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89-415: A great refractor was often the centerpiece of a new 19th century observatory, but was typically used with an entourage of other astronomical instruments such as a Meridian Circle , a Heliometer , an Astrograph , and a smaller refractor such as a Comet Seeker or Equatorial. Great refractors were often used for observing double stars and equipped with a Filar micrometer . Pioneering work on astrophotography

178-400: A lever supported by the pier, counterbalanced so as to leave only a small fraction of the weight on the precision V-shaped bearings. In some cases, the counterweight pushed up on the roller bearings from below. The bearings were set nearly in a true east–west line, but fine adjustment was possible by horizontal and vertical screws. A spirit level was used to monitor for any inclination of

267-420: A "wedge". Many mid-size professional telescopes also have equatorial forks , these are usually in range of 0.5-2.0 meter diameter. The English mount or Yoke mount has a frame or " yoke " with right ascension axis bearings at the top and the bottom ends, and a telescope attached inside the midpoint of the yoke allowing it to swing on the declination axis. The telescope is usually fitted entirely inside

356-478: A 9  Paris inch (about 9.6 in (24 cm)) aperture achromatic lens and a 4 m (13.4 ft) focal length. It was also equipped with the first modern equatorial mount type called a "German equatorial mount" developed by Fraunhofer, a mount that became standard for most large refractors from then on. A Fraunhofer "9-inch" (24 cm) at Berlin Observatory was used by Johann Gottfried Galle in

445-456: A constant speed. Such an arrangement is called a sidereal drive or clock drive . Equatorial mounts achieve this by aligning their rotational axis with the Earth, a process known as polar alignment . In astronomical telescope mounts , the equatorial axis (the right ascension ) is paired with a second perpendicular axis of motion (known as the declination ). The equatorial axis of

534-476: A control "paddle" or supplied through an adjacent laptop computer which is also used to capture images from an electronic camera. The electronics of modern telescope systems often include a port for autoguiding. A special instrument tracks a star and makes adjustment in the telescope's position while photographing the sky. To do so the autoguider must be able to issue commands through the telescope's control system. These commands can compensate for very slight errors in

623-402: A device which allowed matching a vertical crosshair's motion to the star's motion. Set precisely on the moving star, the crosshair would trigger the electrical timing of the meridian crossing, removing the observer's personal equation from the measurement. The field of the wires could be illuminated; the lamps were placed at some distance from the piers in order not to heat the instrument, and

712-418: A fine screw . By this slow motion, the telescope was adjusted until the star moved along the horizontal wire (or if there were two, in the middle between them), from the east side of the field of view to the west. Following this, the circles were read by the microscopes for a measurement of the apparent altitude of the star. The difference between this measurement and the nadir point was the nadir distance of

801-408: A fixed bend in the tube, was detected by arranging that eyepiece and objective lens could be interchanged, and the average of the two observations of the same star was free from this error. Parts of the apparatus, including the circles, pivots and bearings, were sometimes enclosed in glass cases to protect them from dust. These cases had openings for access. The reading microscopes then extended into

890-480: A fixed object in the sky. Also, for astrophotography , the image does not rotate in the focal plane , as occurs with altazimuth mounts when they are guided to track the target's motion, unless a rotating erector prism or other field-derotator is installed. Equatorial telescope mounts come in many designs. In the last twenty years motorized tracking has increasingly been supplemented with computerized object location. There are two main types. Digital setting circles take

979-473: A large meridian quadrant. Meridian circles have been used since the 18th century to accurately measure positions of stars in order to catalog them. This is done by measuring the instant when the star passes through the local meridian. Its altitude above the horizon is noted as well. Knowing one's geographic latitude and longitude these measurements can be used to derive the star's right ascension and declination . Once good star catalogs were available

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1068-428: A meridian circle, fitted with leveling screws. Extremely sensitive levels are attached to the telescope mount to make angle measurements and the telescope has an eyepiece fitted with a micrometer . The idea of having an instrument ( quadrant ) fixed in the plane of the meridian occurred even to the ancient astronomers and is mentioned by Ptolemy , but it was not carried into practice until Tycho Brahe constructed

1157-439: A perfectly horizontal mirror, reflecting an image of the crosshairs back up the telescope tube. The crosshairs could then be adjusted until coincident with their reflection, and the line of sight was then perpendicular to the axis. The line of sight of the telescope needed to be exactly within the plane of the meridian. This was done approximately by building the piers and the bearings of the axis on an east–west line. The telescope

1246-421: A right ascension axis at its base. The telescope is attached to two pivot points at the other end of the fork so it can swing in declination. Most modern mass-produced catadioptric reflecting telescopes (200 mm or larger diameter) tend to be of this type. The mount resembles an Altazimuth mount , but with the azimuth axis tilted and lined up to match earth rotation axis with a piece of hardware usually called

1335-456: A shadow to set the Sun's position. It was mounted vertically and aligned with the meridian. The instrument was used to measure the altitude of the Sun at noon in order to determine the path of the ecliptic . A meridian circle enabled the observer to simultaneously determine right ascension and declination , but it does not appear to have been much used for right ascension during the 17th century,

1424-418: A small computer with an object database that is attached to encoders. The computer monitors the telescope's position in the sky. The operator must push the telescope. Go-to systems use (in most cases) a worm and ring gear system driven by servo or stepper motors, and the operator need not touch the instrument at all to change its position in the sky. The computers in these systems are typically either hand-held in

1513-440: A star of known declination passing from one wire to the other, the pole star being best on account of its slow motion. \ Timings were originally made by an "eye and ear" method, estimating the interval between two beats of a clock. Later, timings were registered by pressing a key, the electrical signal making a mark on a strip recorder . Later still, the eye end of the telescope was usually fitted with an impersonal micrometer ,

1602-467: A transit telescope could be used anywhere in the world to accurately measure local longitude and time by observing local meridian transit times of catalogue stars. Prior to the invention of the atomic clock this was the most reliable source of accurate time. In the Almagest , Ptolemy describes a meridian circle which consisted of a fixed graduated outer ring and a movable inner ring with tabs that used

1691-420: A vast improvement over speculum metal and made reflectors a practical instrument. The era of large reflectors had begun, with telescopes such as the 36-inch (91 cm) Crossley Reflector (1895), 60-inch (1.5 m) Mount Wilson Observatory Hale telescope of 1908, and the 100-inch (2.5 m) Mount Wilson Hooker telescope in 1917. Two other big telescopes that surpassed the largest refractors in aperture were

1780-470: Is a mount for instruments that compensates for Earth's rotation by having one rotational axis , called polar axis , parallel to the Earth's axis of rotation. This type of mount is used for astronomical telescopes and cameras . The advantage of an equatorial mount lies in its ability to allow the instrument attached to it to stay fixed on any celestial object with diurnal motion by driving one axis at

1869-413: Is likewise mounted on a horizontal axis, but the axis need not be fixed in the east–west direction. For instance, a surveyor's theodolite can function as a transit instrument if its telescope is capable of a full revolution about the horizontal axis. Meridian circles are often called by these names, although they are less specific. For many years, transit timings were the most accurate method of measuring

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1958-420: The meridian , the great circle through the north point of the horizon, the north celestial pole , the zenith , the south point of the horizon, the south celestial pole, and the nadir . Meridian telescopes rely on the rotation of the sky to bring objects into their field of view and are mounted on a fixed, horizontal, east–west axis. The similar transit instrument , transit circle , or transit telescope

2047-739: The Dominion Astrophysical Observatory and the David Dunlap Observatory in Canada, which came online in the early 1900s. The largest refractor in Europe, with the exhibition scope dismantled, would be the double telescope, with 33-inch (84 cm) primary, La Grande Lunette at Meudon (later part of Paris Observatory). This was manufactured by the Henry Brothers and Gautier, who had also made

2136-544: The Leviathan of Parsonstown , and work with the Crossley Reflector and increasingly larger silver-and-glass mirrors marked large refractors' obsolescence. The era slowly came to end as large reflecting telescopes superseded the great refractors. In 1856–57, Carl August von Steinheil and Léon Foucault introduced a process of depositing a layer of silver on glass telescope mirrors. Silvered glass mirrors were

2225-551: The Royal Greenwich Observatory (1851) and that at the Royal Observatory, Cape of Good Hope (1855) were made by Ransomes and May of Ipswich. The Greenwich instrument had optical and instrumental work by Troughton and Simms to the design of George Biddell Airy . A modern-day example of this type of telescope is the 8 inch (~0.2m) Flagstaff Astrometric Scanning Transit Telescope (FASTT) at

2314-704: The USNO Flagstaff Station Observatory . Modern meridian circles are usually automated. The observer is replaced with a CCD camera. As the sky drifts across the field of view, the image built up in the CCD is clocked across (and out of) the chip at the same rate. This allows some improvements: The first automated instrument was the Carlsberg Automatic Meridian Circle , which came online in 1984. Attribution: Equatorial mount An equatorial mount

2403-590: The discovery of Neptune . There is tendency to round apertures to the nearest large figure, which can create a sort of drift when conversions are made; the Fraunhofer "9-inch" were nine paris inches which is about 9.6 in or about 24 cm, not exactly nine English inches, and closer to ten inches. (Paris inches are also called pouces ) In 1851, at the Great Exhibition in Hyde Park, one of

2492-443: The mural quadrant continued until the end of the century to be employed for determining declinations. The advantages of using a whole circle, it being less liable to change its figure and not requiring reversal in order to observe stars north of the zenith, were then again recognized by Jesse Ramsden , who also improved the method of reading off angles by means of a micrometer microscope as described below. The making of circles

2581-427: The 18th century, John Dollond (1706–1761) invented and created an achromatic object glass and lens which permitted achromatic telescopes up to 3–5 in (8–13 cm) aperture. The Swiss Pierre-Louis Guinand (1748–1824) discovered and developed a way to make much larger crown and flint glass blanks. He worked with instrument maker Joseph von Fraunhofer (1787–1826) to use this technology for instruments in

2670-677: The Newall telescope to the National Observatory of Athens, who accepted the gift and it has been there ever since. In Greece, it was installed in new custom dome building near the Pendeli mountain. Refracting telescopes would quadruple in size by the end of the century, culminating with the largest practical refractor ever built, the Yerkes Observatory 40-inch (1 meter) aperture of 1895. This great refractor pushed

2759-462: The Yerkes 40-inch objective, said a 45-inch (114 cm) would be possible before he died. In addition to the lens, the rest of the telescope needed to be a practical and highly precise instrument, despite the size. For example, the Yerkes tube alone weighed 75  tons , and had to track stars just as accurately as a smaller instrument. The choice between large refractors or reflectors was driven by

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2848-400: The art of meridian instruments of the late 19th and early 20th century is described here, giving some idea of the precise methods of construction, operation and adjustment employed. The earliest transit telescope was not placed in the middle of the axis, but nearer to one end, to prevent the axis from bending under the weight of the telescope. Later, it was usually placed in the centre of

2937-415: The axis to the horizon. Eccentricity (an off-center condition) or other irregularities of the pivots of the telescope's axis was accounted for, in some cases, by providing another telescope through the axis itself. By observing the motion of an artificial star, located east or west of the center of the main instrument, and seen through this axis telescope and a small collimating telescope, as the main telescope

3026-400: The axis, which consisted of one piece of brass or gun metal with turned cylindrical steel pivots at each end. Several instruments were made entirely of steel , which was much more rigid than brass. The pivots rested on V-shaped bearings , either set into massive stone or brick piers which supported the instrument, or attached to metal frameworks on the tops of the piers. The temperature of

3115-489: The big Expo telescope of 1900. The advent of chemical-based astrophotography in the late 19th century brought difficulties in adapting great refractors to this application. Achromatic lenses were color corrected for what the human eye was sensitive to, yellow light, while photography plates at that time were more sensitive to light at the blue end of the spectrum, requiring a lens with a different color correction and focal plane. Solutions to this problem included: An example of

3204-464: The centerpiece of the observatory. By 1834 it was mounted on an equatorial mounting supplied by Thomas Grubb of Dublin. This was the largest refractor in the world in the early 1830s, and Cooper used the telescope to sketch Halley's comet in 1835 and to view the solar eclipse of 15 May 1836. In 1833 the Duke of Northumberland donated a Cauchoix of Paris objective lens to establish a large telescope for

3293-433: The circle reading after observing a star and the reading corresponding to the zenith was the zenith distance of the star, and this plus the colatitude was the north polar distance. To determine the zenith point of the circle, the telescope was directed vertically downwards at a basin of mercury , the surface of which formed an absolutely horizontal mirror. The observer saw the horizontal wire and its reflected image, and moving

3382-439: The circle. The periodic errors of the screw were accounted for. On some instruments, one of the circles was graduated and read more coarsely than the other, and was used only in finding the target stars. The telescope consisted of two tubes screwed to the central cube of the axis. The tubes were usually conical and as stiff as possible to help prevent flexure . The connection to the axis was also as firm as possible, as flexure of

3471-461: The clocks, recorders, and other equipment for making observations. At the focal plane , the eye end of the telescope had a number of vertical and one or two horizontal wires ( crosshairs ). In observing stars, the telescope was first directed downward at a basin of mercury forming a perfectly horizontal mirror and reflecting an image of the crosshairs back up the telescope tube. The crosshairs were adjusted until coincident with their reflection, and

3560-502: The correct shape. This sometimes proved so difficult, that a telescope mirror was abandoned. In the mid-19th century a technique for coating glass with metal offered a major advantage and this technology became more common in the following decades. In the 21st century metal-coated glass mirrors remain popular, including on space telescopes like the Hubble Space Telescope . The Great Paris Exhibition Telescope of 1900

3649-402: The crosshairs in their foci coincided. The collimators were often permanently mounted in these positions, with their objectives and eyepieces fixed to separate piers. The meridian telescope was pointed to one collimator and then the other, moving through exactly 180°, and by reading the circle the amount of flexure (the amount the readings differed from 180°) was found. Absolute flexure, that is,

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3738-407: The deviation of the star's path from a great circle, and for the inclination of the horizontal wire to the horizon. The amount of this inclination was found by taking repeated observations of the zenith distance of a star during the one transit, the pole star being the most suitable because of its slow motion. Attempts were made to record the transits of a star photographically. A photographic plate

3827-462: The discovery of Neptune, the discovery of the Moons of Mars, and the compilation of various star catalogs. A derivative instrument of refractors, the heliometer was used to measure for the first time the distance to another star by geometric parallax in the mid-1800s. As telescopes became larger and longer, the relatively modest increases in aperture belied their enormous size, with moving weights in

3916-536: The discovery of the moons of Mars in 1877 and the Jovian moon Amalthea in 1892. That was the first new moon of Jupiter to be found since Galileo. In addition, they were used for groundbreaking work on astrophotography and spectroscopy. The discovery of interstellar calcium in 1904, by the Potsdam great refractor, rounded out their discoveries. However, through this time they were overshadowed by large reflectors such as

4005-576: The early 19th century. The era of great refractors started with the first modern, achromatic, refracting telescopes built by Joseph von Fraunhofer in the early 1820s. The first of these was the Dorpat Great Refractor, also known as the Fraunhofer 9-inch, at what was then Dorpat Observatory in the Governorate of Estonia (Estland) (which later became Tartu Observatory in southern Estonia ). This telescope made by Fraunhofer had

4094-435: The enormous Yerkes and Treptow refractors actually debuted at exhibitions, which were major events of the period. Meridian Circle The meridian circle is an instrument for timing of the passage of stars across the local meridian , an event known as a culmination , while at the same time measuring their angular distance from the nadir . These are special purpose telescopes mounted so as to allow pointing only in

4183-764: The first case was the Meudon Great Refractor in Paris, which was finished in 1891. This had a visual objective lens of 32.7 inches on one tube, and alongside it another tube with a lens of 24.4 inches intended for photographic work. An example of converting to photographic work with a third corrector lens is the Lick telescope. A 33-inch corrector lens was used to convert this telescope for photography. Great refractors were admired for their quality, durability, and usefulness which correlated to features such as lens quality, mount quality, aperture, and also length. Length

4272-561: The fork, although there are exceptions such as the Mount Wilson 2.5 m reflector , and there are no counterweights as with the German mount . The original English fork design is disadvantaged in that it does not allow the telescope to point too near the north or south celestial pole. The horseshoe mount overcomes the design disadvantage of English or Yoke mounts by replacing the polar bearing with an open "horseshoe" structure to allow

4361-463: The glass cases, while their eyepiece ends and micrometers were protected from dust by removable silk covers. Certain instrumental errors could be averaged out by reversing the telescope on its mounting. A carriage was provided, which ran on rails between the piers, and on which the axis, circles and telescope could be raised by a screw-jack, wheeled out from between the piers, turned 180°, wheeled back, and lowered again. The observing building housing

4450-422: The instrument and local atmosphere were monitored by thermometers. The piers were usually separate from the foundation of the building, to prevent transmission of vibration from the building to the telescope. To relieve the pivots from the weight of the instrument, which would have distorted their shape and caused rapid wear, each end of the axis was supported by a hook or yoke with friction rollers , suspended from

4539-428: The light passed through holes in the piers and through the hollow axis to the center, whence it was directed to the eye-end by a system of prisms . To determine absolute declinations or polar distances, it was necessary to determine the observatory's colatitude , or distance of the celestial pole from the zenith , by observing the upper and lower culmination of a number of circumpolar stars . The difference between

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4628-519: The limits of technology of the day; the fabrication of the two element achromatic lens (the largest lens ever made at the time), required 18 attempts and cooperation between Alvan Clark & Sons and Charles Feil of Paris. To achieve its optical aperture it was actually slightly bigger physically, at 41 3/8 in. Refractors had reached their technological limit; the problems of lens sagging from gravity meant refractors would not exceed around 1 meter, although Alvan G. Clark , who had made

4717-448: The line of sight was then perfectly vertical; in this position the circles were read for the nadir point . The telescope was next brought up to the approximate declination of the target star by watching the finder circle. The instrument was provided with a clamping apparatus, by which the observer, after having set the approximate declination, could clamp the axis so the telescope could not be moved in declination, except very slowly by

4806-502: The main telescope needed to be exactly horizontal. A sensitive spirit level , designed to rest on the pivots of the axis, performed this function. By adjusting one of the V-shaped bearings, the bubble was centered. The line of sight of the telescope needed to be exactly perpendicular to the axis of rotation. This could be done by sighting a distant, stationary object, lifting and reversing the telescope on its bearings, and again sighting

4895-426: The meridian circle did not have a rotating dome, as is often seen at observatories. Since the telescope observed only in the meridian, a vertical slot in the north and south walls, and across the roof between these, was all that was necessary. The building was unheated and kept as much as possible at the temperature of the outside air, to avoid air currents which would disturb the telescopic view. The building also housed

4984-440: The method of equal altitudes by portable quadrants or measures of the angular distance between stars with an astronomical sextant being preferred. These methods were very inconvenient, and in 1690, Ole Rømer invented the transit instrument. The transit instrument consists of a horizontal axis in the direction east and west resting on firmly fixed supports, and having a telescope fixed at right angles to it, revolving freely in

5073-432: The middle of the 19th century to be the principal instrument in observatories, the first transit circle constructed there being that at Greenwich (mounted in 1850). However, on the continent, the transit circle superseded them from the years 1818–1819, when two circles by Johann Georg Repsold and Georg Friedrich von Reichenbach were mounted at Göttingen , and one by Reichenbach at Königsberg . The firm of Repsold and Sons

5162-412: The modern era aperture and location are important, the older style observatories were often located near towns because astronomy was only one function; major tasks were simply to record the weather, make accurate determinations of location, and to determine the local time. In modern times many of these functions are performed elsewhere and communicated locally. Some noted accomplishments of refractors were

5251-445: The mount is often equipped with a motorized " clock drive ", that rotates that axis one revolution every 23 hours and 56 minutes in exact sync with the apparent diurnal motion of the sky. They may also be equipped with setting circles to allow for the location of objects by their celestial coordinates . Equatorial mounts differ from mechanically simpler altazimuth mounts , which require variable speed motion around both axes to track

5340-509: The multiple tons in domes several stories tall; physically many of the biggest were larger than even some modern reflecting telescopes. In the early 19th century a young Edward Joshua Cooper built in Ireland one of the most richly furnished astronomical observatories of the period. Cooper had acquired the largest lens in the world, made by Cauchoix of Paris , with an objective of 13.3 inches (~34.8 cm) for 1200 pounds, and he placed it as

5429-410: The new Observatory of Northumberland. The telescope was used for over a century with some updates, but the original was an "achromatic doublet of 11.6 inches clear aperture and focal length 19ft 6in". Although there had been very large (and unwieldy) Non-achromatic aerial telescopes of the late 17th century, and Chester Moore Hall and others had experimented with small achromatic telescopes in

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5518-433: The noted exhibits was telescope with 5 m (16 feet) long tube, called the "Trophy telescope" and was featured in the exhibition. The telescope was placed by the astronomer James William Grant , and it had an 11-inch aperture (280mm) and a 16 feet (4.88m) focal length. At the 1861 International Exhibit, the size increased to showing a telescope with 21-inch objective lens. The Buckingham or Walworth Common telescope's objective

5607-420: The object. If the crosshairs did not intersect the object, the line of sight was halfway between the new position of the crosshairs and the distant object; the crosshairs were adjusted accordingly and the process repeated as necessary. Also, if the rotation axis was known to be perfectly horizontal, the telescope could be directed downward at a basin of mercury , and the crosshairs illuminated. The mercury acted as

5696-444: The other. An equatorial platform is a specially designed platform that allows any device sitting on it to track on an equatorial axis. It achieves this by having a surface that pivots about a "virtual polar axis". This gives equatorial tracking to anything sitting on the platform, from small cameras up to entire observatory buildings. These platforms are often used with altazimuth mounted amateur astronomical telescopes, such as

5785-426: The piers or a framework surrounding the axis, at 90° intervals around the circles. By averaging the four readings the eccentricity (from inaccurate centering of the circles) and the errors of graduation were greatly reduced. Each microscope was furnished with a micrometer screw, which moved crosshairs , with which the distance of the circle graduations from the centre of the field of view could be measured. The drum of

5874-445: The plane of the meridian. At the same time Rømer invented the altitude and azimuth instrument for measuring vertical and horizontal angles, and in 1704, he combined a vertical circle with his transit instrument, so as to determine both co-ordinates at the same time. This latter idea was, however, not adopted elsewhere, although the transit instrument soon came into universal use (the first one at Greenwich being mounted in 1721), and

5963-474: The positions of heavenly bodies, and meridian instruments were relied upon to perform this painstaking work. Before spectroscopy , photography , and the perfection of reflecting telescopes , the measuring of positions (and the deriving of orbits and astronomical constants ) was the major work of observatories . Fixing a telescope to move only in the meridian has advantages in the high-precision work for which these instruments are employed: The state of

6052-405: The professional level. At the amateur level, however, equatorial mounts remain popular, particularly for astrophotography. In the German equatorial mount , (sometimes called a " GEM " for short) the primary structure is a T -shape, where the lower bar is the right ascension axis (lower diagonal axis in image), and the upper bar is the declination axis (upper diagonal axis in image). The mount

6141-435: The screw was divided to measure single seconds of arc (0.1" being estimated), while the number of revolutions were counted by a comb like scale in the field of view. The microscopes were given such magnification and placed at such a distance from the circle that one revolution of the micrometer screw corresponded to 1 arcminute (1') on the circle. The error was determined occasionally by measuring standard intervals of 2' or 5' on

6230-493: The second-largest refractors, or otherwise notable. Approximate historical progression of some of the Great refactors of the late 19th century: As long as these were, they were actually much shorter than the longest singlet refractors in aerial telescopes . The Paris exhibition scope, besides from using a mirror to aim, was not really an observatories 'great' refractor in that sense, but its possible it might have been and both

6319-438: The star. A movable horizontal wire or declination-micrometer was also used. Another method of observing the apparent altitude of a star was to take half of the angular distance between the star observed directly and its reflection observed in a basin of mercury. The average of these two readings was the reading when the line of sight was horizontal, the horizontal point of the circle. The small difference in latitude between

6408-408: The technology of the time. For refractors, the difficulties of fabricating two disks of optical glass for a large achromatic lens were formidable. For reflectors in much of the 19th century, the preferred material of a primary mirror was speculum metal , a substance that reflected up to 66 percent of the light that hit it and tarnished in months. They had to be removed, polished, and re-figured to

6497-416: The telescope and the basin of mercury was accounted for. The vertical wires were used for observing transits of stars, each wire furnishing a separate result. The time of transit over the middle wire was estimated, during subsequent analysis of the data, for each wire by adding or subtracting the known interval between the middle wire and the wire in question. These known intervals were predetermined by timing

6586-417: The telescope to access Polaris and stars near it. The Hale Telescope is the most prominent example of a horseshoe mount in use. The Cross-axis or English cross axis mount is like a big "plus" sign ( + ). The right ascension axis is supported at both ends, and the declination axis is attached to it at approximately midpoint with the telescope on one end of the declination axis and a counter weight on

6675-402: The telescope to make these coincide, its optical axis was made perpendicular to the plane of the horizon, and the circle reading was 180° + zenith point. In observations of stars refraction was taken into account as well as the errors of graduation and flexure. If the bisection of the star on the horizontal wire was not made in the centre of the field, allowance was made for curvature, or

6764-595: The time, and the largest telescope in the United States. A 25-inch (63.5 cm) objective refractor was installed in the Newall telescope . This had an objective made by the makers Chance, with the overall telescope made by Thomas Cooke. The telescope was made for Robert Stirling Newall, and when completed in 1869 was the largest refracting telescope in the world. In the 1950s the University of Cambridge donated

6853-416: The tracking performance, such as periodic error caused by the worm drive that makes the telescope move. In new observatory designs, equatorial mounts have been out of favor for decades in large-scale professional applications. Massive new instruments are most stable when mounted in an alt-azimuth (up down, side-to-side) configuration. Computerized tracking and field-derotation are not difficult to implement at

6942-411: The tube would affect declinations deduced from observations. The flexure in the horizontal position of the tube was determined by two collimators —telescopes placed horizontally in the meridian, north and south of the transit circle, with their objective lenses towards it. These were pointed at one another (through holes in the tube of the telescope, or by removing the telescope from its mount) so that

7031-534: Was developed by Joseph von Fraunhofer for the Great Dorpat Refractor that was finished in 1824. The telescope is placed on one end of the declination axis (top left in image), and a suitable counterweight on other end of it (bottom right). The right ascension axis has bearings below the T-joint, that is, it is not supported above the declination axis. The Open Fork mount has a Fork attached to

7120-510: Was done with great refractors. 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 it opened in 1935; this is the most people to have viewed through any telescope. In modern times many large refractors have become important historical items, and are often used for public astronomy outreaches. However, many have also been shut down or moved due to their difficulty of use as telescopes. Whereas in

7209-493: Was fixed in a horizontal position to overcome gravitational distortion on its 1.25 m (49.2 in) lens and was aimed with a 2 m siderostat . This demonstration telescope was scrapped after the Exposition Universelle closed. The Treptow refractor was built for Great Industrial Exposition of Berlin of 1896. In the late 19th century, the big refractors reached some of their great successes including

7298-511: Was for a number of years eclipsed by that of Pistor and Martins in Berlin, who furnished various observatories with first-class instruments. Following the death of Martins, the Repsolds again took the lead and made many transit circles. The observatories of Harvard College , Cambridge University and Edinburgh University had large circles by Troughton and Simms . The Airy Transit Circles at

7387-401: Was important because unlike reflectors (which can be folded and shortened), the focal length of glass lens correlated to the physical length of the telescope and offered some optical and image quality advantages. The progression of largest refracting telescopes in the 19th century, including some telescopes at private observatories that were not really used very much or had problems. Some of

7476-454: Was manufactured by William Wray. On January 31, 1862, American telescope-maker and astronomer Alvan Graham Clark first observed the faint companion, which is now called Sirius B, or affectionately "the Pup". This happened during testing of an 18.5-inch (470 mm) aperture great refractor telescope for Dearborn Observatory , which was one of the largest refracting telescope lens in existence at

7565-518: Was placed in the focus of a transit instrument and a number of short exposures made, their length and the time being registered automatically by a clock. The exposing shutter was a thin strip of steel, fixed to the armature of an electromagnet. The plate thus recorded a series of dots or short lines, and the vertical wires were photographed on the plate by throwing light through the objective lens for one or two seconds. Meridian circles required precise adjustment to do accurate work. The rotation axis of

7654-508: Was rotated, the shape of the pivots, and any wobble of the axis, could be determined. Near each end of the axis, attached to the axis and turning with it, was a circle or wheel for measuring the angle of the telescope to the zenith or horizon. Generally of 1 to 3  feet or more in diameter, it was divided to 2 or 5 arcminutes , on a slip of silver set into the face of the circle near the circumference. These graduations were read by microscopes , generally four for each circle, mounted to

7743-559: Was shortly afterwards taken up by Edward Troughton , who constructed the first modern transit circle in 1806 for Groombridge 's observatory at Blackheath , the Groombridge Transit Circle (a meridian transit circle). Troughton afterwards abandoned the idea and designed the mural circle to take the place of the mural quadrant. In the United Kingdom, the transit instrument and mural circle continued until

7832-420: Was spent in perfecting it. In practice, none of these adjustments were perfect. The small errors introduced by the imperfections were mathematically corrected during the analysis of the data. Some telescopes designed to measure star transits are zenith telescopes designed to point straight up at or near the zenith for extreme precision measurement of star positions. They use an altazimuth mount , instead of

7921-402: Was then brought into the meridian by repeatedly timing the (apparent, incorrect) upper and lower meridian transits of a circumpolar star and adjusting one of the bearings horizontally until the interval between the transits was equal. Another method used calculated meridian crossing times for particular stars as established by other observatories. This was an important adjustment, and much effort

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