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Asolar telescope or asolar observatory is a special-purposetelescope used to observe theSun. Solar telescopes usually detect light with wavelengths in, or not far outside, thevisible spectrum. Obsolete names for Sun telescopes includeheliograph andphotoheliograph.
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Solar telescopes need optics large enough to achieve the best possiblediffraction limit but less so for the associated light-collecting power of other astronomical telescopes. However, recently newer narrowerfilters and higher framerates have also driven solar telescopes towards photon-starved operations.[1] Both theDaniel K. Inouye Solar Telescope as well as the proposedEuropean Solar Telescope (EST) have larger apertures not only to increase the resolution, but also to increase the light-collecting power.
Because solar telescopes operate during the day, seeing is generally worse than for night-time telescopes, because the ground around the telescope is heated, which causesturbulence and degrades the resolution. To alleviate this, solar telescopes are usually built ontowers and the structures are painted white. TheDutch Open Telescope is built on an open framework to allow the wind to pass through the complete structure and provide cooling around the telescope's main mirror.
Another solar telescope-specific problem is the heat generated by the tightly-focused sunlight. For this reason, aheat stop is an integral part of the design of solar telescopes. For theDaniel K. Inouye Solar Telescope, the heat load is 2.5 MW/m2, with peak powers of 11.4 kW.[2] The goal of such a heat stop is not only to survive this heat load, but also to remain cool enough not to induce any additional turbulence inside the telescope's dome.
Professional solar observatories may have main optical elements with very longfocal lengths (although not always,Dutch Open Telescope) and light paths operating in avacuum orhelium to eliminate air motion due toconvection inside the telescope. However, this is not possible for apertures over 1 meter, at which the pressure difference at the entrance window of the vacuum tube becomes too large. Therefore, theDaniel K. Inouye Solar Telescope and theEST have active cooling of the dome to minimize the temperature difference between the air inside and outside the telescope.
Due to the Sun's narrow path across the sky, some solar telescopes are fixed in position (and are sometimes buried underground), with the only moving part being aheliostat to track the Sun. One example of this is theMcMath-Pierce Solar Telescope.
The Sun, being the closest star to earth, allows a unique chance to studystellar physics with high-resolution. It was, until the 1990s,[3] the only star whose surface had been resolved. General topics that interest a solar astronomer are its 11-year periodicity (i.e., theSolar Cycle),sunspots,magnetic field activity (seesolar dynamo),solar flares,coronal mass ejections,differential rotation, andplasma physics.
Most solar observatories observe optically at visible, UV, and near infrared wavelengths, but other solar phenomena can be observed — albeit not from the Earth's surface due to theabsorption of the atmosphere:
In the field ofamateur astronomy there are many methods used to observe the Sun. Amateurs use everything from simple systems to project the Sun on a piece of white paper, light blockingfilters,Herschel wedges which redirect 95% of the light and heat away from the eyepiece,[4] up tohydrogen-alpha filter systems and even home builtspectrohelioscopes. In contrast to professional telescopes, amateur solar telescopes are usually much smaller.[citation needed]
With a conventional telescope, an extremely dark filter at the opening of the primary tube is used to reduce the light of the Sun to tolerable levels. Since the full available spectrum is observed, this is known as "white-light" viewing, and the opening filter is called a "white-light filter". The problem is that even reduced, the full spectrum of white light tends to obscure many of the specific features associated with solar activity, such as prominences and details of thechromosphere. Specialized solar telescopes facilitate clear observation of such H-alpha emissions by using a bandwidth filter implemented with aFabry-Perot etalon.[5]
Asolar tower is a structure used to support equipment for studying the Sun, and is typically part of solar telescope designs. Solar tower observatories are also called vacuum tower telescopes. Solar towers are used to raise the observation equipment aboveatmospheric turbulence caused by solar heating of the ground and the radiation of the heat into the atmosphere. Traditional observatories do not have to be placed high above ground level, as they do most of their observation at night, when ground radiation is at a minimum.
The horizontal Snow solar observatory was built onMount Wilson in 1904. It was soon found that heat radiation was disrupting observations. Almost as soon as the Snow Observatory opened, plans were started for a 60-foot-tall (18 m) tower that opened in 1908 followed by a 150-foot (46 m) tower in 1912. The 60-foot tower is currently used to studyhelioseismology, while the 150-foot tower is active inUCLA's Solar Cycle Program.
The term has also been used to refer to other structures used for experimental purposes, such as the Solar Tower Atmospheric Cherenkov Effect Experiment (STACEE), which is being used to studyCherenkov radiation, and theWeizmann Institutesolar power tower.
Other solar telescopes that have solar towers areRichard B. Dunn Solar Telescope,Solar Observatory Tower Meudon and others.
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