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Great refractor refers to a large telescope with a lens, usually the largest refractor at an observatory with an
equatorial mount An equatorial mount is a mount for instruments that compensates for Earth's rotation by having one rotational axis, the polar axis, parallel to the Earth's axis of rotation. This type of mount is used for astronomical telescopes and cameras. The ...
. The preeminence and success of this style in observational astronomy defines an era in modern telescopy in the 19th and early 20th century. Great refractors were large
refracting telescope 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 an ...
s using
achromatic lens An achromatic lens or achromat is a lens that is designed to limit the effects of chromatic and spherical aberration. Achromatic lenses are corrected to bring two wavelengths (typically red and blue) into focus on the same plane. The most comm ...
es (as opposed to the mirrors of
reflecting telescope A reflecting telescope (also called a reflector) is a telescope that uses a single or a combination of curved mirrors that reflect light and form an image. The reflecting telescope was invented in the 17th century by Isaac Newton as an alternati ...
s). They were often the largest in the world, or largest in a region. Despite typical designs having smaller apertures than reflectors, great refractors offered a number of advantages and were popular for astronomy. It was also popular to exhibit large refractors at international exhibits, and examples of this include the Trophy Telescope at the 1851 Great Exhibition, and the Yerkes Great Refractor at the 1893 World's Fair in Chicago. 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 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 moun ...
, a Heliometer, an
Astrograph An astrograph (or astrographic camera) is a telescope designed for the sole purpose of astrophotography. Astrographs are mostly used in wide-field astronomical surveys of the sky and for detection of objects such as asteroids, meteors, a ...
, 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 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 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 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 Parallax is a displacement or difference in the apparent position of an object viewed along two different lines of sight and is measured by the angle or semi-angle of inclination between those two lines. Due to foreshortening, nearby object ...
in the mid-1800s. As telescopes became larger and longer, the relatively modest increases in aperture belied their enormous size, with moving weights in the multiple tons in domes several stories tall; physically many of the biggest were larger than even some modern reflecting telescopes.


Beginnings

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 Paris () is the capital and most populous city of France, with an estimated population of 2,165,423 residents in 2019 in an area of more than 105 km² (41 sq mi), making it the 30th most densely populated city in the world in 2020. Si ...
, with an objective of 13.3 inches (~34.8 cm) for 1200 pounds, and he placed it as the centerpiece of the observatory.''History of the Cauchoix objective''
/ref> 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 Halley's Comet or Comet Halley, officially designated 1P/Halley, is a short-period comet visible from Earth every 75–79 years. Halley is the only known short-period comet that is regularly visible to the naked eye from Earth, and thus the on ...
in 1835 and to view the solar eclipse of 15 May 1836. In 1833 the
Duke of Northumberland Duke of Northumberland is a noble title that has been created three times in English and British history, twice in the Peerage of England and once in the Peerage of Great Britain. The current holder of this title is Ralph Percy, 12th Duke o ...
donated a Cauchoix of Paris objective lens to establish a large telescope for 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".


Refractors come of age

Although there had been very large (and unwieldy) Non-achromatic
aerial telescope An aerial telescope is a type of very long focal length refracting telescope, built in the second half of the 17th century, that did not use a tube. Instead, the objective was mounted on a pole, tree, tower, building or other structure on a swive ...
s of the late 17th century, and Chester Moore Hall and others had experimented with small achromatic telescopes in the 18th century,
John Dollond John Dollond FRS (10 June O.S. (21 June N.S.) 170630 November 1761) was an English optician, known for his successful optics business and his patenting and commercialization of achromatic doublets. Biography Dollond was the son of a Hugue ...
(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 (1748–1824) discovered and developed a way to make much larger crown and flint glass blanks. He worked with instrument maker
Joseph von Fraunhofer Joseph Ritter von Fraunhofer (; ; 6 March 1787 – 7 June 1826) was a German physicist and optical lens manufacturer. He made optical glass, an achromatic telescope, and objective lenses. He also invented the spectroscope and developed diffr ...
(1787–1826) to use this technology for instruments in the early 19th century. The era of ''great refractors'' started with the first modern, achromatic, refracting telescopes built by
Joseph von Fraunhofer Joseph Ritter von Fraunhofer (; ; 6 March 1787 – 7 June 1826) was a German physicist and optical lens manufacturer. He made optical glass, an achromatic telescope, and objective lenses. He also invented the spectroscope and developed diffr ...
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 The Governorate of Estonia, also known as the Governorate of Esthonia (Pre-reformed rus, Эстля́ндская губе́рнія, r=Estlyandskaya guberniya); et, Eestimaa kubermang was a governorate in the Baltic region, along with the ...
(Estland) (which later became Tartu Observatory in southern
Estonia Estonia, formally the Republic of Estonia, is a country by the Baltic Sea in Northern Europe. It is bordered to the north by the Gulf of Finland across from Finland, to the west by the sea across from Sweden, to the south by Latvia, an ...
). This telescope made by Fraunhofer had a 9 
Paris inch The Paris inch or pouce is an archaic unit of length that, among other uses, was common for giving the measurement of lenses. The Paris inch could be subdivided into 12 Paris lines (''ligne''), and 12 Paris inches made a Paris foot. The abbrev ...
(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 An equatorial mount is a mount for instruments that compensates for Earth's rotation by having one rotational axis, the polar axis, parallel to the Earth's axis of rotation. This type of mount is used for astronomical telescopes and cameras. The ...
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 Johann Gottfried Galle (9 June 1812 – 10 July 1910) was a German astronomer from Radis, Germany, at the Berlin Observatory who, on 23 September 1846, with the assistance of student Heinrich Louis d'Arrest, was the first person to view the pl ...
in the
discovery of Neptune The planet Neptune was mathematically predicted before it was directly observed. With a prediction by Urbain Le Verrier, telescopic observations confirming the existence of a major planet were made on the night of September 23–24, 1846, at the ...
. 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 The Great Exhibition of the Works of Industry of All Nations, also known as the Great Exhibition or the Crystal Palace Exhibition (in reference to the temporary structure in which it was held), was an international exhibition which took pl ...
in Hyde Park, one of 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.http://www.mbgrg.org/newsletters/newsletter20_July13.pdf 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 was manufactured by William Wray. On January 31, 1862, American telescope-maker and astronomer
Alvan Graham Clark Alvan Graham Clark (July 10, 1832 – June 9, 1897) was an American astronomer and telescope-maker. Biography Alvan Graham Clark was born in Fall River, Massachusetts, the son of Alvan Clark, founder of Alvan Clark & Sons. On January 31, 1862, ...
first observed the faint companion, which is now called Sirius B, or affectionately "the Pup". This happened during testing of an aperture great refractor telescope for Dearborn Observatory, which was one of the largest refracting telescope lens in existence at 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 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.


The Golden era

Refracting telescopes would quadruple in size by the end of the century, culminating with the largest practical refractor ever built, the
Yerkes Observatory Yerkes Observatory ( ) is an astronomical observatory located in Williams Bay, Wisconsin, United States. The observatory was operated by the University of Chicago Department of Astronomy and Astrophysics from its founding in 1897 to 2018. Owner ...
40-inch (1 meter) aperture of 1895. This great refractor pushed 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 Alvan Clark & Sons was an American maker of optics that became famous for crafting lenses for some of the largest refracting telescopes of the 19th and early 20th centuries. Founded in 1846 in Cambridgeport, Massachusetts, by Alvan Clark (1804&n ...
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 sag Lens sag is a problem that sometimes afflicts very large refracting telescopes. It is the equivalent of mirror sag in reflecting telescopes. It occurs when the physical weight of the glass causes a distortion in the shape of the lens A len ...
ging from
gravity In physics, gravity () is a fundamental interaction which causes mutual attraction between all things with mass or energy. Gravity is, by far, the weakest of the four fundamental interactions, approximately 1038 times weaker than the stro ...
meant refractors would not exceed around 1 meter, although Alvan G. Clark, who had made 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 Tons can refer to: * Tons River, a major river in India * Tamsa River, locally called Tons in its lower parts (Allahabad district, Uttar pradesh, India). * the plural of ton, a unit of mass, force, volume, energy or power :* short ton, 2,000 poun ...
, and had to track stars just as accurately as a smaller instrument. The choice between large refractors or reflectors was driven by 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 A primary mirror (or primary) is the principal light-gathering surface (the objective) of a reflecting telescope. Description The primary mirror of a reflecting telescope is a spherical or parabolic shaped disks of polished reflective meta ...
was
speculum metal Speculum metal is a mixture of around two-thirds copper and one-third tin, making a white brittle alloy that can be polished to make a highly reflective surface. It was used historically to make different kinds of mirrors from personal grooming ...
, 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 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 Hubble Space Telescope (often referred to as HST or Hubble) is a space telescope that was launched into low Earth orbit in 1990 and remains in operation. It was not the first space telescope, but it is one of the largest and most vers ...
. The Great Paris Exhibition Telescope of 1900 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 the discovery of the
moons of Mars The two moons of Mars are Phobos (moon), Phobos and Deimos (moon), Deimos. They are irregular in shape. Both were discovered by American astronomer Asaph Hall in August 1877 and are named after the Greek mythology, Greek mythological twin charac ...
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 the
Leviathan of Parsonstown Leviathan of Parsonstown, or Rosse six-foot telescope, is a historic reflecting telescope of aperture, which was the largest telescope in the world from 1845 until the construction of the Hooker Telescope in California in 1917. The Rosse six-f ...
, and work with the
Crossley Reflector The Crossley telescope is a reflecting telescope located at Lick Observatory in the U.S. state of California. It was used between 1895 to 2010, and was donated to the observatory by Edward Crossley, its namesake. It was the largest glass ref ...
and increasingly larger silver-and-glass mirrors marked large refractors' obsolescence.


End of the era

The era slowly came to end as large reflecting telescopes superseded the great refractors. In 1856–57,
Carl August von Steinheil Carl August von Steinheil (12 October 1801 – 14 September 1870) was a German physicist, inventor, engineer and astronomer. Biography Steinheil was born in Ribeauvillé, Alsace. He studied law in Erlangen since 1821. He then studied astronomy ...
and
Léon Foucault Jean Bernard Léon Foucault (, ; ; 18 September 1819 – 11 February 1868) was a French physicist best known for his demonstration of the Foucault pendulum, a device demonstrating the effect of Earth's rotation. He also made an early measurement ...
introduced a process of depositing a layer of silver on glass telescope mirrors. Silvered glass mirrors were 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 The Crossley telescope is a reflecting telescope located at Lick Observatory in the U.S. state of California. It was used between 1895 to 2010, and was donated to the observatory by Edward Crossley, its namesake. It was the largest glass ref ...
(1895), 60-inch (1.5 m)
Mount Wilson Observatory The Mount Wilson Observatory (MWO) is an astronomical observatory in Los Angeles County, California, United States. The MWO is located on Mount Wilson, a peak in the San Gabriel Mountains near Pasadena, northeast of Los Angeles. The observat ...
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 the
Dominion Astrophysical Observatory The Dominion Astrophysical Observatory, located on Observatory Hill, in Saanich, British Columbia, was completed in 1918 by the Canadian government. The Dominion architect responsible for the building was Edgar Lewis Horwood. The main instrument ...
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 the big Expo telescope of 1900.


Photographic vs. Visual

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: * mounting two telescopes side by side, one with a visual objective and one with a photographic objective * mounting a single telescope but having an interchangeable visual objective and photographic objective * creating objective lenses for visual use which had an additional correcting lens that could be added for photographic work * make special objective lenses that could be reversed, one way it was a visual objective, flipped over it was a photographic objective An example of the first case was the
Meudon Great Refractor Meudon Great Refractor (also known as the ''Grande Lunette'') is a double telescope with lenses (83 cm + 62 cm), in Meudon, France. It is a twin refracting telescope built in 1891, with one visual and one photographic, on a single square ...
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.


Examples

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 was important because unlike reflectors (which can be folded and shortened), the
focal length The focal length of an optical system is a measure of how strongly the system converges or diverges light; it is the inverse of the system's optical power. A positive focal length indicates that a system converges light, while a negative foc ...
of glass lens correlated to the physical length of the telescope and offered some optical and image quality advantages.


Aperture

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 the second-largest refractors, or otherwise notable.


Focal length

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 the enormous Yerkes and Treptow refractors actually debuted at exhibitions, which were major events of the period.


Gallery

File:Archenhold-Observatory-Refractor.ogv, Modern day video of ''Großer Refraktor'' of 1896 at Treptow Observatory, later renamed
Archenhold Observatory The Archenhold Observatory () was named in honour of Friedrich Simon Archenhold, is an observatory in Berlin-Treptow. It houses the ''Großer Refraktor'' (Great Refractor), which is the longest pointable telescope in the world. It is also called th ...
File:Newall Telescope (The Engineer, May 13, 1870).jpg, Newall refractor (1870 diagram) File:Meyers b1 s0983a.jpg, Four astronomical instruments of the Strasbourg Observatory, including its ''Großer Refraktor'' File:Refractor Cincinnati observatory.jpg, Cincinnati Observatory, Illustration of the 11 inch "Merz and Mahler" refracting telescope (from "Smith's Illustrated Astronomy" 1848).


Observations


See also

*
Achromatic lens An achromatic lens or achromat is a lens that is designed to limit the effects of chromatic and spherical aberration. Achromatic lenses are corrected to bring two wavelengths (typically red and blue) into focus on the same plane. The most comm ...
*
Aerial telescope An aerial telescope is a type of very long focal length refracting telescope, built in the second half of the 17th century, that did not use a tube. Instead, the objective was mounted on a pole, tree, tower, building or other structure on a swive ...
*
Apochromat An apochromat, or apochromatic lens (apo), is a photographic or other lens that has better correction of chromatic and spherical aberration than the much more common achromat lenses. Explanation Chromatic aberration is the phenomenon of differen ...
*
Extremely large telescope The Extremely Large Telescope (ELT) is an astronomical observatory currently under construction. When completed, it is planned to be the world's largest optical/near-infrared extremely large telescope. Part of the European Southern Observatory ...
* List of largest optical refracting telescopes * List of largest optical telescopes in the 18th century * List of largest optical telescopes in the 19th century * List of largest optical telescopes in the 20th century *
List of telescope types The following are lists of devices categorized as types of telescopes or devices associated with telescopes. They are broken into major classifications with many variations due to professional, amateur, and commercial sub-types. Telescopes can be ...


References


Further reading


"The History of the Development of the Telescope", Authors: Schirach, W. F. H."The Great 19th Century Refractors"
*


External links


Great Refractors EraModern Day Image of the Harvard Great Refractor
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