold Dennis; Gill, David (1911). "Telescope".
Encyclopædia Britannica. 26 (11th ed.). pp. 557–573.
Outside the Optical: Other Kinds of Telescopes
Gray, Meghan; Merrifield, Michael (2009). "
Telescope Diameter". Sixty
Brady Haran for the University of Nottingham.