In physics , absorption of electromagnetic radiation is how matter (typically electrons bound in atoms ) takes up a photon 's energy — and so transforms electromagnetic energy into internal energy of the absorber (for example, thermal energy ).
76-408: A telescope is a device used to observe distant objects by their emission, absorption , or reflection of electromagnetic radiation . Originally, it was an optical instrument using lenses , curved mirrors , or a combination of both to observe distant objects – an optical telescope . Nowadays, the word "telescope" is defined as a wide range of instruments capable of detecting different regions of
152-449: 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
228-550: 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
304-413: 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
380-480: 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
456-469: 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
532-419: 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
608-769: 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
684-490: A few bands can be observed from the Earth's surface. These bands are visible – near-infrared and a portion of the radio-wave part of the spectrum. For this reason there are no X-ray or far-infrared ground-based telescopes as these have to be observed from orbit. Even if a wavelength is observable from the ground, it might still be advantageous to place a telescope on a satellite due to issues such as clouds, astronomical seeing and light pollution . The disadvantages of launching
760-605: A few decades of the first refracting telescope. In the 20th century, many new types of telescopes were invented, including radio telescopes in the 1930s and infrared telescopes in the 1960s. The word telescope was coined in 1611 by the Greek mathematician Giovanni Demisiani for one of Galileo Galilei 's instruments presented at a banquet at the Accademia dei Lincei . In the Starry Messenger , Galileo had used
836-407: 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
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#1732779962781912-496: A large dish to collect radio waves. The dishes are sometimes constructed of a conductive wire mesh whose openings are smaller than the wavelength being observed. Unlike an optical telescope, which produces a magnified image of the patch of sky being observed, a traditional radio telescope dish contains a single receiver and records a single time-varying signal characteristic of the observed region; this signal may be sampled at various frequencies. In some newer radio telescope designs,
988-475: 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
1064-701: A mirror (reflecting optics). Also using reflecting optics, the Hubble Space Telescope with Wide Field Camera 3 can observe in the frequency range from about 0.2 μm (0.0002 mm) to 1.7 μm (0.0017 mm) (from ultra-violet to infrared light). With photons of the shorter wavelengths, with the higher frequencies, glancing-incident optics, rather than fully reflecting optics are used. Telescopes such as TRACE and SOHO use special mirrors to reflect extreme ultraviolet , producing higher resolution and brighter images than are otherwise possible. A larger aperture does not just mean that more light
1140-417: A single dish contains an array of several receivers; this is known as a focal-plane array . By collecting and correlating signals simultaneously received by several dishes, high-resolution images can be computed. Such multi-dish arrays are known as astronomical interferometers and the technique is called aperture synthesis . The 'virtual' apertures of these arrays are similar in size to the distance between
1216-908: A space telescope include cost, size, maintainability and upgradability. Some examples of space telescopes from NASA are the Hubble Space Telescope that detects visible light, ultraviolet, and near-infrared wavelengths, the Spitzer Space Telescope that detects infrared radiation, and the Kepler Space Telescope that discovered thousands of exoplanets. The latest telescope that was launched was the James Webb Space Telescope on December 25, 2021, in Kourou, French Guiana. The Webb telescope detects infrared light. The name "telescope" covers
1292-474: 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 the size of an imaginary telescope aperture for which the diffraction limited angular resolution is equal to the resolution limited by seeing. Both
1368-702: A telescope could be built using only this kind of mirror. Examples of space observatories using this type of telescope are the Einstein Observatory , ROSAT , and the Chandra X-ray Observatory . In 2012 the NuSTAR X-ray Telescope was launched which uses Wolter telescope design optics at the end of a long deployable mast to enable photon energies of 79 keV. Higher energy X-ray and gamma ray telescopes refrain from focusing completely and use coded aperture masks:
1444-409: 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
1520-419: A wide range of angles." Astronomical seeing In astronomy , seeing is the degradation of the image of an astronomical object due to turbulence in 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
1596-453: A wide range of instruments. Most detect electromagnetic radiation , but there are major differences in how astronomers must go about collecting light (electromagnetic radiation) in different frequency bands. As wavelengths become longer, it becomes easier to use antenna technology to interact with electromagnetic radiation (although it is possible to make very tiny antenna). The near-infrared can be collected much like visible light; however, in
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#17327799627811672-410: 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
1748-413: 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
1824-404: Is a major limitation to the angular resolution in astronomical observations with telescopes that would otherwise be limited through diffraction by 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
1900-412: Is a proposed ultra-lightweight design for a space telescope that uses a Fresnel lens to focus light. Beyond these basic optical types there are many sub-types of varying optical design classified by the task they perform such as astrographs , comet seekers and solar telescopes . Most ultraviolet light is absorbed by the Earth's atmosphere, so observations at these wavelengths must be performed from
1976-549: 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
2052-468: Is collected, it also enables a finer angular resolution. Telescopes may also be classified by location: ground telescope, space telescope , or flying telescope . They may also be classified by whether they are operated by professional astronomers or amateur astronomers . A vehicle or permanent campus containing one or more telescopes or other instruments is called an observatory . Radio telescopes are directional radio antennas that typically employ
2128-462: 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
2204-401: 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
2280-693: 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}}
2356-405: Is underway on several 30–40m designs. The 20th century also saw the development of telescopes that worked in a wide range of wavelengths from radio to gamma-rays . The first purpose-built radio telescope went into operation in 1937. Since then, a large variety of complex astronomical instruments have been developed. Since the atmosphere is opaque for most of the electromagnetic spectrum, only
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2432-577: Is unknown but word of it spread through Europe. Galileo heard about it and, in 1609, built his own version, and made his telescopic observations of celestial objects. The idea that the objective , or light-gathering element, could be a mirror instead of a lens was being investigated soon after the invention of the refracting telescope. The potential advantages of using parabolic mirrors —reduction of spherical aberration and no chromatic aberration —led to many proposed designs and several attempts to build reflecting telescopes . In 1668, Isaac Newton built
2508-553: 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
2584-714: The Imaging Atmospheric Cherenkov Telescopes (IACTs) or with Water Cherenkov Detectors (WCDs). Examples of IACTs are H.E.S.S. and VERITAS with the next-generation gamma-ray telescope, the Cherenkov Telescope Array ( CTA ), currently under construction. HAWC and LHAASO are examples of gamma-ray detectors based on the Water Cherenkov Detectors. A discovery in 2012 may allow focusing gamma-ray telescopes. At photon energies greater than 700 keV,
2660-577: The Latin term perspicillum . The root of the word is from the Ancient Greek τῆλε, romanized tele 'far' and σκοπεῖν, skopein 'to look or see'; τηλεσκόπος, teleskopos 'far-seeing'. The earliest existing record of a telescope was a 1608 patent submitted to the government in the Netherlands by Middelburg spectacle maker Hans Lipperhey for a refracting telescope . The actual inventor
2736-403: The electromagnetic spectrum , and in some cases other types of detectors. The first known practical telescopes were refracting telescopes with glass lenses and were invented in the Netherlands at the beginning of the 17th century. They were used for both terrestrial applications and astronomy . The reflecting telescope , which uses mirrors to collect and focus light, was invented within
2812-438: 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
2888-576: 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
2964-542: The advantage of being able to pass through the atmosphere and interstellar gas and dust clouds. Some radio telescopes such as the Allen Telescope Array are used by programs such as SETI and the Arecibo Observatory to search for extraterrestrial life. An optical telescope gathers and focuses light mainly from the visible part of the electromagnetic spectrum. Optical telescopes increase
3040-436: 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
3116-666: The apparent angular size of distant objects as well as their apparent brightness . For the image to be observed, photographed, studied, and sent to a computer, telescopes work by employing one or more curved optical elements, usually made from glass lenses and/or mirrors , to gather light and other electromagnetic radiation to bring that light or radiation to a focal point. Optical telescopes are used for astronomy and in many non-astronomical instruments, including: theodolites (including transits ), spotting scopes , monoculars , binoculars , camera lenses , and spyglasses . There are three main optical types: A Fresnel imager
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3192-638: 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
3268-535: 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
3344-416: 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
3420-404: 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
3496-400: 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
3572-475: The convention. The absorbance of an object quantifies how much of the incident light is absorbed by it (instead of being reflected or refracted ). This may be related to other properties of the object through the Beer–Lambert law . Precise measurements of the absorbance at many wavelengths allow the identification of a substance via absorption spectroscopy , where a sample is illuminated from one side, and
3648-544: 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}}
3724-525: 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
3800-682: 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
3876-557: The far-infrared and submillimetre range, telescopes can operate more like a radio telescope. For example, the James Clerk Maxwell Telescope observes from wavelengths from 3 μm (0.003 mm) to 2000 μm (2 mm), but uses a parabolic aluminum antenna. On the other hand, the Spitzer Space Telescope , observing from about 3 μm (0.003 mm) to 180 μm (0.18 mm) uses
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#17327799627813952-457: The first practical reflecting telescope, of a design which now bears his name, the Newtonian reflector . The invention of the achromatic lens in 1733 partially corrected color aberrations present in the simple lens and enabled the construction of shorter, more functional refracting telescopes. Reflecting telescopes, though not limited by the color problems seen in refractors, were hampered by
4028-417: 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
4104-442: 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
4180-400: 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
4256-444: The index of refraction starts to increase again. Absorption (electromagnetic radiation) A notable effect of the absorption of electromagnetic radiation is attenuation of the radiation; attenuation is the gradual reduction of the intensity of light waves as they propagate through the medium. Although the absorption of waves does not usually depend on their intensity (linear absorption), in certain conditions ( optics )
4332-456: The intensity of the light that exits from the sample in every direction is measured. A few examples of absorption are ultraviolet–visible spectroscopy , infrared spectroscopy , and X-ray absorption spectroscopy . Understanding and measuring the absorption of electromagnetic radiation has a variety of applications. In scientific literature is known a system of mirrors and lenses that with a laser "can enable any material to absorb all light from
4408-403: 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
4484-425: The medium's transparency changes by a factor that varies as a function of wave intensity, and saturable absorption (or nonlinear absorption) occurs. Many approaches can potentially quantify radiation absorption, with key examples following. All these quantities measure, at least to some extent, how well a medium absorbs radiation. Which among them practitioners use varies by field and technique, often due simply to
4560-407: 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
4636-769: The patterns of the shadow the mask creates can be reconstructed to form an image. X-ray and Gamma-ray telescopes are usually installed on high-flying balloons or Earth-orbiting satellites since the Earth's atmosphere is opaque to this part of the electromagnetic spectrum. An example of this type of telescope is the Fermi Gamma-ray Space Telescope which was launched in June 2008. The detection of very high energy gamma rays, with shorter wavelength and higher frequency than regular gamma rays, requires further specialization. Such detections can be made either with
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#17327799627814712-421: 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,
4788-759: 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
4864-419: 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
4940-495: 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
5016-606: 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 ()}
5092-402: 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 ,
5168-512: 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
5244-491: 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
5320-418: 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
5396-647: The telescopes. As of 2005, the record array size is many times the diameter of the Earth – using 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 reflecting telescopes. Radio telescopes are also used to collect microwave radiation , which has
5472-401: 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
5548-466: 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
5624-458: The upper atmosphere or from space. X-rays are much harder to collect and focus than electromagnetic radiation of longer wavelengths. X-ray telescopes can use X-ray optics , such as Wolter telescopes composed of ring-shaped 'glancing' mirrors made of heavy metals that are able to reflect the rays just a few degrees . The mirrors are usually a section of a rotated parabola and a hyperbola , or ellipse . In 1952, Hans Wolter outlined 3 ways
5700-538: The use of fast tarnishing speculum metal mirrors employed during the 18th and early 19th century—a problem alleviated by the introduction of silver coated glass mirrors in 1857, and aluminized mirrors in 1932. The maximum physical size limit for refracting telescopes is about 1 meter (39 inches), dictating that the vast majority of large optical researching telescopes built since the turn of the 20th century have been reflectors. The largest reflecting telescopes currently have objectives larger than 10 meters (33 feet), and work
5776-408: 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
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