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94-654: Astro-Physics, Inc. is a manufacturer of amateur telescopes , mounts , and accessories. Founded in 1975 by former Sundstrand Corporation engineer Roland Christen, the company is noted for its line of apochromatic refractors as well as high-end mounts. The company is located in Machesney Park, Illinois . The company is owned by Roland and Marjorie Christen. Author and Astronomy magazine contributing editor Philip S. Harrington wrote in his 2011 book Star Ware: The Amateur Astronomer's Guide to Choosing, Buying, and Using Telescopes and Accessories that Astro-Physics

188-411: 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

282-515: 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

376-416: 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

470-618: 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

564-419: A refractor ) is a type of optical telescope that uses a lens as its objective to form an image (also referred to a dioptric telescope ). The refracting telescope design was originally used in spyglasses and astronomical telescopes but is also used for long-focus camera lenses . Although large refracting telescopes were very popular in the second half of the 19th century, for most research purposes,

658-424: 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

752-434: A 200-millimetre (8 in) objective and a 46-metre (150 ft) focal length , and even longer tubeless " aerial telescopes " were constructed). The design also allows for use of a micrometer at the focal plane (to determine the angular size and/or distance between objects observed). Huygens built an aerial telescope for Royal Society of London with a 19 cm (7.5″) single-element lens. The next major step in

846-516: A 92mm refractor and a 130mm refractor, priced in the $ 4000–$ 7000 range. Their more active line of products is three series of computer-controlled "German"-style equatorial mounts priced in the $ 10,000–$ 20,000 range. (Complete telescopes consisting of both optical tube and mount are not currently offered.) Both the telescopes and the mounts, despite being sought-after, are manufactured in small numbers on an irregular basis. None are maintained in stock, and all typically have long lead times. When available on

940-543: A century later, two and even three element lenses were made. Refracting telescopes use technology that has often been applied to other optical devices, such as binoculars and zoom lenses / telephoto lens / long-focus lens . Refractors were the earliest type of optical telescope . The first record of a refracting telescope appeared in the Netherlands about 1608, when a spectacle maker from Middelburg named Hans Lippershey unsuccessfully tried to patent one. News of

1034-424: 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,

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1128-430: A couple of years. Apochromatic refractors have objectives built with special, extra-low dispersion materials. They are designed to bring three wavelengths (typically red, green, and blue) into focus in the same plane. The residual color error (tertiary spectrum) can be an order of magnitude less than that of an achromatic lens. Such telescopes contain elements of fluorite or special, extra-low dispersion (ED) glass in

1222-404: 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

1316-400: 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

1410-406: 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

1504-404: 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

1598-402: 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

1692-468: 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

1786-399: 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

1880-563: A refracting telescope is around 1 meter (39 in). There is a further problem of glass defects, striae or small air bubbles trapped within the glass. In addition, glass is opaque to certain wavelengths , and even visible light is dimmed by reflection and absorption when it crosses the air-glass interfaces and passes through the glass itself. Most of these problems are avoided or diminished in reflecting telescopes , which can be made in far larger apertures and which have all but replaced refractors for astronomical research. The ISS-WAC on

1974-547: 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

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2068-402: 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

2162-501: Is "a name immediately recognizable to the connoisseur of fine refractors on rock-steady mounts". Astro-Physics was profiled by Travel Channel ' s Made in America , a TV program hosted by John Ratzenberger , on October 25, 2006. In 2006, the company employed 18 people. The company's finished telescopes, which include mounting and lens , cost between $ 10,000 and $ 25,000. Astro-Physics has offered many models of telescopes over

2256-447: Is called the angular magnification. It equals the ratio between the retinal image sizes obtained with and without the telescope. Refracting telescopes can come in many different configurations to correct for image orientation and types of aberration. Because the image was formed by the bending of light, or refraction, these telescopes are called refracting telescopes or refractors . The design Galileo Galilei used c.  1609

2350-502: Is commonly called a Galilean telescope . It used a convergent (plano-convex) objective lens and a divergent (plano-concave) eyepiece lens (Galileo, 1610). A Galilean telescope, because the design has no intermediary focus, results in a non-inverted (i.e., upright) image. Galileo's most powerful telescope, with a total length of 980 millimeters (39 in; 3 ft 3 in; 1.07 yd; 98 cm; 9.8 dm; 0.98 m), magnified objects about 30 times. Galileo had to work with

2444-425: 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

2538-479: 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\ } ),

2632-463: 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

2726-477: 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

2820-435: Is ground and polished , and then the two pieces are assembled together. Achromatic lenses are corrected to bring two wavelengths (typically red and blue) into focus in the same plane. Chester More Hall is noted as having made the first twin color corrected lens in 1730. Dollond achromats were quite popular in the 18th century. A major appeal was they could be made shorter. However, problems with glass making meant that

2914-427: Is likely to show considerable color fringing (generally a purple halo around bright objects); an f / 16 achromat has much less color fringing. In very large apertures, there is also a problem of lens sagging , a result of gravity deforming glass . Since a lens can only be held in place by its edge, the center of a large lens sags due to gravity, distorting the images it produces. The largest practical lens size in

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3008-431: 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

3102-663: 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

3196-513: Is that the rays of light emerging from the eyepiece are converging. This allows for a much wider field of view and greater eye relief , but the image for the viewer is inverted. Considerably higher magnifications can be reached with this design, but, like the Galilean telescope, it still uses simple single element objective lens so needs to have a very high focal ratio to reduce aberrations ( Johannes Hevelius built an unwieldy f/225 telescope with

3290-682: Is the Shuckburgh telescope (dating to the late 1700s). A famous refractor was the "Trophy Telescope", presented at the 1851 Great Exhibition in London. The era of the ' great refractors ' in the 19th century saw large achromatic lenses, culminating with the largest achromatic refractor ever built, the Great Paris Exhibition Telescope of 1900 . In the Royal Observatory, Greenwich an 1838 instrument named

3384-400: 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

3478-412: 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

3572-550: The Galilean satellites of Jupiter in 1610 with a refracting telescope. The planet Saturn's moon, Titan , was discovered on March 25, 1655, by the Dutch astronomer Christiaan Huygens . In 1861, the brightest star in the night sky, Sirius, was found to have smaller stellar companion using the 18 and half-inch Dearborn refracting telescope. By the 18th century refractors began to have major competition from reflectors, which could be made quite large and did not normally suffer from

3666-535: 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

3760-577: The Sheepshanks telescope includes an objective by Cauchoix. The Sheepshanks had a 6.7-inch (17 cm) wide lens, and was the biggest telescope at Greenwich for about twenty years. An 1840 report from the Observatory noted of the then-new Sheepshanks telescope with the Cauchoix doublet: The power and general goodness of this telescope make it a most welcome addition to the instruments of

3854-606: The Voyager 1 / 2 used a 6 centimetres (2.4 in) lens, launched into space in the late 1970s, an example of the use of refractors in space. Refracting telescopes were noted for their use in astronomy as well as for terrestrial viewing. Many early discoveries of the Solar System were made with singlet refractors. The use of refracting telescopic optics are ubiquitous in photography, and are also used in Earth orbit. One of

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3948-444: 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

4042-431: 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

4136-414: The phases of Venus . Parallel rays of light from a distant object ( y ) would be brought to a focus in the focal plane of the objective lens ( F′ L1 / y′ ). The (diverging) eyepiece ( L2 ) lens intercepts these rays and renders them parallel once more. Non-parallel rays of light from the object traveling at an angle α1 to the optical axis travel at a larger angle ( α2 > α1 ) after they passed through

4230-506: 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

4324-516: 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

4418-426: 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

4512-461: 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

4606-402: 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

4700-496: 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

4794-596: 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

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4888-629: The evolution of refracting telescopes was the invention of the achromatic lens , a lens with multiple elements that helped solve problems with chromatic aberration and allowed shorter focal lengths. It was invented in 1733 by an English barrister named Chester Moore Hall , although it was independently invented and patented by John Dollond around 1758. The design overcame the need for very long focal lengths in refracting telescopes by using an objective made of two pieces of glass with different dispersion , ' crown ' and ' flint glass ', to reduce chromatic and spherical aberration . Each side of each piece

4982-446: 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

5076-439: 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

5170-471: The eyepiece. This leads to an increase in the apparent angular size and is responsible for the perceived magnification. The final image ( y″ ) is a virtual image, located at infinity and is the same way up (i.e., non-inverted or upright) as the object. The Keplerian telescope , invented by Johannes Kepler in 1611, is an improvement on Galileo's design. It uses a convex lens as the eyepiece instead of Galileo's concave one. The advantage of this arrangement

5264-640: The famous triplet objectives is the Cooke triplet , noted for being able to correct the Seidal aberrations. It is recognized as one of the most important objective designs in the field of photography. The Cooke triplet can correct, with only three elements, for one wavelength, spherical aberration , coma , astigmatism , field curvature , and distortion . Refractors suffer from residual chromatic and spherical aberration . This affects shorter focal ratios more than longer ones. An f /6 achromatic refractor

5358-440: 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,

5452-459: 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

5546-405: 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

5640-578: The glass objectives were not made more than about four inches (10 cm) in diameter. In the late 19th century, the Swiss optician Pierre-Louis Guinand developed a way to make higher quality glass blanks of greater than four inches (10 cm). He passed this technology to his apprentice Joseph von Fraunhofer , who further developed this technology and also developed the Fraunhofer doublet lens design. The breakthrough in glass making techniques led to

5734-578: The great refractors of the 19th century, that became progressively larger through the decade, eventually reaching over 1 meter by the end of that century before being superseded by silvered-glass reflecting telescopes in astronomy. Noted lens makers of the 19th century include: Some famous 19th century doublet refractors are the James Lick telescope (91 cm/36 in) and the Greenwich 28 inch refractor (71 cm). An example of an older refractor

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5828-500: 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

5922-413: 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 )

6016-409: 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

6110-528: 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

6204-405: 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

6298-408: The more famous applications of the refracting telescope was when Galileo used it to discover the four largest moons of Jupiter in 1609. Furthermore, early refractors were also used several decades later to discover Titan, the largest moon of Saturn, along with three more of Saturn's moons. In the 19th century, refracting telescopes were used for pioneering work on astrophotography and spectroscopy, and

6392-567: 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

6486-445: 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

6580-433: The objective and produce a very crisp image that is virtually free of chromatic aberration. Due to the special materials needed in the fabrication, apochromatic refractors are usually more expensive than telescopes of other types with a comparable aperture. In the 18th century, Dollond, a popular maker of doublet telescopes, also made a triplet, although they were not really as popular as the two element telescopes. One of

6674-470: The observatory In the 1900s a noted optics maker was Zeiss. An example of prime achievements of refractors, over 7 million people have been able to view through the 12-inch Zeiss refractor at Griffith Observatory since its opening in 1935; this is the most people to have viewed through any telescope. Achromats were popular in astronomy for making star catalogs, and they required less maintenance than metal mirrors. Some famous discoveries using achromats are

6768-403: 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

6862-498: The patent spread fast and Galileo Galilei , happening to be in Venice in the month of May 1609, heard of the invention, constructed a version of his own , and applied it to making astronomical discoveries. All refracting telescopes use the same principles. The combination of an objective lens 1 and some type of eyepiece 2 is used to gather more light than the human eye is able to collect on its own, focus it 5 , and present

6956-461: 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

7050-527: The planet Neptune and the Moons of Mars . The long achromats, despite having smaller aperture than the larger reflectors, were often favored for "prestige" observatories. In the late 18th century, every few years, a larger and longer refractor would debut. For example, the Nice Observatory debuted with 77-centimeter (30.31 in) refractor, the largest at the time, but was surpassed within only

7144-482: The poor lens technology of the time, and found he had to use aperture stops to reduce the diameter of the objective lens (increase its focal ratio ) to limit aberrations, so his telescope produced blurry and distorted images with a narrow field of view. Despite these flaws, the telescope was still good enough for Galileo to explore the sky. He used it to view craters on the Moon , the four largest moons of Jupiter , and

7238-430: The pre-1925 astronomical convention that began the day at noon, give the time of discovery as 11 August 14:40 and 17 August 16:06 Washington mean time respectively). The telescope used for the discovery was the 26-inch (66 cm) refractor (telescope with a lens) then located at Foggy Bottom . In 1893 the lens was remounted and put in a new dome, where it remains into the 21st century. Jupiter's moon Amalthea

7332-424: 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

7426-543: 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,

7520-461: The refracting telescope has been superseded by the reflecting telescope , which allows larger apertures . A refractor's magnification is calculated by dividing the focal length of the objective lens by that of the eyepiece . Refracting telescopes typically have a lens at the front, then a long tube , then an eyepiece or instrumentation at the rear, where the telescope view comes to focus. Originally, telescopes had an objective of one element, but

7614-552: The refractors. Despite this, some discoveries include the Moons of Mars, a fifth Moon of Jupiter, and many double star discoveries including Sirius (the Dog star). Refractors were often used for positional astronomy, besides from the other uses in photography and terrestrial viewing. The Galilean moons and many other moons of the solar system, were discovered with single-element objectives and aerial telescopes. Galileo Galilei 's discovered

7708-408: The related instrument, the heliometer, was used to calculate the distance to another star for the first time. Their modest apertures did not lead to as many discoveries and typically so small in aperture that many astronomical objects were simply not observable until the advent of long-exposure photography, by which time the reputation and quirks of reflecting telescopes were beginning to exceed those of

7802-452: 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,

7896-441: 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. Refracting telescope A refracting telescope (also called

7990-610: The same inherent problem with chromatic aberration. Nevertheless, the astronomical community continued to use doublet refractors of modest aperture in comparison to modern instruments. Noted discoveries include the Moons of Mars and a fifth moon of Jupiter, Amalthea . Asaph Hall discovered Deimos on 12 August 1877 at about 07:48 UTC and Phobos on 18 August 1877, at the US Naval Observatory in Washington, D.C. , at about 09:14 GMT (contemporary sources, using

8084-561: 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

8178-756: 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

8272-401: 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

8366-408: 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

8460-403: The used market, Astro-Physics' telescopes and mounts command high prices, telescopes often selling for multiples of their original cost. The company also offers a wide array of telescope, mount and imaging accessories available from stock. Optical telescope There are three primary types of optical telescope: An optical telescope's ability to resolve small details is directly related to

8554-428: The viewer with a brighter , clearer , and magnified virtual image 6 . The objective in a refracting telescope refracts or bends light . This refraction causes parallel light rays to converge at a focal point ; while those not parallel converge upon a focal plane . The telescope converts a bundle of parallel rays to make an angle α, with the optical axis to a second parallel bundle with angle β. The ratio β/α

8648-415: 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

8742-428: The years, distinctive for their apertures and optical designs as well as for their brief periods of production and irregular availability before being discontinued. Past product lines include refractors up to 206mm in aperture, a catadioptric design of 254mm, and an astrograph of 305mm. As of the end of 2021, the company's line of telescopes (optical tubes only, consisting of objective lens, tube, and focuser) included only

8836-438: Was discovered on 9 September 1892, by Edward Emerson Barnard using the 36 inches (91 cm) refractor telescope at Lick Observatory . It was discovered by direct visual observation with the doublet-lens refractor. In 1904, one of the discoveries made using Great Refractor of Potsdam (a double telescope with two doublets) was of the interstellar medium . The astronomer Professor Hartmann determined from observations of

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