Telescope

Similar dependencies affect the remaining aberrations in the list. However, in New York City, lines on the New York City Subway have been referred to as "trains". 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. The term rapid transit is used for public transport such as commuter trains, metro and light rail. 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). Maglev trains and monorails represent minor technologies in the train field. They are always listed in the above order since this expresses their interdependence as first order aberrations via moves of the exit/entrance pupils. They may also be called a trolley.

They are now commonly referred to as the five Seidel Aberrations. These are often protected with crossing gates. In 1857, Philipp Ludwig von Seidel (1821-1896) decomposed the first order monochromatic aberrations into five constituent aberrations. The term light rail is sometimes used for a modern tram, but it may also mean an intermediate form between a tram and a train, similar to metro except that it may have level crossings. Image aberrations can be broken down into two main classes, monochromatic, and polychromatic. In some countries such as the United Kingdom the distinction between a tramway and a railway is precise and defined in law. In reality, perfect mirrors and perfect lenses do not exist, so image aberrations in addition to aperture diffraction must be taken into account. A light one- or two-car rail vehicle running through the streets is not called a train but a tram, trolley, light rail vehicle or streetcar, but the distinction is not always strict.

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. They can accelerate and decelerate faster than heavier, long-distance trains. No telescope can form a perfect image. Usually they run in tunnels in the city center and sometimes on elevated structures in the outer parts of the city. 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. The trains are electrically powered, usually by third rail, and their railroads are separate from other traffic, without level crossings. This means that a telescope with a certain mirror diameter can resolve up to a certain limit at a certain wavelength. Large cities often have a metro system, also called underground, subway or tube.

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. Abuse is punished by a fine. This absolute limit is called the diffraction limit (or sometimes the Rayleigh criterion, Dawes limit or Sparrow's resolution limit). Passenger trains usually have emergency brake handles (or a "communication cord") that the public can operate. 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. Double deck high speed and sleeper trains are becoming more common in Europe. See adaptive optics, speckle imaging and optical interferometry. Some countries have some double-decked passenger trains for use in conurbations.

In recent years, some technologies to overcome the distortions caused by atmosphere on ground-based telescopes were developed, with good results. Some carriages may be laid out to have more standing room than seats, or to facilitate the carrying of prams, cycles or wheelchairs. Current research telescopes have several instruments to choose from such as:. For shorter distances many cities have networks of commuter trains, serving the city and its suburbs. 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. For trains connecting cities, we can distinguish inter-city trains, which do not halt at small stations, and trains that serve all stations, usually known as local trains or "stoppers" (and sometimes an intermediate kind, see also limited-stop). Later, the sensitized photographic plate took its place, and the spectrograph was introduced, allowing the gathering of spectral information. Tilting is a system where the passenger cars automatically lean into curves, reducing the centrifugal forces acting on passengers and permitting higher speeds on curves in the track with greater passenger comfort.

Initially the detector used in telescopes was the human eye. Very fast trains sometimes tilt, like the Pendolino or Talgo. the Liverpool Telescope and the Faulkes Telescope North and South), allowing automated follow-up of astronomical events. Very long distance trains such as those on the Trans-Siberian railway are usually not high-speed. Many are robotic telescopes, computer controlled over the internet (see e.g. Long-distance trains, sometimes crossing several countries, may have a dining or restaurant car; they may also have sleeping cars, but not in the case of high-speed rail, these arrive at their destination before the night falls and are in competition with airplanes in speed. These allow many astronomical targets to be monitored continuously, and for large areas of sky to be surveyed. Passenger trains travel between stations; the distance between stations may vary from under 1 km to much more.

Relatively cheap, mass-produced ~2 meter telescopes have recently been developed and have made a significant impact on astronomy research. Passenger trains have Passenger cars. This technology has driven new designs for future telescopes with diameters of 30, 50 and even 100 meters. Electric trains receive their current via overhead lines or through a third rail electric system. 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). Since the cost per mile of construction is much higher, electric traction is less favored on long-distance lines. The largest current ground-based telescopes have primary mirrors of between 6 and 11 meters in diameter. Electric traction offers a lower cost per mile of train operation but at a very high initial cost, which can only be justified on high traffic lines.

Its example was followed by the Keck telescopes with 10 m segmented mirrors. Historic steam trains still run in many other countries, for the leisure and enthusiast market. This has now been replaced by a single 6.5m mirror. A few countries, most notably the People's Republic of China where coal is in cheap and plentiful supply, still use steam locomotives, but this is being gradually phased out. 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. Most countries had replaced steam locomotives for day-to-day use by the 1970s. They have a pierced primary mirror, a Newtonian focus, and a spider to mount a variety of replaceable secondary mirrors. From the 1920s onwards they began to be replaced by less labor intensive and cleaner (but more expensive) diesel locomotives and electric locomotives, while at about the same time self-propelled multiple unit vehicles of either power system became much more common in passenger service.

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). The first trains were rope-hauled, gravity powered or pulled by horses, but from the early 19th century almost all were powered by steam locomotives. For example:. A single uncoupled rail vehicle is not technically a train, but is usually referred to as such for signaling reasons. There are mountings even simpler than altazimuth, typically used for specialized instruments. Special trains are also used for track maintenance; in some places, this is called maintenance of way. 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. Such mixed trains became rare in many countries, but were commonplace on the first 19th-century railroads.

This is known as an equatorial mount. Transportation in Mauritania. 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. Trains can also be mixed, hauling both passengers and freight, see e.g. The last effect makes an altazimuth mount especially impractical for long-exposure photography with small telescopes. Where the second locomotive is attached temporarily to assist a train up steep banks (or down them by providing breaking power) it is referred to as 'banking'. 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. This practice typically being used when there are no reversing facilities available.

When using an altazimuth for astronomy, both axes must be continuously adjusted to compensate for the Earth's rotation. A train with a locomotive attached each end is described as 'top and tailed'. A Dobsonian mount is a type of altazimuth mount which has proven to be very popular as it is simple and inexpensive. In the United Kingdom, a train hauled by two locomotives is said to be "double-headed", and in Canada and the United States it is quite common for a long freight train to be headed by three, four, or even five locomotives. 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). Freight trains comprise wagons or trucks rather than carriages, though some parcel and mail trains (especially Travelling Post Offices) are outwardly more like passenger trains. It is similar to that of a surveying transit. In many parts of the world, particularly Japan and Europe, high-speed rail is utilized extensively for passenger travel.

A simple telescope mount is an altitude-azimuth or altazimuth mount. Alternatively, a train may consist entirely of passenger carrying coaches, some or all of which are powered as a "multiple unit". These are more useful for astronomical viewing. A passenger train may consist of one or several locomotives, and one or more coaches. Newtonian or reflecting telescopes employ the reflective properties of light, and use mirrors and lenses. Special kinds of trains running on corresponding special 'railways' are atmospheric railways, monorails, high-speed railways, Dinky Trains, maglev, rubber-tired underground, funicular and cog railways. These can be used for both terrestrial and astronomical viewing. Trains can also be hauled by horses, pulled by a cable, or run downhill by gravity.

Galilean or refracting telescopes employ the refractive properties of light, and are constructed of lenses. A train can consist of a combination of a locomotive and attached railroad cars, or a self-propelled multiple unit (or occasionally a single powered coach, called a railcar). Optical telescopes are also divided into two types. There are various types of trains designed for particular purposes. Telescopes are broadly classified into two main types. . Optical interferometer arrays and arrays of radio telescopes were developed much more recently. In American railway terminology, and increasingly in the United Kingdom, a consist is used to describe the group of rail vehicles which make up a train.

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). Historically the steam engine was the dominant form of locomotive power, and other sources of power (such as horses, rope, gravitiy, pneumatics, or gas turbines) are possible as well. 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). Power is usually derived from diesel engines or from electricity supplied by trackside systems. Galileo is generally credited with being the first to use a telescope for astronomical purposes. Propulsion for the train is typically provided by a separate locomotive, or from individual motors in self-propelled multiple units. 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). The guideway (permanent way) usually consists of conventional rail tracks, but might also be monorail or maglev.

Even if Lippershey did not make the first one, he publicized it. In rail transport, a train consists of a single or several connected rail vehicles that are capable of being moved together along a guideway to transport freight or passengers from one place to another along a planned route. Some name that person as Hans Lippershey (c1570-c1619), but Jacob Metius and Zacharias Jansen also claimed to have invented a telescope during the same time period. Leonard Digges is sometimes credited with the invention in England in the 1570s, but usually credit for assembling the first telescope is usually given to an unknown Dutch spectacle maker in about 1608. The Visby lenses tentatively suggest that the technology was known to the Arabs and Persians then to the Vikings in the 10th century.

Article. The first telescopes may have been Assyrian crystal lenses. . The mirrors are usually a section of a rotated parabola.

They use ring-shaped "glancing" mirrors, made of heavy metals, that reflect the rays just a few degrees. X-ray and gamma-ray telescopes have a problem because these rays go through most metals and glasses. Aperture synthesis is now also being applied to optical telescopes using optical interferometers (arrays of optical telescopes) and Aperture Masking Interferometry at single telescopes. As of 2005, the current record 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].

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 dish is sometimes constructed of a conductive wire mesh whose openings are smaller than a wavelength. Radio telescopes are focused radio antennas, usually shaped like large dishes. The word "telescope" usually refers to optical telescopes, but there are telescopes for most of the spectrum of electromagnetic radiation.

Telescopes are used for astronomy and in many non-astronomical instruments including theodolites, transits, spotting scopes, monoculars, binoculars, camera lenses and spyglasses. 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. Telescopes increase the apparent angular size of distant objects, as well as their apparent brightness. A telescope (from the Greek tele = 'far' and skopein = 'to look or see'; teleskopos = 'far-seeing') is an optical tool that gathers and focuses electromagnetic radiation.

The 1-meter refracting Swedish Solar Telescope (SST) on La Palma, is currently the highest-resolution solar telescope in the world. It was a failure. The horizontal tube was 60 m long and the objective had 1.25 m in diameter. 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.

Its lens was stationary, prefigured so as to sag into the correct shape. It was on display at the 1900 Paris Exposition. The largest refractor ever constructed was French. It was exceeded in size one year later by the 0.91 m refractor at the Lick Observatory.

This was the last time the most powerful operational telescope in the world was located in Europe. The 0.76 m Nice refractor (in France) that became operational in 1888 was at that time the world's largest telescope. The 1.02 m Yerkes Telescope (in Wisconsin) is the largest aimable refracting telescope in use. The telescope now has an adaptive optics system, and is still useful for advanced research.

In 1919, the telescope was used for the first stellar diameter measurements using interferometry. The mirror was made of green glass by Saint-Gobain. 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 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.

It has a single borosilicate (Pyrex™) mirror that was famously difficult to construct. The 200 inch (5.08 m) Hale telescope on Palomar Mountain was the largest conventional research telescope for many years. One of them is the Overwhelmingly Large Telescope (OWL), which is intended to have a single aperture of 100 meters in diameter. There are many plans for even larger telescopes.

The CHARA (Center for High Angular Resolution Astronomy) array is the telescope array that can currently (2005) produce the highest resolution images at near-infrared wavelengths. The Navy Prototype Optical Interferometer is the optical telescope (array) that can currently (2005) produce the highest resolution images at visible wavelengths. The four telescopes, belonging to the European Southern Observatory (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 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 meters in diameter.

The Keck telescopes are currently (2005) the largest, but will soon be superseded by the Gran Telescopio Canarias and Southern African Large Telescope. In this way the images can be diffraction limited, and used for coverage in the ultraviolet (UV) and infrared. The Hubble Space Telescope is in orbit beyond Earth's atmosphere to allow for observations not distorted by astronomical seeing. polarimeters, that detect light polarization.

spectrographs, useful in different regions of the spectrum. imagers, of different spectral responses. ball-and-socket (ancient and useless for astronomy). fixed with movable plane mirror for solar observing.

meridian transit (altitude only). Newtonian reflecting telescopes. Galilean refracting telescopes. Radio telescopes.

Optical telescopes.