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82-469: The Southern African Large Telescope ( SALT ) is a 9.2-metre optical telescope designed mainly for spectroscopy. It consists of 91 hexagonal mirror segments each with a 1-metre inscribed diameter, resulting in a total hexagonal mirror of 11.1 by 9.8 m. However, its effective aperture is only 9.2 m. It is located close to the town of Sutherland in the semi-desert region of the Karoo , South Africa . It

164-411: A = 313 Π 10800 {\displaystyle D_{a}={\frac {313\Pi }{10800}}} radians to arcsecs is given by: D a = 313 Π 10800 ⋅ 206265 = 1878 {\displaystyle D_{a}={\frac {313\Pi }{10800}}\cdot 206265=1878} . An example using a telescope with an aperture of 130 mm observing

246-426: A 2 {\displaystyle a_{2}} coefficients are typically expressed as a fraction of a chosen wavelength of light. Piston and tilt are not actually true optical aberrations , as they do not represent or model curvature in the wavefront. Defocus is the lowest order true optical aberration. If piston and tilt are subtracted from an otherwise perfect wavefront, a perfect, aberration-free image

328-511: A curved mirror in place of the objective lens, theory preceded practice. The theoretical basis for curved mirrors behaving similar to lenses was probably established by Alhazen , whose theories had been widely disseminated in Latin translations of his work. Soon after the invention of the refracting telescope, Galileo, Giovanni Francesco Sagredo , and others, spurred on by their knowledge that curved mirrors had similar properties to lenses, discussed

410-416: A focal ratio slower (bigger number) than f/12 is generally considered slow, and any telescope with a focal ratio faster (smaller number) than f/6, is considered fast. Faster systems often have more optical aberrations away from the center of the field of view and are generally more demanding of eyepiece designs than slower ones. A fast system is often desired for practical purposes in astrophotography with

492-618: A pupil diameter of 7 mm. Younger persons host larger diameters, typically said to be 9 mm, as the diameter of the pupil decreases with age. An example gathering power of an aperture with 254 mm compared to an adult pupil diameter being 7 mm is given by: P = ( D D p ) 2 = ( 254 7 ) 2 ≈ 1316.7 {\displaystyle P=\left({\frac {D}{D_{p}}}\right)^{2}=\left({\frac {254}{7}}\right)^{2}\approx 1316.7} Light-gathering power can be compared between telescopes by comparing

574-424: A "normal" or standard value of 7 mm for most adults aged 30–40, to 5–6 mm for retirees in their 60s and 70s. A lifetime spent exposed to chronically bright ambient light, such as sunlight reflected off of open fields of snow, or white-sand beaches, or cement, will tend to make individuals' pupils permanently smaller. Sunglasses greatly help, but once shrunk by long-time over-exposure to bright light, even

656-415: A compact star commonly pulls in gas from a companion star, thus radiation (especially X-ray ) is emitted. Scientists used this as an indirect way to locate black holes. Another phenomenon that SALT has helped astronomers investigate is the way that masses build up on some compact stars until supernova explosions blow them apart, which gives scientists a "Type 1a" supernovae used to show that the expansion of

738-424: A computer ( smartphone , pad , or laptop) is required to make astronomical observations from the telescopes. The digital technology allows multiple images to be stacked while subtracting the noise component of the observation producing images of Messier objects and faint stars as dim as an apparent magnitude of 15 with consumer-grade equipment. The basic scheme is that the primary light-gathering element,

820-458: A few hundred nanoradians . This almost totally eliminates image blurring due to motion, and far-field laser beam jitter. Limitations on the degree of line-of-sight stabilization arise from the limited dynamic range of the FSM tilt, and the highest frequency the mirror tilt angle can be changed. Most FSM's can be driven to several wavelengths of tilt, and at frequencies exceeding one kilohertz . As

902-404: A few weeks later by claims by Jacob Metius , and a third unknown applicant, that they also knew of this "art". Word of the invention spread fast and Galileo Galilei , on hearing of the device, was making his own improved designs within a year and was the first to publish astronomical results using a telescope. Galileo's telescope used a convex objective lens and a concave eye lens , a design

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984-400: A focus in a shorter distance. In astronomy, the f-number is commonly referred to as the focal ratio notated as N {\displaystyle N} . The focal ratio of a telescope is defined as the focal length f {\displaystyle f} of an objective divided by its diameter D {\displaystyle D} or by the diameter of an aperture stop in

1066-479: A half to complete an orbit. Also, the SALT telescope allows scientists to study the rapid brightness changes in exotic stars. More research using SALT has aided astronomers to investigate the structure and evolution of our galaxy , such as quasars , Magellanic clouds , the galactic structure and stellar astrophysics . SALT released its first color images, which marked the achievement of the "first light". This also marked

1148-406: A larger field of view. Design specifications relate to the characteristics of the telescope and how it performs optically. Several properties of the specifications may change with the equipment or accessories used with the telescope; such as Barlow lenses , star diagonals and eyepieces . These interchangeable accessories do not alter the specifications of the telescope, however they alter the way

1230-404: A limit related to something called the exit pupil . The exit pupil is the cylinder of light exiting the eyepiece and entering the pupil of the eye; hence the lower the magnification , the larger the exit pupil . It is the image of the shrunken sky-viewing aperture of the telescope, reduced by the magnification factor,   M   , {\displaystyle \ M\ ,} of

1312-402: A minimum and maximum. A wider field of view eyepiece may be used to keep the same eyepiece focal length whilst providing the same magnification through the telescope. For a good quality telescope operating in good atmospheric conditions, the maximum usable magnification is limited by diffraction. The visual magnification M {\displaystyle M} of the field of view through

1394-463: A more convenient viewing location, and in that case the image is erect, but still reversed left to right. In terrestrial telescopes such as spotting scopes , monoculars and binoculars , prisms (e.g., Porro prisms ) or a relay lens between objective and eyepiece are used to correct the image orientation. There are telescope designs that do not present an inverted image such as the Galilean refractor and

1476-399: A multitude of lenses that increase or decrease effective focal length. The quality of the image generally depends on the quality of the optics (lenses) and viewing conditions—not on magnification. Magnification itself is limited by optical characteristics. With any telescope or microscope, beyond a practical maximum magnification, the image looks bigger but shows no more detail. It occurs when

1558-401: A position corresponding to the center of curvature of the mirror is reflected and refocused to the same position. Therefore, the telescope employs a center-of-curvature alignment sensor (CCAS) situated at the top of a tall tower adjacent to the dome. Laser light is shone down on all the segments, and the position of the reflections from each mirror measured. A process called "stacking" thus allows

1640-531: A survey of a given area, the field of view is just as important as raw light gathering power. Survey telescopes such as the Large Synoptic Survey Telescope try to maximize the product of mirror area and field of view (or etendue ) rather than raw light gathering ability alone. The magnification through a telescope makes an object appear larger while limiting the FOV. Magnification is often misleading as

1722-402: A telescope can be determined by the telescope's focal length f {\displaystyle f} divided by the eyepiece focal length f e {\displaystyle f_{e}} (or diameter). The maximum is limited by the focal length of the eyepiece . An example of visual magnification using a telescope with a 1200 mm focal length and 3 mm eyepiece

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1804-408: Is a deviation in the direction a beam of light propagates. Tilt quantifies the average slope in both the X and Y directions of a wavefront or phase profile across the pupil of an optical system. In conjunction with piston (the first Zernike polynomial term), X and Y tilt can be modeled using the second and third Zernike polynomials: where ρ {\displaystyle \rho }

1886-633: Is a facility of the South African Astronomical Observatory , the national optical observatory of South Africa. SALT is the largest optical telescope in the southern hemisphere . It enables spectroscopic and polarimetric analysis and imaging of the radiation from astronomical objects that are out of reach of northern hemisphere telescopes. It is closely based on the Hobby–Eberly Telescope (HET) at McDonald Observatory , with some changes in its design, especially to

1968-619: Is directly related to the diameter (or aperture ) of its objective (the primary lens or mirror that collects and focuses the light), and its light-gathering power is related to the area of the objective. The larger the objective, the more light the telescope collects and the finer detail it resolves. People use optical telescopes (including monoculars and binoculars ) for outdoor activities such as observational astronomy , ornithology , pilotage , hunting and reconnaissance , as well as indoor/semi-outdoor activities such as watching performance arts and spectator sports . The telescope

2050-418: Is formed. Rapid optical tilts in both X and Y directions are termed jitter . Jitter can arise from three-dimensional mechanical vibration, and from rapidly varying 3D refraction in aerodynamic flowfields. Jitter may be compensated in an adaptive optics system by using a flat mirror mounted on a dynamic two-axis mount that allows small, rapid, computer-controlled changes in the mirror X and Y angles. This

2132-425: Is given by where λ {\displaystyle \lambda } is the wavelength and D {\displaystyle D} is the aperture. For visible light ( λ {\displaystyle \lambda } = 550 nm) in the small-angle approximation , this equation can be rewritten: Here, α R {\displaystyle \alpha _{R}} denotes

2214-479: Is given by:   M m i n = D   d e p =   254   7 ≈ 36 ×   . {\displaystyle \ M_{\mathsf {min}}={\frac {D}{\ d_{\mathsf {ep}}}}={\frac {\ 254\ }{7}}\approx 36\!\times ~.} If the telescope happened to have a 1 200 mm focal length (   L   {\displaystyle \ L\ } ),

2296-463: Is given by: M = f f e = 1200 3 = 400 {\displaystyle M={\frac {f}{f_{e}}}={\frac {1200}{3}}=400} There are two issues constraining the lowest useful magnification on a telescope: Both constraints boil down to approximately the same rule: The magnification of the viewed image,   M   , {\displaystyle \ M\ ,} must be high enough to make

2378-473: Is given by: R = λ 10 6 = 550 10 6 = 0.00055 {\displaystyle R={\frac {\lambda }{10^{6}}}={\frac {550}{10^{6}}}=0.00055} . The constant Φ {\displaystyle \Phi } is derived from radians to the same unit as the object's apparent diameter ; where the Moon's apparent diameter of D

2460-431: Is more a discovery of optical craftsmen than an invention of a scientist. The lens and the properties of refracting and reflecting light had been known since antiquity , and theory on how they worked was developed by ancient Greek philosophers, preserved and expanded on in the medieval Islamic world , and had reached a significantly advanced state by the time of the telescope's invention in early modern Europe . But

2542-643: Is now called a Galilean telescope . Johannes Kepler proposed an improvement on the design that used a convex eyepiece , often called the Keplerian Telescope . The next big step in the development of refractors was the advent of the Achromatic lens in the early 18th century, which corrected the chromatic aberration in Keplerian telescopes up to that time—allowing for much shorter instruments with much larger objectives. For reflecting telescopes , which use

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2624-463: Is often termed a " fast steering mirror ", or FSM. A gimbaled optical pointing system cannot mechanically track an object or stabilize a projected laser beam to much better than several hundred microradians . Buffeting due to aerodynamic turbulence further degrades the pointing stability. Light, however, has no appreciable momentum, and by reflecting from a computer-driven FSM, an image or laser beam can be stabilized to single microradians, or even

2706-400: Is the ability of a telescope to collect a lot more light than the human eye. Its light-gathering power is probably its most important feature. The telescope acts as a light bucket , collecting all of the photons that come down on it from a far away object, where a larger bucket catches more photons resulting in more received light in a given time period, effectively brightening the image. This

2788-426: Is the normalized radius with 0 ≤ ρ ≤ 1 {\displaystyle 0\leq \rho \leq 1} and θ {\displaystyle \theta } is the azimuthal angle with 0 ≤ θ ≤ 2 π {\displaystyle 0\leq \theta \leq 2\pi } . The a 1 {\displaystyle a_{1}} and

2870-412: Is why the pupils of your eyes enlarge at night so that more light reaches the retinas. The gathering power P {\displaystyle P} compared against a human eye is the squared result of the division of the aperture D {\displaystyle D} over the observer's pupil diameter D p {\displaystyle D_{p}} , with an average adult having

2952-472: The Arecibo Radio Telescope . Although this results in only a limited observing window per target, it greatly simplifies the primary mirror mount, when compared to a fully steerable telescope, transferring the complexity to the smaller and lighter payload tracking system, providing for an overall reduction in total telescope construction cost. SALT has a fixed zenith angle of 37 degrees, optimised for

3034-530: The Gregorian reflector . These are referred to as erecting telescopes . Many types of telescope fold or divert the optical path with secondary or tertiary mirrors. These may be integral part of the optical design ( Newtonian telescope , Cassegrain reflector or similar types), or may simply be used to place the eyepiece or detector at a more convenient position. Telescope designs may also use specially designed additional lenses or mirrors to improve image quality over

3116-475: The Keck Telescopes , the primary mirror is composed of an array of mirrors designed to act as a single larger mirror; however, the SALT mirrors produce a spherical primary, rather than the paraboloid shape associated with a classical Cassegrain telescope. Each SALT mirror is a 1-meter hexagon, and the array of 91 identical mirrors produces a hexagonally shaped primary 11 by 9.8 meters in size. To compensate for

3198-456: The Lagoon Nebula . The official opening by President Thabo Mbeki took place during the inauguration ceremony on 10 November 2005. South Africa contributed about a third of the total of US$ 36 million that will finance SALT for its first 10 years (US$ 20 million for the construction of the telescope, US$ 6 million for instruments, and US$ 10 million for operations). The rest was contributed by

3280-444: The areas A {\displaystyle A} of the two different apertures. As an example, the light-gathering power of a 10-meter telescope is 25x that of a 2-meter telescope: p = A 1 A 2 = π 5 2 π 1 2 = 25 {\displaystyle p={\frac {A_{1}}{A_{2}}}={\frac {\pi 5^{2}}{\pi 1^{2}}}=25} For

3362-431: The objective (1) (the convex lens or concave mirror used to gather the incoming light), focuses that light from the distant object (4) to a focal plane where it forms a real image (5). This image may be recorded or viewed through an eyepiece (2), which acts like a magnifying glass . The eye (3) then sees an inverted, magnified virtual image (6) of the object. Most telescope designs produce an inverted image at

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3444-420: The spherical aberration corrector. The main purpose for these changes was to improve the telescope's field of view . It shares the same fixed mirror altitude design, which limits access to 70% of the visible sky. First light with the full mirror was declared on 1 September 2005, with 1-arc-second resolution images of globular cluster 47 Tucanae , open cluster NGC 6152 , spiral galaxy NGC 6744 , and

3526-501: The 19th century, long-lasting aluminum coatings in the 20th century, segmented mirrors to allow larger diameters, and active optics to compensate for gravitational deformation. A mid-20th century innovation was catadioptric telescopes such as the Schmidt camera , which uses both a lens (corrector plate) and mirror as primary optical elements, mainly used for wide field imaging without spherical aberration. The late 20th century has seen

3608-489: The FSM mirror is optically flat, FSM's need not be located at pupil images . Two FSM's can be combined to create an anti-beamwalk pair, which stabilizes not only the beam pointing angle but the location of the beam center. Anti-beamwalk FSM's are positioned prior to a deformable mirror (which must be located at a pupil image) to stabilize the position of the pupil image on the deformable mirror and minimize correction errors resulting from wavefront movement, or shearing , on

3690-569: The Magellanic clouds, but because of the full range of azimuths and the celestial rotation, SALT has access to a good fraction of the sky available at the Sutherland site. Another consequence of this design is that the entrance pupil varies in size during the tracking of a target. The first generation instrumentation for SALT includes the SALT Imaging Camera (SALTICAM), designed and built by

3772-516: The Moon in a 550 nm wavelength , is given by: F = 2 R D ⋅ D o b ⋅ Φ D a = 2 ⋅ 0.00055 130 ⋅ 3474.2 ⋅ 206265 1878 ≈ 3.22 {\displaystyle F={\frac {{\frac {2R}{D}}\cdot D_{ob}\cdot \Phi }{D_{a}}}={\frac {{\frac {2\cdot 0.00055}{130}}\cdot 3474.2\cdot 206265}{1878}}\approx 3.22} The unit used in

3854-680: The South African Astronomical Observatory (SAAO); the Robert Stobie Spectrograph (RSS) (née Prime Focus Imaging Spectrograph), a multi-purpose long-slit and multi-object imaging spectrograph and spectropolarimeter, designed and built by the University of Wisconsin–Madison , Rutgers University, and the SAAO; and a fiber-fed High Resolution Spectrograph (HRS), designed by the University of Canterbury (New Zealand). SALTICAM

3936-426: The bright cores of active galaxies . The focal length of an optical system is a measure of how strongly the system converges or diverges light . For an optical system in air, it is the distance over which initially collimated rays are brought to a focus. A system with a shorter focal length has greater optical power than one with a long focal length; that is, it bends the rays more strongly, bringing them to

4018-461: The captured light gets into the eye. The minimum   M m i n   {\displaystyle \ M_{\mathsf {min}}\ } can be calculated by dividing the telescope aperture   D   {\displaystyle \ D\ } over the largest tolerated exit pupil diameter   d e p   . {\displaystyle \ d_{\mathsf {ep}}~.} Decreasing

4100-402: The constant Φ {\displaystyle \Phi } all divided by the objects apparent diameter D a {\displaystyle D_{a}} . Resolving power R {\displaystyle R} is derived from the wavelength λ {\displaystyle {\lambda }} using the same unit as aperture; where 550 nm to mm

4182-504: The debut of the fully operating SALTICAM, which is a $ 600,000 digital camera designed and built for SALT. Despite initial estimates by SAAO that SALT would bring up to 30,000 tourists to Sutherland, the telescope has so far only resulted in about 14,000 annual visitors, which has nevertheless resulted in the creation of at least 300 jobs in the town of 5,000. Optical telescope There are three primary types of optical telescope: An optical telescope's ability to resolve small details

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4264-496: The development of adaptive optics and space telescopes to overcome the problems of astronomical seeing . The electronics revolution of the early 21st century led to the development of computer-connected telescopes in the 2010s that allow non-professional skywatchers to observe stars and satellites using relatively low-cost equipment by taking advantage of digital astrophotographic techniques developed by professional astronomers over previous decades. An electronic connection to

4346-446: The eyepiece exit pupil,   d e p   , {\displaystyle \ d_{\mathsf {ep}}\ ,} no larger than the pupil of the observer's own eye. The formula for the eypiece exit pupil is where   D   {\displaystyle \ D\ } is the light-collecting diameter of the telescope's aperture. Dark-adapted pupil sizes range from 8–9 mm for young children, to

4428-439: The eyepiece-telescope combination: where   L   {\displaystyle \ L\ } is the focal length of the telescope and   ℓ   {\displaystyle \ \ell \ } is the focal length of the eyepiece. Ideally, the exit pupil of the eyepiece,   d e p   , {\displaystyle \ d_{\mathsf {ep}}\ ,} matches

4510-440: The finest detail the instrument can resolve is magnified to match the finest detail the eye can see. Magnification beyond this maximum is sometimes called empty magnification . To get the most detail out of a telescope, it is critical to choose the right magnification for the object being observed. Some objects appear best at low power, some at high power, and many at a moderate magnification. There are two values for magnification,

4592-454: The first practical reflecting telescopes, the Newtonian telescope , in 1668 although due to their difficulty of construction and the poor performance of the speculum metal mirrors used it took over 100 years for reflectors to become popular. Many of the advances in reflecting telescopes included the perfection of parabolic mirror fabrication in the 18th century, silver coated glass mirrors in

4674-405: The focal plane; these are referred to as inverting telescopes . In fact, the image is both turned upside down and reversed left to right, so that altogether it is rotated by 180 degrees from the object orientation. In astronomical telescopes the rotated view is normally not corrected, since it does not affect how the telescope is used. However, a mirror diagonal is often used to place the eyepiece in

4756-715: The following new partners joined the SALT consortium: Research using SALT at the South African Astronomical Observatory has led the facility to important discoveries. By using the Southern African Large Telescope, SAAO has the ability to take "snapshots" of stars in very quick succession. It is optimized for wavelengths and observing modes not available on other very large telescopes. As a result, astronomers can study rapidly changing properties of compact stars, primarily as they pull in gas from their companion stars or surroundings. The significance of this discovery allows us to detect black holes. The gravitational field of

4838-500: The idea of building a telescope using a mirror as the image forming objective. The potential advantages of using parabolic mirrors (primarily a reduction of spherical aberration with elimination of chromatic aberration ) led to several proposed designs for reflecting telescopes, the most notable of which was published in 1663 by James Gregory and came to be called the Gregorian telescope , but no working models were built. Isaac Newton has been generally credited with constructing

4920-413: The image by turbulence in the atmosphere ( atmospheric seeing ) and optical imperfections of the telescope, the angular resolution of an optical telescope is determined by the diameter of the primary mirror or lens gathering the light (also termed its "aperture"). The Rayleigh criterion for the resolution limit α R {\displaystyle \alpha _{R}} (in radians )

5002-409: The larger the aperture, the better the angular resolution. The resolution is not given by the maximum magnification (or "power") of a telescope. Telescopes marketed by giving high values of the maximum power often deliver poor images. For large ground-based telescopes, the resolution is limited by atmospheric seeing . This limit can be overcome by placing the telescopes above the atmosphere, e.g., on

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5084-528: The longest recommended eyepiece focal length (   ℓ   {\displaystyle \ \ell \ } ) would be   ℓ =   L   M ≈   1   200   m m   36 ≈ 33   m m   . {\displaystyle \ \ell ={\frac {\ L\ }{M}}\approx {\frac {\ 1\ 200{\mathsf {\ mm\ }}}{36}}\approx 33{\mathsf {\ mm}}~.} An eyepiece of

5166-405: The magnification past this limit will not increase brightness nor improve clarity: Beyond this limit there is no benefit from lower magnification. Likewise calculating the exit pupil   d e p   {\displaystyle \ d_{\mathsf {ep}}\ } is a division of the aperture diameter   D   {\displaystyle \ D\ } and

5248-610: The most significant step cited in the invention of the telescope was the development of lens manufacture for spectacles , first in Venice and Florence in the thirteenth century, and later in the spectacle making centers in both the Netherlands and Germany. It is in the Netherlands in 1608 where the first documents describing a refracting optical telescope surfaced in the form of a patent filed by spectacle maker Hans Lippershey , followed

5330-445: The object diameter results in the smallest resolvable features at that unit. In the above example they are approximated in kilometers resulting in the smallest resolvable Moon craters being 3.22 km in diameter. The Hubble Space Telescope has a primary mirror aperture of 2400 mm that provides a surface resolvability of Moon craters being 174.9 meters in diameter, or sunspots of 7365.2 km in diameter. Ignoring blurring of

5412-403: The optical power of the telescope, its characteristic is the most misunderstood term used to describe the observable world. At higher magnifications the image quality significantly reduces, usage of a Barlow lens increases the effective focal length of an optical system—multiplies image quality reduction. Similar minor effects may be present when using star diagonals , as light travels through

5494-568: The other partners: Germany , Poland , the United States , the United Kingdom , and New Zealand . SALT is located on a hilltop 1837 m above sea level in a nature reserve in the Hantam, Karoo 370 km (230 mi) north-east of Cape Town , near the small town of Sutherland . In March 2004, installation of the massive mirror began. The last of the 91 smaller mirrored hexagon segments

5576-461: The physical area that can be resolved. A familiar way to express the characteristic is the resolvable ability of features such as Moon craters or Sun spots. Expression using the formula is given by twice the resolving power R {\displaystyle R} over aperture diameter D {\displaystyle D} multiplied by the objects diameter D o b {\displaystyle D_{ob}} multiplied by

5658-424: The pupil of the observer's eye: If the exit pupil from the eyepiece is larger than the pupil of individual observer's eye, some of the light delivered from the telescope will be cut off. If the eyepiece exit pupil is the same or smaller than the pupil of the observer's eye, then all of the light collected by the telescope aperture will enter the eye, with lower magnification producing a brighter image, as long as all of

5740-543: The purpose of gathering more photons in a given time period than a slower system, allowing time lapsed photography to process the result faster. Wide-field telescopes (such as astrographs ), are used to track satellites and asteroids , for cosmic-ray research, and for astronomical surveys of the sky. It is more difficult to reduce optical aberrations in telescopes with low f-ratio than in telescopes with larger f-ratio. The light-gathering power of an optical telescope, also referred to as light grasp or aperture gain,

5822-448: The resolution limit in arcseconds and D {\displaystyle D} is in millimeters. In the ideal case, the two components of a double star system can be discerned even if separated by slightly less than α R {\displaystyle \alpha _{R}} . This is taken into account by the Dawes limit The equation shows that, all else being equal,

5904-412: The same apparent field-of-view but longer focal-length will deliver a wider true field of view, but dimmer image. If the telescope has a central obstruction (e.g. a Newtonian , Maksutov , or Schmidt–Cassegrain telescope ) it is also likely that the low magnification will make the obstruction come into focus enough to make a black spot in the middle of the image. Tilt (optics) In optics , tilt

5986-418: The spherical primary, the telescope has a four-mirror spherical aberration corrector (SAC) that provides a corrected, flat focal plane with a field of view of 8 arcminutes at prime focus. Each of the 91 mirrors is made of low-expansion Sitall glass and can be adjusted in tip, tilt and piston in order to properly align them so as to act as a single mirror. Because the mirror is spherical, light emitted from

6068-561: The summits of high mountains, on balloons and high-flying airplanes, or in space . Resolution limits can also be overcome by adaptive optics , speckle imaging or lucky imaging for ground-based telescopes. Recently, it has become practical to perform aperture synthesis with arrays of optical telescopes. Very high resolution images can be obtained with groups of widely spaced smaller telescopes, linked together by carefully controlled optical paths, but these interferometers can only be used for imaging bright objects such as stars or measuring

6150-756: The system. The focal length controls the field of view of the instrument and the scale of the image that is presented at the focal plane to an eyepiece , film plate, or CCD . An example of a telescope with a focal length of 1200 mm and aperture diameter of 254 mm is given by: N = f D = 1200 254 ≈ 4.7 {\displaystyle N={\frac {f}{D}}={\frac {1200}{254}}\approx 4.7} Numerically large Focal ratios are said to be long or slow . Small numbers are short or fast . There are no sharp lines for determining when to use these terms, and an individual may consider their own standards of determination. Among contemporary astronomical telescopes, any telescope with

6232-453: The telescope operator to optimize the adjustments of the mirrors. The telescope is also unusual in that during an observation, the mirror remains at a fixed altitude and azimuth, and the image of an astronomical target produced by the telescope is tracked by the "payload", which resides at the position of prime focus and includes the SAC and prime-focus instrumentation. This is similar in operation to

6314-401: The telescope's properties function, typically magnification , apparent field of view (FOV) and actual field of view. The smallest resolvable surface area of an object, as seen through an optical telescope, is the limited physical area that can be resolved. It is analogous to angular resolution , but differs in definition: instead of separation ability between point-light sources it refers to

6396-484: The universe is speeding up. Other note-worthy research the South African Astronomical Observatory has achieved using SALT include the discovery of a class of stars known as "polar", or a pair of stars. The "polar" binary star system, where a compactor star called a "white dwarf" whose volume has shrunk about one millionth of a star like the Sun. Studies using SALT concluded that these polar binary star systems take only an hour and

6478-408: The use of opthamalogic drugs cannot restore lost pupil size. Most observers' eyes instantly respond to darkness by widening the pupil to almost its maximum, although complete adaption to night vision generally takes at least a half-hour. (There is usually a slight extra widening of the pupil the longer the pupil remains dilated / relaxed.) The improvement in brightness with reduced magnification has

6560-415: The visual magnification   M   {\displaystyle \ M\ } used. The minimum often may not be reachable with some telescopes, a telescope with a very long focal length may require a longer focal length eyepiece than is available. An example of the lowest usable magnification using a fairly common 10″ (254 mm) aperture and the standard adult 7 mm maximum exit pupil

6642-771: Was installed in early 2005, while the RSS was installed on 11 October 2005. The telescope is connected to the SAAO site in Cape Town via a 1 Gbit/s fibre connection over the SANREN network. The SAAO has a 1 Gbit/s connection to the SANREN network with 30 Mbit/s of that link being the international portion. Membership of the SALT science working group: David Buckley, Gerald Cecil, Brian Chaboyer, Richard Griffiths, Janusz Kałużny, Michael Albrow, Karen Pollard, Kenneth Nordsieck, Darragh O'Donoghue, Larry Ramsey, Anne Sansom, Pat Cote. In 2007,

6724-546: Was put in place in May 2005. Korea , Japan , Poland and Google have telescopes at the site and South Africa has at least five optical telescopes there. The University of Birmingham has a solar telescope to help monitor the Sun . SALT will probe quasars and enable scientists to view stars and galaxies a billion times too faint to be seen by the naked eye . Both SALT and HET have an unusual design for an optical telescope. Similar to

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