Telescope

Categories: Telescopes | Measuring instruments | Optical devices | Navigation | Astronomical imaging

For other senses of this word, see telescope (disambiguation).
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50 cm refracting telescope at Nice Observatory.

A telescope (from the Greek tele = 'far' and skopein = 'to look or see'; teleskopos = 'far-seeing') is an astronomical tool which gathers and focuses electromagnetic radiation. Telescopes increase the apparent angular size of ingerso objects, as well as their apparent brightness. Telescopes used for non-astronomical purposes are often referred to as theodolites, transits, spotting scopes, monoculars, binoculars, camera lenses, microscopes or spyglasses.

The word "telescope" usually refers to optical telescopes, but there are telescopes for most of the spectrum of electromagnetic radiation.

Radio telescopes are focused radio antennas, usually shaped like large dishes. The dish is sometimes constructed of a conductive wire mesh whose openings are smaller than a wavelength. Radio telescopes are often operated in pairs, or larger groups to synthesize large "virtual" apertures that are similar in size to the separation between the telescopes: see aperture synthesis. The current record is many times the width of the Earth, utilizing space-based VLBI telescopes such as the Japanese HALCA VSOP satellite. Aperture synthesis is now also being applied to optical telescopes using optical interferometers (arrays of optical telescopes) 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.

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History

The first telescopes may have been Assyrian crystal lenses. Article

Though the Visby lenses tentatively suggest that the technology was known to the Arabs and then to the Vikings in the 10th century, credit for assembling the first telescope is usually given to an unknown Dutch spectacle maker in about 1608. Some name that person as Hans Lippershey (c1570-c1619). 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 (from which the English word derives). 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 (used as a viewfinder in many simple cameras). Later, Johannes Kepler described the optics of lenses (see his books Astronomiae Pars Optica and Dioptrice), including a new kind of astronomical telescope with two convex lenses (a principle often called Kepler telescope). Optical interferometer arrays and arrays of radio telescopes were developed much more recently.

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Telescope mountings

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In modern altazimuth telescopes the Cassegrain instruments usually sit on a rotating assembly which keeps the images stationary during long exposures

A simple telescope mount is an altitude-azimuth or altazimuth mount. It is similar to that of a surveying transit. A fork rotates in azimuth (in the horizontal plane), and bearings on the tips of the fork allow the telescope to vary in altitude (in a vertical plane). A dobsonian mount is a type of altazimuth mount which has proven to be very popular as it is simple and cheap to make.

The major problem with using an altazimuth for astronomy is that both axes must be continuously adjusted to compensate for the Earth's rotation. Even if this is done, by computer control, the image rotates at a rate that varies depending on the angle of the star from the celestial pole. The last effect especially makes an altazimuth mount impractical for long-exposure photography with small telescopes.

The preferred solution for small astronomical telescopes is to tip the altazimuth mount so that the azimuth axis is parallel with the axis of the Earth's rotation; this is known as an equatorial mount.

Modern large telescopes use computer-controlled altazimuth mounts, and for long exposures they rotate the instruments or have variable-rate image rotators in an image of the telescope pupil.

There are mountings even simpler than altazimuth, typically for specialised instruments. A few are: meridian transit (altitude only); fixed with movable plane mirror for solar observing; ball-and-socket (ancient and useless for astronomy).

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Research telescopes

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Harlan J. Smith Telescope at McDonald Observatory, TX

Most large research telescopes can operate as either a cassegrain telescope (longer focal length, and a narrower field with higher magnification) or newtonian telescope (brighter field). They have a pierced primary, a newtonian focus, and a spider to mount a variety of replaceable secondaries.

A new era of telescope making was inaugurated by the MMT, with a mirror composed of six segments synthesizing a mirror of 4.5 metres 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 telescopes have a primary mirror of between 6 and 11 meters in diameter (for ground-based telescopes). In this generation of telescopes, the mirror is usually very thin, and is kept in an optimal shape by an array of actuators (see active optics). This technology has driven new designs for future telescopes with diameters of 30, 50 and even 100 metres.

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 (see e.g. the Liverpool Telescope and the Faulkes Telescope North and South), 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 (CCDs), 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: imagers, of different spectral responses; spectrographs, useful in different regions of the spectrum; polarimeters, that detect light polarization, etc.

In recent years, some technologies to overcome the bad effect of 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 we may place two such discs. This absolute limit is called the diffraction limit (or sometimes the Rayleigh criterion, Dawes limit or Sparrow's resolution limit). This limit depends on the wavelength of the studied light (so that the limit for red light comes much earlier than the limit for blue 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, so if you want more resolution at that very wavelength, you have to build a wider mirror or perform aperture synthesis using an array of nearby telescopes.

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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 (1821-1896) decomposed the first order monochromatic abberations into five constituent aberrations. They are now commonly referred to as the five Seidel Aberrations.

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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. It's 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 (in the absence of coma) an eliptical 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, causing the same image shape errors as the eponymous controls on your monitor!

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 abberation, Spherical Aberration is independent of the position of the exit pupil (as it is the same for axial and extra-axial pencils). The second, coma is 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 remaing aberrations in the list.

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The chromatic aberrations

Longitudinal Chromatic Aberration 
As with spherical aberration this is the same for axial and oblique pencils.
Transverse Chromatic Aberration (Chromatic Aberration of Magnification)
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Famous optical telescopes

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The Hubble Space Telescope orbits above Earth.
  • The Hubble Space Telescope is in orbit outside of the Earth's atmosphere to allow for observations not distorted by astronomical seeing, in this way they can be diffraction limited, and used for coverage in the ultraviolet (UV) and infrared.
  • The Keck telescopes are currently (2005) the largest, but will soon be superseded by the Gran Telescopio Canarias and Southern African Large Telescope.
  • The Very Large Telescope array (VLT) is currently (2002) the record holder for total collecting area in an array of telescopes, with four telescopes each 8 metres in diameter. The four telescopes, belonging to ESO 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 (array) which can currently (2005) produce the highest resolution images at visible wavelengths.
  • The CHARA array is the telescope (array) which can currently (2005) 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 or OWL, which is intended to have a single aperture of 100 metres in diameter.
  • The 200 inch (5.08 m) Hale telescope on Palomar Mountain is a conventional research telescope that was the largest for many years. It has a single borosilicate (Pyrex™) mirror that was famously difficult to construct. The mounting is also unique, an equatorial mount that is not a fork, yet permits the telescope to image near the north celestial pole.
  • The 100 inch (2.54 m) 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 1.02 m Yerkes Telescope (in Wisconsin) is the largest aimable refractor in use.
  • The 0.76 m Nice refractor (in France) that became operational in 1888 was at that time the world's largest telescope. This was the last time the most powerful operational telescope in the world was located in Europe. It was outperformed one year later by the 0.91 m 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 m 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 1-meter refracting Swedish Solar Telescope (SST) on La Palma, currently the highest-resolution solar telescope in the world.
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See also

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Related lists

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External links

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