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73-507: The BTA-6 achieved first light in late 1975, making it the largest telescope in the world until 1990, when it was surpassed by the partially constructed Keck 1 . It pioneered the technique, now standard in large astronomical telescopes, of using an altazimuth mount with a computer-controlled derotator. For a variety of reasons, BTA-6 has never been able to operate near its theoretical limits. Early problems with poorly fabricated mirror glass were addressed in 1978, improving but not eliminating

146-449: A ( ρ ) {\displaystyle D_{\phi _{a}}\left({\mathbf {\rho } }\right)} is the atmospherically induced variance between the phase at two parts of the wavefront separated by a distance ρ {\displaystyle {\boldsymbol {\rho }}} in the aperture plane, and ⟨ ⋅ ⟩ {\displaystyle \langle \cdot \rangle } represents

219-540: A ( r ) {\displaystyle \phi _{a}\left(\mathbf {r} \right)} describe the effect of the Earth's atmosphere, and the timescales for any changes in these functions will be set by the speed of refractive index fluctuations in the atmosphere. A description of the nature of the wavefront perturbations introduced by the atmosphere is provided by the Kolmogorov model developed by Tatarski, based partly on

292-413: A ( r ) {\displaystyle \phi _{a}\left(\mathbf {r} \right)} , but any amplitude fluctuations are only brought about as a second-order effect while the perturbed wavefronts propagate from the perturbing atmospheric layer to the telescope. For all reasonable models of the Earth's atmosphere at optical and infrared wavelengths the instantaneous imaging performance is dominated by

365-480: A ( r ) e i ϕ a ( r ) ) ψ 0 ( r ) {\displaystyle \psi _{p}\left(\mathbf {r} \right)=\left(\chi _{a}\left(\mathbf {r} \right)e^{i\phi _{a}\left(\mathbf {r} \right)}\right)\psi _{0}\left(\mathbf {r} \right)} where χ a ( r ) {\displaystyle \chi _{a}\left(\mathbf {r} \right)} represents

438-463: A ( r ) {\displaystyle \phi _{a}(\mathbf {r} )} is the optical phase error introduced by atmospheric turbulence, R (k) is a two-dimensional square array of independent random complex numbers which have a Gaussian distribution about zero and white noise spectrum, K (k) is the (real) Fourier amplitude expected from the Kolmogorov (or Von Karman) spectrum, Re[] represents taking

511-419: A ( r ) = Re ⁡ [ FT [ ( R ( k ) ⊗ I ( k ) ) K ( k ) ] ] {\displaystyle \phi _{a}(\mathbf {r} )=\operatorname {Re} [{\mbox{FT}}[(R(\mathbf {k} )\otimes I(\mathbf {k} ))K(\mathbf {k} )]]} where I ( k ) is a two-dimensional array which represents the spectrum of intermittency, with

584-463: A Cassegrain design, albeit without the traditional Cassegrain-style focus. Due to its large primary, the image scale at the prime focus is 8.6 arc seconds per millimeter, about the same as the Cassegrainian focus of a 4 m telescope. When working at the prime focus, a Ross coma corrector is used. The field of view, with coma and astigmatism corrected at a level of less than 0.5 arcseconds,

657-769: A commonly used definition for r 0 {\displaystyle r_{0}} , a parameter frequently used to describe the atmospheric conditions at astronomical observatories. r 0 {\displaystyle r_{0}} can be determined from a measured C N profile (described below) as follows: r 0 = ( 16.7 λ − 2 ( cos ⁡ γ ) − 1 ∫ 0 ∞ C N 2 ( h ) d h ) − 3 / 5 {\displaystyle r_{0}=\left(16.7\lambda ^{-2}(\cos \gamma )^{-1}\int _{0}^{\infty }C_{N}^{2}(h)dh\right)^{-3/5}} where

730-407: A filled disc called the "seeing disc". The diameter of the seeing disk, most often defined as the full width at half maximum (FWHM), is a measure of the astronomical seeing conditions. It follows from this definition that seeing is always a variable quantity, different from place to place, from night to night, and even variable on a scale of minutes. Astronomers often talk about "good" nights with

803-467: A first light is always a moment of great excitement, both for the people who design and build the telescope and for the astronomical community, who may have anticipated the moment for many years while the telescope was under construction. A well-known and spectacular astronomical object is usually chosen as a subject. The famous 5.08-metre (200 in) Hale Telescope of Palomar Observatory saw first light on 26 January 1949, targeting NGC 2261 under

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876-475: A low average seeing disc diameter, and "bad" nights where the seeing diameter was so high that all observations were worthless. The FWHM of the seeing disc (or just "seeing") is usually measured in arcseconds , abbreviated with the symbol (″). A 1.0″ seeing is a good one for average astronomical sites. The seeing of an urban environment is usually much worse. Good seeing nights tend to be clear, cold nights without wind gusts. Warm air rises ( convection ), degrading

949-450: A resolution of about 0.021 arcseconds. Atmospheric effects overwhelm this, so it becomes important to locate high-resolution instruments at high altitudes in order to avoid as much of the atmosphere as possible. The Pulkovo site, at 75 m above sea level, was simply not suitable for a high-quality instrument. While BTA was being designed another instrument, the RATAN-600 radio telescope ,

1022-409: A telescope. The perturbed wavefront ψ p {\displaystyle \psi _{p}} may be related at any given instant to the original planar wavefront ψ 0 ( r ) {\displaystyle \psi _{0}\left(\mathbf {r} \right)} in the following way: ψ p ( r ) = ( χ

1095-456: A test image from the JWST's Fine Guidance Sensor. NASA released the first official JWST image on 11 July 2022. Later, in an official ceremony, the first collection of five JWST science images were released on Tuesday, 12 July 2022 (NASA-TV live; 10:30 am/et/usa). Astronomical seeing In astronomy , seeing is the degradation of the image of an astronomical object due to turbulence in

1168-567: A usable figure. If the temperatures of the primary and the outside air differ by even 10 degrees, observations become impossible. The large size of the dome itself means there are thermal gradients within it that compound these problems. Refrigeration within the dome offsets some of these issues. Speckle interferometry techniques today allow the diffraction-limited resolution of 0.02 arcseconds of 15th magnitude objects under good seeing conditions ( EMCCD -based speckle interferometer – PhotonMAX-512B camera – in active use since 2007). "In contrast to

1241-522: A variety of factors, while a number of much larger instruments were built around the world over the next few decades. In the 1950s the Soviet Academy of Sciences decided to build a new telescope that would allow first-rate deep space observation. Design work started at Pulkovo in 1959 under the leadership of future Lenin Prize winner Bagrat K. Ioannisiani . With the goal of building the largest telescope in

1314-410: Is 10–20 cm at visible wavelengths under the best conditions) and this limits the resolution of telescopes to be about the same as given by a space-based 10–20 cm telescope. The distortion changes at a high rate, typically more frequently than 100 times a second. In a typical astronomical image of a star with an exposure time of seconds or even minutes, the different distortions average out as

1387-413: Is a commonly used measurement of the astronomical seeing at observatories. At visible wavelengths, r 0 {\displaystyle r_{0}} varies from 20 cm at the best locations to 5 cm at typical sea-level sites. In reality, the pattern of blobs ( speckles ) in the images changes very rapidly, so that long-exposure photographs would just show a single large blurred blob in

1460-476: Is about 14 arcminutes. It takes about three to four minutes to switch from one focus to another, making it possible to use several different instrument sets in a short period of time. BTA-6 is enclosed in a massive dome, 53 m tall at the peak, and 48 m tall from the cylindrical base it sits on. The dome is much larger than required, and there is a gap of 12 m between the telescope and dome. First light (astronomy) In astronomy , first light

1533-549: Is assumed to occur on slow timescales, then the timescale t 0 is simply proportional to r 0 divided by the mean wind speed. The refractive index fluctuations caused by Gaussian random turbulence can be simulated using the following algorithm: ϕ a ( r ) = Re [ FT [ R ( k ) K ( k ) ] ] {\displaystyle \phi _{a}(\mathbf {r} )={\mbox{Re}}[{\mbox{FT}}[R(\mathbf {k} )K(\mathbf {k} )]]} where ϕ

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1606-432: Is not recorded as having taken place. With a Sitall primary mirror it would be possible to reduce the thickness from 65 to 40 cm, reducing thermal inertia. By 2007 the operational mirror, the third to be produced, had become heavily corroded by the use of nitric acid to neutralise the alkali-based solvents used to clean the glass before applying a new layer of reflective aluminium . A major overhaul in order to re-grind

1679-458: Is the Dirac delta function . A more thorough description of the astronomical seeing at an observatory is given by producing a profile of the turbulence strength as a function of altitude, called a C n 2 {\displaystyle C_{n}^{2}} profile. C n 2 {\displaystyle C_{n}^{2}} profiles are generally performed when deciding on

1752-401: Is the complex field at position r {\displaystyle \mathbf {r} } and time t {\displaystyle t} , with real and imaginary parts corresponding to the electric and magnetic field components, ϕ u {\displaystyle \phi _{u}} represents a phase offset, ν {\displaystyle \nu } is

1825-450: Is the first use of a telescope (or, in general, a new instrument) to take an astronomical image after it has been constructed. This is often not the first viewing using the telescope; optical tests will probably have been performed to adjust the components. The first light image is normally of little scientific interest and is of poor quality, since the various telescope elements are yet to be adjusted for optimum efficiency. Despite this,

1898-693: Is treated as an oscillation in a field ψ {\displaystyle \psi } . For monochromatic plane waves arriving from a distant point source with wave-vector k {\displaystyle \mathbf {k} } : ψ 0 ( r , t ) = A u e i ( ϕ u + 2 π ν t + k ⋅ r ) {\displaystyle \psi _{0}\left(\mathbf {r} ,t\right)=A_{u}e^{i\left(\phi _{u}+2\pi \nu t+\mathbf {k} \cdot \mathbf {r} \right)}} where ψ 0 {\displaystyle \psi _{0}}

1971-435: Is ≈1 arcsecond for 20% of observational nights. Weather is another significant factor; on average, observing takes place on fewer than half of the nights throughout the year. Perhaps the most annoying problem is the huge thermal mass of the primary mirror, the telescope as a whole, and the enormous dome. Thermal effects are so significant in the primary that it can tolerate only a 2 °C change per day and still retain

2044-553: The C n 2 {\displaystyle C_{n}^{2}} profile. Some are empirical fits from measured data and others attempt to incorporate elements of theory. One common model for continental land masses is known as Hufnagel-Valley after two workers in this subject. The first answer to this problem was speckle imaging , which allowed bright objects with simple morphology to be observed with diffraction-limited angular resolution. Later came space telescopes , such as NASA 's Hubble Space Telescope , working outside

2117-483: The Hubble Space Telescope in 1990 soon gave way to initial disappointment when a flaw prevented adjustments for proper operation. The expected first light image quality was finally achieved after a 1993 servicing mission by Space Shuttle Endeavour . The Large Binocular Telescope had its first light with a single primary mirror on 12 October 2005, which was a view of NGC 891 . The second primary mirror

2190-408: The atmosphere of Earth that may become visible as blurring, twinkling or variable distortion . The origin of this effect is rapidly changing variations of the optical refractive index along the light path from the object to the detector. Seeing is a major limitation to the angular resolution in astronomical observations with telescopes that would otherwise be limited through diffraction by

2263-438: The strength of the phase fluctuations as it corresponds to the diameter of a circular telescope aperture at which atmospheric phase perturbations begin to seriously limit the image resolution. Typical r 0 {\displaystyle r_{0}} values for I band (900 nm wavelength) observations at good sites are 20–40 cm. r 0 {\displaystyle r_{0}} also corresponds to

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2336-567: The 1990s, many telescopes have developed adaptive optics systems that partially solve the seeing problem. The best systems so far built, such as SPHERE on the ESO VLT and GPI on the Gemini telescope, achieve a Strehl ratio of 90% at a wavelength of 2.2 micrometers, but only within a very small region of the sky at a time. A wider field of view can be obtained by using multiple deformable mirrors conjugated to several atmospheric heights and measuring

2409-629: The BTA-6 and RATAN-600. The first attempt to fabricate the primary mirror was made by the Lytkarino Optical Glass Plant , near Moscow. They annealed the glass too quickly, causing cracks and bubbles to form, making the mirror useless. A second attempt fared better and was installed in 1975. BTA's first images were obtained on the night of 28/29 December 1975. After a break-in period, BTA was declared fully operational in January 1977. However, it

2482-508: The West that something was seriously wrong with the telescope. It was not long before many dismissed it as a white elephant , so much so that it was even discussed in James Oberg 's 1988 book Uncovering Soviet Disasters . A third mirror, with an improved figure and no cracks, was installed in 1978. Although this improved the major problems, a number of unrelated issues continued to seriously degrade

2555-448: The adaptive optics, which is effective today mainly in the infrared, speckle interferometry can be used for observations in visible and near UV bands. In addition, speckle interferometry is realizable under poor atmospheric conditions, while the adaptive optics always needs the best seeing". SAO astronomers planned to address one of the main problems with a new mirror made of the ultra-low expansion glass-ceramic Sitall , but this upgrade

2628-436: The aperture diameter for which the variance σ 2 {\displaystyle \sigma ^{2}} of the wavefront phase averaged over the aperture comes approximately to unity: σ 2 = 1.0299 ( d r 0 ) 5 / 3 {\displaystyle \sigma ^{2}=1.0299\left({\frac {d}{r_{0}}}\right)^{5/3}} This equation represents

2701-623: The atmosphere and thus not having any seeing problems and allowing observations of faint targets for the first time (although with poorer resolution than speckle observations of bright sources from ground-based telescopes because of Hubble's smaller telescope diameter). The highest resolution visible and infrared images currently come from imaging optical interferometers such as the Navy Prototype Optical Interferometer or Cambridge Optical Aperture Synthesis Telescope , but those can only be used on very bright stars. Starting in

2774-577: The atmosphere. The parameters r 0 and t 0 vary with the wavelength used for the astronomical imaging, allowing slightly higher resolution imaging at longer wavelengths using large telescopes. The seeing parameter r 0 is often known as the Fried parameter , named after David L. Fried . The atmospheric time constant t 0 is often referred to as the Greenwood time constant , after Darryl Greenwood . Mathematical models can give an accurate model of

2847-416: The belief that there were canals on Mars . In viewing a bright object such as Mars, occasionally a still patch of air will come in front of the planet, resulting in a brief moment of clarity. Before the use of charge-coupled devices , there was no way of recording the image of the planet in the brief moment other than having the observer remember the image and draw it later. This had the effect of having

2920-404: The center for each telescope diameter. The diameter (FWHM) of the large blurred blob in long-exposure images is called the seeing disc diameter, and is independent of the telescope diameter used (as long as adaptive optics correction is not applied). It is first useful to give a brief overview of the basic theory of optical propagation through the atmosphere. In the standard classical theory, light

2993-400: The changes in the dancing speckle patterns is substantially lower. There are three common descriptions of the astronomical seeing conditions at an observatory: These are described in the sub-sections below: Without an atmosphere, a small star would have an apparent size, an " Airy disk ", in a telescope image determined by diffraction and would be inversely proportional to the diameter of

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3066-570: The direction of American astronomer Edwin Powell Hubble . The image was published in many magazines and is available on Caltech Archives. The Isaac Newton Telescope had two first lights: one in England in 1965 with its original mirror, and another in 1984 at La Palma island. The second first light was done with a video camera that showed the Crab Pulsar flashing. Elation at first light images by

3139-540: The effects of astronomical seeing on images taken through ground-based telescopes. Three simulated short-exposure images are shown at the right through three different telescope diameters (as negative images to highlight the fainter features more clearly—a common astronomical convention). The telescope diameters are quoted in terms of the Fried parameter r 0 {\displaystyle r_{0}} (defined below). r 0 {\displaystyle r_{0}}

3212-525: The effects of the atmosphere will be negligible, and hence by recording large numbers of images in real-time, a 'lucky' excellent image can be picked out. This happens more often when the number of r0-size patches over the telescope pupil is not too large, and the technique consequently breaks down for very large telescopes. It can nonetheless outperform adaptive optics in some cases and is accessible to amateurs. It does require very much longer observation times than adaptive optics for imaging faint targets, and

3285-677: The ensemble average. For the Gaussian random approximation, the structure function of Tatarski (1961) can be described in terms of a single parameter r 0 {\displaystyle r_{0}} : D ϕ a ( ρ ) = 6.88 ( | ρ | r 0 ) 5 / 3 {\displaystyle D_{\phi _{a}}\left({\mathbf {\rho } }\right)=6.88\left({\frac {\left|\mathbf {\rho } \right|}{r_{0}}}\right)^{5/3}} r 0 {\displaystyle r_{0}} indicates

3358-541: The first light viewed by the James Webb Space Telescope (JWST) was from the star HD 84406 for the purpose of testing and aligning the focus of the telescope's 18 mirrors . On 11 February 2022. The New York Times reported that "first light" images from the James Webb Space Telescope were released - as well as a related NASA alignment video (2/11/2022; 3:00) . On 6 July 2022, NASA released

3431-417: The fractional change in wavefront amplitude and ϕ a ( r ) {\displaystyle \phi _{a}\left(\mathbf {r} \right)} is the change in wavefront phase introduced by the atmosphere. It is important to emphasise that χ a ( r ) {\displaystyle \chi _{a}\left(\mathbf {r} \right)} and ϕ

3504-442: The frequency of the light determined by ν = 1 π c | k | {\textstyle \nu ={\frac {1}{\pi }}c\left|\mathbf {k} \right|} , and A u {\displaystyle A_{u}} is the amplitude of the light. The photon flux in this case is proportional to the square of the amplitude A u {\displaystyle A_{u}} , and

3577-400: The image of the planet be dependent on the observer's memory and preconceptions which led the belief that Mars had linear features. The effects of atmospheric seeing are qualitatively similar throughout the visible and near infrared wavebands. At large telescopes the long exposure image resolution is generally slightly higher at longer wavelengths, and the timescale ( t 0 - see below) for

3650-403: The length-scale over which the turbulence becomes significant (10–20 cm at visible wavelengths at good observatories), and t 0 corresponds to the time-scale over which the changes in the turbulence become significant. r 0 determines the spacing of the actuators needed in an adaptive optics system, and t 0 determines the correction speed required to compensate for the effects of

3723-445: The mirror was needed, but this would have cut into the packed observing schedule. Instead, the second mirror, abandoned due to imperfections but sitting in storage throughout, was returned to Lytkarino for refurbishment. In 2012 a milling machine removed 8 mm of glass from the upper surface, taking with that all of the optical imperfections. Work was supposed to be finished in 2013, but was delayed due to funding shortage. The mirror

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3796-406: The most serious issue. But due to its location downwind of numerous large mountain peaks, astronomical seeing is rarely good. The telescope also suffers from serious thermal expansion problems due to the large thermal mass of the mirror, and the dome as a whole, which is much larger than necessary. Upgrades have taken place throughout the system's history and are ongoing to this day. For many years

3869-407: The optical phase corresponds to the complex argument of ψ 0 {\displaystyle \psi _{0}} . As wavefronts pass through the Earth's atmosphere they may be perturbed by refractive index variations in the atmosphere. The diagram at the top-right of this page shows schematically a turbulent layer in the Earth's atmosphere perturbing planar wavefronts before they enter

3942-455: The overall performance of the telescope. In particular, the site is downwind of a number of other peaks in the Caucasus, so the site's astronomical seeing is rarely better than one arcsecond resolution, and anything under 2 arcseconds is considered good. In comparison, most major astronomical sites average seeing under one arcsecond. Under favourable conditions the width of the seeing disc ( FWHM )

4015-421: The phase fluctuations ϕ a ( r ) {\displaystyle \phi _{a}\left(\mathbf {r} \right)} . The amplitude fluctuations described by χ a ( r ) {\displaystyle \chi _{a}\left(\mathbf {r} \right)} have negligible effect on the structure of the images seen in the focus of a large telescope. For simplicity,

4088-754: The phase fluctuations in Tatarski's model are often assumed to have a Gaussian random distribution with the following second-order structure function: D ϕ a ( ρ ) = ⟨ | ϕ a ( r ) − ϕ a ( r + ρ ) | 2 ⟩ r {\displaystyle D_{\phi _{a}}\left(\mathbf {\rho } \right)=\left\langle \left|\phi _{a}\left(\mathbf {r} \right)-\phi _{a}\left(\mathbf {r} +\mathbf {\rho } \right)\right|^{2}\right\rangle _{\mathbf {r} }} where D ϕ

4161-592: The primary world-class observatory in the Soviet Union was the Pulkovo Observatory outside Saint Petersburg , originally built in 1839. Like many observatories of its era, it was primarily dedicated to timekeeping, weather, navigation and similar practical tasks, with a secondary role for scientific research. Around its 50th anniversary a new 76 cm telescope, then the world's largest, was installed for deep space observation. Further upgrades were limited due to

4234-419: The real part, and FT[] represents a discrete Fourier transform of the resulting two-dimensional square array (typically an FFT). The assumption that the phase fluctuations in Tatarski's model have a Gaussian random distribution is usually unrealistic. In reality, turbulence exhibits intermittency. These fluctuations in the turbulence strength can be straightforwardly simulated as follows: ϕ

4307-491: The resolution of long-exposure images is determined primarily by diffraction and the size of the Airy pattern and thus is inversely proportional to the telescope diameter. For telescopes with diameters larger than r 0 , the image resolution is determined primarily by the atmosphere and is independent of telescope diameter, remaining constant at the value given by a telescope of diameter equal to r 0 . r 0 also corresponds to

4380-606: The same dimensions as R ( k ) , and where ⊗ {\displaystyle \otimes } represents convolution . The intermittency is described in terms of fluctuations in the turbulence strength C n 2 {\displaystyle C_{n}^{2}} . It can be seen that the equation for the Gaussian random case above is just the special case from this equation with: I ( k ) = δ ( | k | ) {\displaystyle I(k)=\delta (|k|)} where δ ( ) {\displaystyle \delta ()}

4453-402: The seeing, as do wind and clouds. At the best high-altitude mountaintop observatories , the wind brings in stable air which has not previously been in contact with the ground, sometimes providing seeing as good as 0.4". The astronomical seeing conditions at an observatory can be conveniently described by the parameters r 0 and t 0 . For telescopes with diameters smaller than r 0 ,

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4526-660: The size of an imaginary telescope aperture for which the diffraction limited angular resolution is equal to the resolution limited by seeing. Both the size of the seeing disc and the Fried parameter depend on the optical wavelength, but it is common to specify them for 500 nanometers. A seeing disk smaller than 0.4 arcseconds or a Fried parameter larger than 30 centimeters can be considered excellent seeing. The best conditions are typically found at high-altitude observatories on small islands, such as those at Mauna Kea or La Palma . Astronomical seeing has several effects: The effects of atmospheric seeing were indirectly responsible for

4599-524: The size of the telescope aperture . Today, many large scientific ground-based optical telescopes include adaptive optics to overcome seeing. The strength of seeing is often characterized by the angular diameter of the long-exposure image of a star ( seeing disk ) or by the Fried parameter r 0 . The diameter of the seeing disk is the full width at half maximum of its optical intensity. An exposure time of several tens of milliseconds can be considered long in this context. The Fried parameter describes

4672-486: The studies of turbulence by the Russian mathematician Andrey Kolmogorov . This model is supported by a variety of experimental measurements and is widely used in simulations of astronomical imaging. The model assumes that the wavefront perturbations are brought about by variations in the refractive index of the atmosphere. These refractive index variations lead directly to phase fluctuations described by ϕ

4745-465: The telescope. However, when light enters the Earth's atmosphere , the different temperature layers and different wind speeds distort the light waves, leading to distortions in the image of a star. The effects of the atmosphere can be modeled as rotating cells of air moving turbulently. At most observatories, the turbulence is only significant on scales larger than r 0 (see below—the seeing parameter r 0

4818-401: The turbulence strength C N 2 ( h ) {\displaystyle C_{N}^{2}(h)} varies as a function of height h {\displaystyle h} above the telescope, and γ {\displaystyle \gamma } is the angular distance of the astronomical source from the zenith (from directly overhead). If turbulent evolution

4891-466: The type of adaptive optics system which will be needed at a particular telescope, or in deciding whether or not a particular location would be a good site for setting up a new astronomical observatory. Typically, several methods are used simultaneously for measuring the C n 2 {\displaystyle C_{n}^{2}} profile and then compared. Some of the most common methods include: There are also mathematical functions describing

4964-408: The vertical structure of the turbulence, in a technique known as Multiconjugate Adaptive Optics. Another cheaper technique, lucky imaging , has had good results on smaller telescopes. This idea dates back to pre-war naked-eye observations of moments of good seeing, which were followed by observations of the planets on cine film after World War II . The technique relies on the fact that every so often

5037-512: The world, a title long held by the 200 inch (5 m) Hale Telescope at the Palomar Observatory , the team settled on a new design of 6 m (236 inches). This is about the maximum size a solid mirror can have without suffering from major distortion when tilted. A telescope's theoretical angular resolution is defined by its aperture, which in the case of the BTA's 6 m leads to

5110-597: Was also designed. It was decided that the two instruments should be co-located, allowing the construction of a single site to house the crews. To select the site, sixteen expeditions were dispatched to various regions of the USSR, and the final selection was in the North Caucasus Mountains near Zelenchukskaya at a height of 2,070 m. In 1966 the Special Astrophysical Observatory was formed to host

5183-429: Was clear the second mirror was only marginally better than the first, and contained major imperfections. Crews took to blocking off portions of the mirror using large pieces of black cloth to cover over the roughest areas. According to Ioannisiani, the primary directed only 61% of the incoming light into a 0.5- arcsecond circle and 91% into one with twice the diameter. Almost immediately after it opened, rumors started in

5256-555: Was finally completed in November 2017, and mirror replacement took place in May 2018. However, the refurbished mirror was found to be still inadequate, and after a few months of testing, it was decided to replace it with the previous mirror. The BTA primary is a 605 cm f/4 mirror. This is a relatively slow primary compared to similar instruments; the Hale is a 5 m f/3.3. The telescope optics are

5329-429: Was installed in January 2006 and became fully operational in January 2008. The 10.4-metre (1,040 cm) Gran Telescopio Canarias had a first light image of Tycho 1205081 on 14 July 2007. The IRIS solar space observatory achieved first light on 17 July 2013. The PI noted: "The quality of images and spectra we are receiving from IRIS is amazing. This is just what we were hoping for ..." On 4 February 2022,

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