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A Telescope gathers and focuses visible light and other electromagnetic radiation
A telescope is an instrument designed for the observation of remote objects. The term 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 gathers and focuses visible light and other 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 (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 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. High energy particle telescopes 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 (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. Types of telescopeTelescopes are frequently characterised by their design details, and usually named after the principal designer or designers of that type of telescope. For optical telescopes, especially optical astronomical telescopes, there are three main types:
Where the telescope is used for direct viewing by the human eye, then very few telescopes are pure reflectors, as it is usual to use an eyepiece to view the image, and most, if not all eyepiece design use an arrangement of lenses. Much astronomy is performed using photographic film or digital sensors which are usually used without an eyepiece, so for mechanised operation more telescopes are used as pure reflectors. For more details see List of telescope types Research telescopes
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 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 a primary mirror 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 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 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 imagesNo 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
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, 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
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