Encyclopedia
The word "
telescope" usually refers to
optical telescopes, but there are telescopes for most of the
spectrum of
electromagnetic radiation and for other signal types.
An optical telescope is an
optical tool that gathers and focuses
electromagnetic radiation. Telescopes increase the apparent angular size of distant objects, as well as their apparent brightness. Telescopes work by employing one or more curved optical elements - lenses or
mirrors - to gather light or other electromagnetic radiation and bring that light or radiation to a focus, where the image can be observed, photographed or studied.
Optical telescopes are used for
astronomy and in many non-astronomical instruments including
theodolites,
transits,
spotting scopes,
monoculars,
binoculars, camera lenses and
spyglasses.
Single-dish
Radio telescopes are focusing
radio antennae often having a parabolic shape. The dishes are sometimes constructed of a conductive wire mesh whose openings are smaller than a
wavelength. 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: see
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 . Aperture synthesis is now also being applied to optical telescopes using
optical interferometers and
Aperture Masking Interferometry at single telescopes.
X-ray and
gamma-ray telescopes have a problem because these rays go through most metals and glasses. They use ring-shaped "glancing"
mirrors, made of heavy metals, that reflect the rays just a few degrees. The mirrors are usually a section of a rotated
parabola. detect a flux of particles, usually originating at an astronomical source.
History
The first telescopes may have been
Assyrian crystal lenses, but the Visby lenses tentatively suggest that the technology was known to the Arabs and
Persians. Leonard Digges is sometimes credited with the invention in England in the 1570s, but usually credit for assembling the first telescope is given to an unknown
Dutch spectacle maker in about 1608. Some name that person as
Hans Lippershey , but Jacob Metius and Zacharias Jansen also claimed to have invented a telescope during the same period. Even if Lippershey did not make the first one, he publicized it.
Galileo Galilei made his own telescope in 1609, calling it at first a "
perspicillum," and then using the terms "
telescopium" in Latin and "
telescopio" in Italian . Galileo is generally credited with being the first to use a telescope for astronomical purposes. Galileo's telescope consisted of a convex object lens and a concave eye lens, which is universally called a Galilean telescope . Later,
Johannes Kepler described the
optics of lenses , including a new kind of astronomical telescope with two convex lenses . Optical
interferometer arrays and arrays of radio telescopes were developed much more recently.
Research telescopes
Most large research telescopes can operate as either a
Cassegrain telescope or a
Newtonian telescope . They have a pierced primary mirror, a Newtonian focus, and a spider to mount a variety of replaceable secondary mirrors.
A new era of telescope making was inaugurated by the
Multiple Mirror Telescope , with a mirror composed of six segments synthesizing a mirror of 4.5
meters diameter. This has now been replaced by a single 6.5m mirror. Its example was followed by the
Keck telescopes with 10 m segmented mirrors.
The largest current ground-based telescopes have primary mirrors of between 6 and 11 meters in diameter. In this generation of telescopes, the mirror is usually very thin, and is kept in an optimal shape by an array of actuators . This technology has driven new designs for future telescopes with diameters of 30, 50 and even 100 meters.
Relatively cheap, mass-produced ~2 meter telescopes have recently been developed and have made a significant impact on astronomy research. These allow many astronomical targets to be monitored continuously, and for large areas of sky to be surveyed. Many are robotic telescopes, computer controlled over the internet , allowing automated follow-up of astronomical events.
Initially the detector used in telescopes was the human eye. Later, the sensitized photographic plate took its place, and the
spectrograph was introduced, allowing the gathering of spectral information. After the photographic plate, successive generations of electronic detectors, such as the
charge-coupled device , have been perfected, each with more sensitivity and resolution, and often with a wider wavelength coverage.
Current research telescopes have several instruments to choose from such as:
- imagers, of different spectral responses
- spectrographs, useful in different regions of the spectrum
- polarimeters, that detect light polarization.
In recent years, some technologies to overcome the distortions caused by
atmosphere on ground-based telescopes were developed, with good results. See
adaptive optics, speckle imaging and
optical interferometry.
The phenomenon of optical
diffraction sets a limit to the resolution and image quality that a telescope can achieve, which is the effective area of the
Airy disc, which limits how close two such discs can be placed. This absolute limit is called the
diffraction limit . This limit depends on the wavelength of the studied light and on the
diameter of the telescope mirror. This means that a telescope with a certain mirror diameter can resolve up to a certain limit at a certain wavelength. If greater resolution is needed at that wavelength, a wider mirror has to be built or
aperture synthesis performed using an array of nearby telescopes.
Imperfect images
No telescope can form a perfect image. Even if a reflecting telescope could have a perfect mirror, or a refracting telescope could have a perfect lens, the effects of aperture diffraction could still not be escaped. In reality, perfect mirrors and perfect lenses do not exist, so image
aberrations in addition to aperture diffraction must be taken into account. Image aberrations can be broken down into two main classes, monochromatic, and polychromatic. In 1857, Philipp Ludwig von Seidel decomposed the first order monochromatic aberrations into five constituent aberrations. They are now commonly referred to as the five Seidel Aberrations.
The five Seidel aberrations
Spherical aberration : The difference in focal length between paraxial rays and marginal rays, proportional to the square of the aperture.
Coma : A most objectionable defect by which points are imaged as comet-like asymmetrical patches of light with tails, which makes measurement very imprecise. Its magnitude is usually deduced from the optical sine theorem.
Astigmatism : The image of a point forms focal lines at the sagittal and tangiental foci and in between an elliptical shape.
Curvature of Field : The
Petzval curvature means that the image instead of lying in a plane actually lies on a curved surface which is described as hollow or round. This causes problems when a flat imaging device is used e.g. a photographic plate or CCD image sensor.
Distortion : Either barrel or pincushion, a radial distortion which must be corrected for if multiple images are to be combined .
They are always listed in the above order since this expresses their interdependence as first order aberrations via moves of the exit/entrance pupils. The first Seidel aberration, Spherical Aberration, is independent of the position of the exit pupil . The second, coma, changes as a function of pupil distance and spherical aberration, hence the well-known result that it is impossible to correct the coma in a lens free of spherical aberration by simply moving the pupil. Similar dependencies affect the remaining aberrations in the list.
The chromatic aberrations
Longitudinal Chromatic Aberration : As with spherical aberration this is the same for axial and oblique pencils.
Transverse Chromatic Aberration
Famous optical telescopes
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s are currently the largest, but will soon be superseded by the
Gran Telescopio Canarias and
Southern African Large Telescope.
- The Very Large Telescope
- List of astronomical interferometers at visible and infrared wavelengths [i]
...
array is currently the record holder for total collecting area in an array of telescopes, with four telescopes each 8
meters in diameter. The four telescopes, belonging to the
European Southern Observatory and located in the
Atacama desert in
Chile, are usually operated independently for faint astronomical observations, but up to three telescopes can be operated together for
aperture synthesis observations of bright objects.
- The Navy Prototype Optical Interferometer is the optical telescope that can currently produce the highest resolution images at visible wavelengths.
- The CHARA array is the telescope array that can currently produce the highest resolution images at near-infrared wavelengths.
- There are many plans for even larger telescopes. One of them is the Overwhelmingly Large Telescope , which is intended to have a single aperture of 100 meters in diameter.
- The 200-inch Hale telescope on Palomar Mountain was the largest conventional research telescope for many years. It has a single borosilicate mirror that was famously difficult to construct. The mounting is a special design of equatorial mount called a yoke mount, which permits the telescope to be pointed at and near the north celestial pole.
- The 100-inch Hooker Telescope at the Mount Wilson Observatory was used by Edwin Hubble to discover galaxies and the redshift. The mirror was made of green glass by Saint-Gobain. In 1919, the telescope was used for the first stellar diameter measurements using interferometry. The telescope now has an adaptive optics system, and is still useful for advanced research.
- The 72-inch Leviathan at Birr Castle was the largest telescope in the world from 1845 until it was dismanlted in 1908. It was not exceeded in size until the construction of the Hooker Telescope.
- The 1.02-meter Yerkes Telescope
- List of observatories [i] ...
is the largest aimable refracting telescope in use. - The 0.76-meter Nice refractor that became operational in 1888 was at that time the world's largest refractor. This was the last time the most powerful operational telescope in the world was located in Europe. It was exceeded in size one year later by the 0.91-meter refractor at the Lick Observatory.
- The largest refractor ever constructed was French. It was on display at the 1900 Paris Exposition. Its lens was stationary, prefigured so as to sag into the correct shape. The telescope was aimed by the aid of a Foucault sidérostat, which is a movable plane mirror with a 2-meter diameter, mounted in a large cast-iron frame. The horizontal tube was 60 m long and the objective had 1.25 m in diameter. It was a failure.
- The Gran Telescopio CANARIAS , is a high performance segmented 10.4 meter telescope that is being installed in one of the best sites of the Northern Hemisphere: the Roque de los Muchachos Observatory .
- The 1-meter refracting Swedish Solar Telescope on La Palma , is currently the highest-resolution solar telescope in the world.
Other famous telescopes
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See also
External links
