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135-411: A refracting telescope (also called 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

270-411: 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

405-515: 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

540-481: 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 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

675-416: 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

810-433: A paraboloid (bowl-shaped) crater in which the centre has been pushed down, a significant volume of material has been ejected, and a topographically elevated crater rim has been pushed up. When this cavity has reached its maximum size, it is called the transient cavity. The depth of the transient cavity is typically a quarter to a third of its diameter. Ejecta thrown out of the crater do not include material excavated from

945-618: 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

1080-424: 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

1215-468: A certain altitude (retardation point), and start to accelerate again due to Earth's gravity until the body reaches its terminal velocity of 0.09 to 0.16 km/s. The larger the meteoroid (i.e. asteroids and comets) the more of its initial cosmic velocity it preserves. While an object of 9,000 kg maintains about 6% of its original velocity, one of 900,000 kg already preserves about 70%. Extremely large bodies (about 100,000 tonnes) are not slowed by

1350-424: 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,

1485-428: 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

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1620-404: 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

1755-400: 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

1890-455: A hole in the surface without filling in nearby craters. This may explain the 'sponge-like' appearance of that moon. It is convenient to divide the impact process conceptually into three distinct stages: (1) initial contact and compression, (2) excavation, (3) modification and collapse. In practice, there is overlap between the three processes with, for example, the excavation of the crater continuing in some regions while modification and collapse

2025-452: A large impact. The subsequent excavation of the crater occurs more slowly, and during this stage the flow of material is largely subsonic. During excavation, the crater grows as the accelerated target material moves away from the point of impact. The target's motion is initially downwards and outwards, but it becomes outwards and upwards. The flow initially produces an approximately hemispherical cavity that continues to grow, eventually producing

2160-406: 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

2295-404: 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

2430-402: 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

2565-468: 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

2700-399: 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

2835-536: 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 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

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2970-493: 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 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

3105-527: 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 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

3240-562: 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

3375-558: A regular sequence with increasing size: small complex craters with a central topographic peak are called central peak craters, for example Tycho ; intermediate-sized craters, in which the central peak is replaced by a ring of peaks, are called peak-ring craters , for example Schrödinger ; and the largest craters contain multiple concentric topographic rings, and are called multi-ringed basins , for example Orientale . On icy (as opposed to rocky) bodies, other morphological forms appear that may have central pits rather than central peaks, and at

3510-409: A result, the impactor is compressed, its density rises, and the pressure within it increases dramatically. Peak pressures in large impacts exceed 1 T Pa to reach values more usually found deep in the interiors of planets, or generated artificially in nuclear explosions . In physical terms, a shock wave originates from the point of contact. As this shock wave expands, it decelerates and compresses

3645-441: A significant crater volume may also be formed by the permanent compaction of the pore space . Such compaction craters may be important on many asteroids, comets and small moons. In large impacts, as well as material displaced and ejected to form the crater, significant volumes of target material may be melted and vaporized together with the original impactor. Some of this impact melt rock may be ejected, but most of it remains within

3780-406: A small angle, and high-temperature highly shocked material is expelled from the convergence zone with velocities that may be several times larger than the impact velocity. In most circumstances, the transient cavity is not stable and collapses under gravity. In small craters, less than about 4 km diameter on Earth, there is some limited collapse of the crater rim coupled with debris sliding down

3915-547: 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

4050-402: 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

4185-539: Is a depression in the surface of a solid astronomical body formed by the hypervelocity impact of a smaller object. In contrast to volcanic craters , which result from explosion or internal collapse, impact craters typically have raised rims and floors that are lower in elevation than the surrounding terrain. Impact craters are typically circular, though they can be elliptical in shape or even irregular due to events such as landslides. Impact craters range in size from microscopic craters seen on lunar rocks returned by

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4320-402: Is already underway in others. In the absence of atmosphere , the impact process begins when the impactor first touches the target surface. This contact accelerates the target and decelerates the impactor. Because the impactor is moving so rapidly, the rear of the object moves a significant distance during the short-but-finite time taken for the deceleration to propagate across the impactor. As

4455-402: 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 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

4590-619: 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

4725-512: Is ejected from close to the center of impact, and the slowest material is ejected close to the rim at low velocities to form an overturned coherent flap of ejecta immediately outside the rim. As ejecta escapes from the growing crater, it forms an expanding curtain in the shape of an inverted cone. The trajectory of individual particles within the curtain is thought to be largely ballistic. Small volumes of un-melted and relatively un-shocked material may be spalled at very high relative velocities from

4860-459: Is estimated that the value of materials mined from impact structures is five billion dollars/year just for North America. The eventual usefulness of impact craters depends on several factors, especially the nature of the materials that were impacted and when the materials were affected. In some cases, the deposits were already in place and the impact brought them to the surface. These are called "progenetic economic deposits." Others were created during

4995-425: 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

5130-479: 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\ } ),

5265-463: 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

5400-477: 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

5535-430: 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

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5670-427: 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

5805-431: 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

5940-663: 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

6075-404: Is sufficient to melt the impactor, and in larger impacts to vaporize most of it and to melt large volumes of the target. As well as being heated, the target near the impact is accelerated by the shock wave, and it continues moving away from the impact behind the decaying shock wave. Contact, compression, decompression, and the passage of the shock wave all occur within a few tenths of a second for

6210-681: 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

6345-400: 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

6480-437: Is the largest goldfield in the world, which has supplied about 40% of all the gold ever mined in an impact structure (though the gold did not come from the bolide). The asteroid that struck the region was 9.7 km (6 mi) wide. The Sudbury Basin was caused by an impacting body over 9.7 km (6 mi) in diameter. This basin is famous for its deposits of nickel , copper , and platinum group elements . An impact

6615-412: 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

6750-764: The Apollo Program to simple bowl-shaped depressions and vast, complex, multi-ringed impact basins . Meteor Crater is a well-known example of a small impact crater on Earth. Impact craters are the dominant geographic features on many solid Solar System objects including the Moon , Mercury , Callisto , Ganymede , and most small moons and asteroids . On other planets and moons that experience more active surface geological processes, such as Earth , Venus , Europa , Io , Titan , and Triton , visible impact craters are less common because they become eroded , buried, or transformed by tectonic and volcanic processes over time. Where such processes have destroyed most of

6885-547: 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

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7020-535: 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

7155-497: The Nevada Test Site , notably Jangle U in 1951 and Teapot Ess in 1955. In 1960, Edward C. T. Chao and Shoemaker identified coesite (a form of silicon dioxide ) at Meteor Crater, proving the crater was formed from an impact generating extremely high temperatures and pressures. They followed this discovery with the identification of coesite within suevite at Nördlinger Ries , proving its impact origin. Armed with

7290-574: 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

7425-605: 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

7560-444: 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

7695-431: 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

7830-429: The speed of sound in those objects. Such hyper-velocity impacts produce physical effects such as melting and vaporization that do not occur in familiar sub-sonic collisions. On Earth, ignoring the slowing effects of travel through the atmosphere, the lowest impact velocity with an object from space is equal to the gravitational escape velocity of about 11 km/s. The fastest impacts occur at about 72 km/s in

7965-399: The stable interior regions of continents . Few undersea craters have been discovered because of the difficulty of surveying the sea floor, the rapid rate of change of the ocean bottom, and the subduction of the ocean floor into Earth's interior by processes of plate tectonics . Daniel M. Barringer, a mining engineer, was convinced already in 1903 that the crater he owned, Meteor Crater ,

8100-441: The "worst case" scenario in which an object in a retrograde near-parabolic orbit hits Earth. The median impact velocity on Earth is about 20 km/s. However, the slowing effects of travel through the atmosphere rapidly decelerate any potential impactor, especially in the lowest 12 kilometres where 90% of the Earth's atmospheric mass lies. Meteorites of up to 7,000 kg lose all their cosmic velocity due to atmospheric drag at

8235-411: The 19th century, for most research purposes, 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

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8370-506: 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

8505-455: 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 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

8640-516: 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

8775-478: The actual impact. The great energy involved caused melting. Useful minerals formed as a result of this energy are classified as "syngenetic deposits." The third type, called "epigenetic deposits," is caused by the creation of a basin from the impact. Many of the minerals that our modern lives depend on are associated with impacts in the past. The Vredeford Dome in the center of the Witwatersrand Basin

8910-469: The association of volcanic flows and other volcanic materials. Impact craters produce melted rocks as well, but usually in smaller volumes with different characteristics. The distinctive mark of an impact crater is the presence of rock that has undergone shock-metamorphic effects, such as shatter cones , melted rocks, and crystal deformations. The problem is that these materials tend to be deeply buried, at least for simple craters. They tend to be revealed in

9045-432: The atmosphere at all, and impact with their initial cosmic velocity if no prior disintegration occurs. Impacts at these high speeds produce shock waves in solid materials, and both impactor and the material impacted are rapidly compressed to high density. Following initial compression, the high-density, over-compressed region rapidly depressurizes, exploding violently, to set in train the sequence of events that produces

9180-599: The binary star Mintaka in Orion, that there was the element calcium in the intervening space. Planet Pluto was discovered by looking at photographs (i.e. 'plates' in astronomy vernacular) in a blink comparator taken with a refracting telescope, an astrograph with a 3 element 13-inch lens. Examples of some of the largest achromatic refracting telescopes, over 60 cm (24 in) diameter. Optical telescope There are three primary types of optical telescope: An optical telescope's ability to resolve small details

9315-426: 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

9450-461: 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

9585-451: The collapse and modification of the transient cavity is much more extensive, and the resulting structure is called a complex crater . The collapse of the transient cavity is driven by gravity, and involves both the uplift of the central region and the inward collapse of the rim. The central uplift is not the result of elastic rebound, which is a process in which a material with elastic strength attempts to return to its original geometry; rather

9720-468: The collapse is a process in which a material with little or no strength attempts to return to a state of gravitational equilibrium . Complex craters have uplifted centers, and they have typically broad flat shallow crater floors, and terraced walls . At the largest sizes, one or more exterior or interior rings may appear, and the structure may be labeled an impact basin rather than an impact crater. Complex-crater morphology on rocky planets appears to follow

9855-402: 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

9990-451: The crater walls and drainage of impact melts into the deeper cavity. The resultant structure is called a simple crater, and it remains bowl-shaped and superficially similar to the transient crater. In simple craters, the original excavation cavity is overlain by a lens of collapse breccia , ejecta and melt rock, and a portion of the central crater floor may sometimes be flat. Above a certain threshold size, which varies with planetary gravity,

10125-405: The craters on the Moon as logical impact sites that were formed not gradually, in eons , but explosively, in seconds." For his PhD degree at Princeton University (1960), under the guidance of Harry Hammond Hess , Shoemaker studied the impact dynamics of Meteor Crater. Shoemaker noted that Meteor Crater had the same form and structure as two explosion craters created from atomic bomb tests at

10260-496: 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

10395-407: The expanding vapor cloud may rise to many times the scale height of the atmosphere, effectively expanding into free space. Most material ejected from the crater is deposited within a few crater radii, but a small fraction may travel large distances at high velocity, and in large impacts it may exceed escape velocity and leave the impacted planet or moon entirely. The majority of the fastest material

10530-446: 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

10665-439: 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

10800-636: 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

10935-440: 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,

11070-459: 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

11205-435: 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 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

11340-405: 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

11475-401: The full depth of the transient cavity; typically the depth of maximum excavation is only about a third of the total depth. As a result, about one third of the volume of the transient crater is formed by the ejection of material, and the remaining two thirds is formed by the displacement of material downwards, outwards and upwards, to form the elevated rim. For impacts into highly porous materials,

11610-448: The geologists John D. Boon and Claude C. Albritton Jr. revisited Bucher's studies and concluded that the craters that he studied were probably formed by impacts. Grove Karl Gilbert suggested in 1893 that the Moon's craters were formed by large asteroid impacts. Ralph Baldwin in 1949 wrote that the Moon's craters were mostly of impact origin. Around 1960, Gene Shoemaker revived the idea. According to David H. Levy , Shoemaker "saw

11745-574: 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

11880-576: 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

12015-500: 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

12150-413: 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 )

12285-421: 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 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

12420-599: The impact crater. Impact-crater formation is therefore more closely analogous to cratering by high explosives than by mechanical displacement. Indeed, the energy density of some material involved in the formation of impact craters is many times higher than that generated by high explosives. Since craters are caused by explosions , they are nearly always circular – only very low-angle impacts cause significantly elliptical craters. This describes impacts on solid surfaces. Impacts on porous surfaces, such as that of Hyperion , may produce internal compression without ejecta, punching

12555-521: The impactor, and it accelerates and compresses the target. Stress levels within the shock wave far exceed the strength of solid materials; consequently, both the impactor and the target close to the impact site are irreversibly damaged. Many crystalline minerals can be transformed into higher-density phases by shock waves; for example, the common mineral quartz can be transformed into the higher-pressure forms coesite and stishovite . Many other shock-related changes take place within both impactor and target as

12690-764: The knowledge of shock-metamorphic features, Carlyle S. Beals and colleagues at the Dominion Astrophysical Observatory in Victoria, British Columbia , Canada and Wolf von Engelhardt of the University of Tübingen in Germany began a methodical search for impact craters. By 1970, they had tentatively identified more than 50. Although their work was controversial, the American Apollo Moon landings, which were in progress at

12825-409: 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

12960-649: The largest sizes may contain many concentric rings. Valhalla on Callisto is an example of this type. Long after an impact event, a crater may be further modified by erosion, mass wasting processes, viscous relaxation, or erased entirely. These effects are most prominent on geologically and meteorologically active bodies such as Earth, Titan, Triton, and Io. However, heavily modified craters may be found on more primordial bodies such as Callisto, where many ancient craters flatten into bright ghost craters, or palimpsests . Non-explosive volcanic craters can usually be distinguished from impact craters by their irregular shape and

13095-528: 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

13230-405: 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

13365-408: 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

13500-567: 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

13635-445: 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

13770-448: The object traveling at an angle α1 to the optical axis travel at a larger angle ( α2 > α1 ) after they passed through 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,

13905-431: 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

14040-468: 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

14175-403: 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

14310-411: The original crater topography , the terms impact structure or astrobleme are more commonly used. In early literature, before the significance of impact cratering was widely recognised, the terms cryptoexplosion or cryptovolcanic structure were often used to describe what are now recognised as impact-related features on Earth. The cratering records of very old surfaces, such as Mercury, the Moon, and

14445-711: The outer Solar System could be different from the inner Solar System. Although Earth's active surface processes quickly destroy the impact record, about 190 terrestrial impact craters have been identified. These range in diameter from a few tens of meters up to about 300 km (190 mi), and they range in age from recent times (e.g. the Sikhote-Alin craters in Russia whose creation was witnessed in 1947) to more than two billion years, though most are less than 500 million years old because geological processes tend to obliterate older craters. They are also selectively found in

14580-461: 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

14715-525: 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

14850-479: The planet than have been discovered so far. The cratering rate in the inner solar system fluctuates as a consequence of collisions in the asteroid belt that create a family of fragments that are often sent cascading into the inner solar system. Formed in a collision 80 million years ago, the Baptistina family of asteroids is thought to have caused a large spike in the impact rate. The rate of impact cratering in

14985-424: 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

15120-424: 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

15255-543: 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,

15390-552: 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

15525-408: 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

15660-452: 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,

15795-414: 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. Impact crater An impact crater

15930-610: 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

16065-411: 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 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

16200-419: The shock wave passes through, and some of these changes can be used as diagnostic tools to determine whether particular geological features were produced by impact cratering. As the shock wave decays, the shocked region decompresses towards more usual pressures and densities. The damage produced by the shock wave raises the temperature of the material. In all but the smallest impacts this increase in temperature

16335-464: The southern highlands of Mars, record a period of intense early bombardment in the inner Solar System around 3.9 billion years ago. The rate of crater production on Earth has since been considerably lower, but it is appreciable nonetheless. Earth experiences, on average, from one to three impacts large enough to produce a 20-kilometre-diameter (12 mi) crater every million years. This indicates that there should be far more relatively young craters on

16470-561: 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

16605-425: The surface of the target and from the rear of the impactor. Spalling provides a potential mechanism whereby material may be ejected into inter-planetary space largely undamaged, and whereby small volumes of the impactor may be preserved undamaged even in large impacts. Small volumes of high-speed material may also be generated early in the impact by jetting. This occurs when two surfaces converge rapidly and obliquely at

16740-756: 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

16875-404: The telescope view comes to focus. Originally, telescopes had an objective of one element, but 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

17010-401: 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

17145-473: The time, provided supportive evidence by recognizing the rate of impact cratering on the Moon . Because the processes of erosion on the Moon are minimal, craters persist. Since the Earth could be expected to have roughly the same cratering rate as the Moon, it became clear that the Earth had suffered far more impacts than could be seen by counting evident craters. Impact cratering involves high velocity collisions between solid objects, typically much greater than

17280-411: The transient crater, initially forming a layer of impact melt coating the interior of the transient cavity. In contrast, the hot dense vaporized material expands rapidly out of the growing cavity, carrying some solid and molten material within it as it does so. As this hot vapor cloud expands, it rises and cools much like the archetypal mushroom cloud generated by large nuclear explosions. In large impacts,

17415-403: The uplifted center of a complex crater, however. Impacts produce distinctive shock-metamorphic effects that allow impact sites to be distinctively identified. Such shock-metamorphic effects can include: On Earth, impact craters have resulted in useful minerals. Some of the ores produced from impact related effects on Earth include ores of iron , uranium , gold , copper , and nickel . It

17550-408: 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

17685-415: 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

17820-434: 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

17955-423: 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

18090-684: Was involved in making the Carswell structure in Saskatchewan , Canada; it contains uranium deposits. Hydrocarbons are common around impact structures. Fifty percent of impact structures in North America in hydrocarbon-bearing sedimentary basins contain oil/gas fields. On Earth, the recognition of impact craters is a branch of geology, and is related to planetary geology in the study of other worlds. Out of many proposed craters, relatively few are confirmed. The following twenty are

18225-519: Was of cosmic origin. Most geologists at the time assumed it formed as the result of a volcanic steam eruption. In the 1920s, the American geologist Walter H. Bucher studied a number of sites now recognized as impact craters in the United States. He concluded they had been created by some great explosive event, but believed that this force was probably volcanic in origin. However, in 1936,

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