A telescope is an instrument that aids in the observation of remote objects by collecting
electromagnetic radiation. 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.
The word "telescope" 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".
History
The earliest recorded working telescopes were the refracting telescopes that appeared in
the Netherlands in 1608. Their development is credited to three individuals: Hans Lippershey
and Zacharias Janssen, who were spectacle makers in Middelburg, and Jacob Metius of
Alkmaar. Galileo heard about the Dutch telescope in June 1609, built his own within a month,
and greatly improved upon the design in the following year.
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, dictating
that the vast majority of large optical researching telescopes built since the turn of the 20th
century have been reflectors. The largest reflecting telescopes currently have objectives
larger than 10 m, and work is underway on several 30-40m designs.
The 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 tremendous variety of complex astronomical instruments
have been developed. Types
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 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
As wavelengths become longer, it becomes easier to use antenna technology to interact with
electromagnetic radiation. The near-infrared can be handled 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 observes from wavelengths from 3 μm to 2000 μm,
but uses a parabolic aluminum antenna. On the other hand, the Spitzer Space Telescope,
observing from about 3 μm to 180 μm uses a mirror. Also using reflecting optics, the
Hubble Space Telescope with Wide Field Camera 3 can observe from about 0.2 μm to 1.7 μm.
Fresnel Imager, an optical lens technology X-ray optics, optics for certain X-ray wavelengths
Another threshold in telescope design, as photon energy increases is the use of fully
reflecting optics rather than glancing-incident optics. Telescopes such as TRACE and SOHO
use special mirrors to reflect Extreme ultraviolet, producing higher resolution and brighter images
then otherwise possible. A larger aperture does not just mean more light is collected,
it is collected at a higher diffraction limit. 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. Optical telescopes
An optical telescope gathers and focuses light mainly from the visible part of the electromagnetic
spectrum. 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, spotting
scopes, monoculars, binoculars, camera lenses, and spyglasses. There are three main optical
types: The refracting telescope which uses lenses
to form an image. The reflecting telescope which uses an arrangement
of mirrors to form an image. 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. 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 telescopes such as the Japanese HALCA VSOP satellite.
Aperture synthesis is now also being applied to optical telescopes using optical interferometers
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
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. 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
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. Types of mount
A telescope mount is a mechanical structure which supports a telescope. 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: Altazimuth mount
Equatorial mount Atmospheric electromagnetic opacity
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 X-ray
or far-infrared ground-based telescopes as these have to be flown in space to observe.
Even if a wavelength is observable from the ground, it might still be advantageous to
fly it on a satellite due to astronomical seeing.
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.
By spectrum Telescopes that operate in the electromagnetic
spectrum: *Links to categories.
Lists of telescopes See also
References
Further reading Contemporary Astronomy – Second Edition,
Jay M. Pasachoff, Saunders Colleges Publishing – 1981, ISBN 0-03-057861-2
Elliott, Robert S., Electromagnetics, McGraw-Hill Rashed, Roshdi; Morelon, Régis, Encyclopedia
of the History of Arabic Science, 1 & 3, Routledge, ISBN 0-415-12410-7
Wade, Nicholas J.; Finger, Stanley, "The eye as an optical instrument: from camera obscura
to Helmholtz's perspective", Perception 30: 1157–1177, doi:10.1068/p3210, PMID 11721819
Sabra, A. I. & Hogendijk, J. P., The Enterprise of Science in Islam: New Perspectives, MIT
Press, pp. 85–118, ISBN 0-262-19482-1 External links
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 Timeline of telescopic technology
Outside the Optical: Other Kinds of Telescopes Gray, Meghan; Merrifield, Michael. "Telescope
Diameter". Sixty Symbols. Brady Haran for the University of Nottingham.