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.]] The word "telescope" (from the Greek ''tele'' = 'far' and ''skopein'' = 'to look or see'; ''teleskopos'' = 'far-seeing') usually refers to Optical Telescope s, 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 Focus es 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 Mirror s - 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 '' Theodolite s'', '' Transits '', '' Spotting Scope s'', '' Monocular s'', '' Binoculars ,'' '' Camera Lens es'' and '' Spyglass es''. Single-dish (arrays of optical telescopes) and Aperture Masking Interferometry at single telescopes. detect a flux of particles, usually originating at an astronomical source. HISTORY See Also: History of telescopes The first telescopes may have been tentatively suggest that the technology was known to the Arabs and Persians , then to the Vikings , in the 10th Century . 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 (c. 1570 – c. 1619), 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 (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 the Kepler telescope). Optical Interferometer arrays and arrays of radio telescopes were developed much more recently. Telescopes have been around for a while. TYPES See Also: List of telescope types Telescopes are broadly classified into two main types. # Optical telescopes # Radio telescopes Optical telescopes are also divided into three types. # Galilean Refracting telescopes (also known as dioptrics) # Newtonian Reflecting telescopes (also known as catoptrics) # Catadioptrics (i.e. Schmidt-Cassegrain, and Maksutov-Cassegrain) Galilean or refracting telescopes employ the Refractive properties of light, and are constructed of lenses. These can be used for both Terrestrial and Astronomical viewing. Newtonian or reflecting telescopes employ the Reflective properties of light, using a concave paraboic primary mirror to collect and focus incoming light onto a flat secondary (diagonal) mirror that in turn reflects the image ot of an opening at the side of the main tube and into the eyepiece. Catadioptrics (generally referred to as Cassegrains) use a combination of mirrors and lenses to fold the optics and form an image. TELESCOPE MOUNTINGS 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). Telescopes built on the popular Dobsonian design use an altazimuth mount because of it's simplicity and suitability to the design. When using an altazimuth for astronomy, 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 target from the celestial pole. The last effect makes an altazimuth mount especially 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 used for specialized instruments. For example:
RESEARCH TELESCOPES 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 a Newtonian Telescope (brighter field). 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 (MMT), with a mirror composed of six segments synthesizing a mirror of 4.5 Meter s diameter. This has now been replaced by a single 6.5m mirror. Its example was followed by the Keck Telescope s 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 (see Active Optics ). 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 Telescope s, 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 Detector s, 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 such as:
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 (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. 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 (1821-1896) 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. ; . ; Astigmatism : The image of a point forms focal lines at the sagittal and tangiental foci and in between (in the absence of coma) 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 (similar to stitching multiple photos into a Panoramic Photo ). 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 (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 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 (Chromatic Aberration of Magnification) FAMOUS OPTICAL TELESCOPES orbits above Earth.]]
OTHER FAMOUS TELESCOPES
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