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 spy glasses and astronomical telescopes but is also used for long focus camera lenses. Although large refracting telescopes were very popular in the second half of the 19th century, for most research purposes the refracting telescope has been superseded by the reflecting telescope which allows larger apertures.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.[1]

Refractor Cincinnati observatory
Image of a refracting telescope from the Cincinnati Observatory in 1848


Refractors were the earliest type of optical telescope. The first practical refracting telescopes appeared in the Netherlands about 1608, and were credited to three individuals, Hans Lippershey and Zacharias Janssen, spectacle-makers in Middelburg, and Jacob Metius of Alkmaar. Galileo Galilei, happening to be in Venice in about the month of May 1609, heard of the invention and constructed a version of his own. Galileo then communicated the details of his invention to the public, and presented the instrument itself to the Doge Leonardo Donato, sitting in full council.[2][3]

Refracting telescope designs


All refracting telescopes use the same principles. The combination of an objective lens 1 and some type of eyepiece 2 is used to gather more light than the human eye is able to collect on its own, focus it 5, and present the viewer with a brighter, clearer, and magnified virtual image 6.

The objective in a refracting telescope refracts or bends light. This refraction causes parallel light rays to converge at a focal point; while those not parallel converge upon a focal plane. The telescope converts a bundle of parallel rays to make an angle α, with the optical axis to a second parallel bundle with angle β. The ratio β/α is called the angular magnification. It equals the ratio between the retinal image sizes obtained with and without the telescope.[4]

Refracting telescopes can come in many different configurations to correct for image orientation and types of aberration. Because the image was formed by the bending of light, or refraction, these telescopes are called refracting telescopes or refractors.

Galileo's telescope

Optical diagram of Galilean telescope y – Distant object ; y′ – Real image from objective ; y″ – Magnified virtual image from eyepiece ; D – Entrance pupil diameter ; d – Virtual exit pupil diameter ; L1 – Objective lens ; L2 – Eyepiece lens e – Virtual exit pupil – Telescope equals [5]

The design Galileo Galilei used in 1609 is commonly called a Galilean telescope. It used a convergent (plano-convex) objective lens and a divergent (plano-concave) eyepiece lens (Galileo, 1610).[6] A Galilean telescope, because the design has no intermediary focus, results in a non-inverted and upright image.

Galileo’s best telescope magnified objects about 30 times. Because of flaws in its design, such as the shape of the lens and the narrow field of view, the images were blurry and distorted. Despite these flaws, the telescope was still good enough for Galileo to explore the sky. The Galilean telescope could view the phases of Venus, and was able to see craters on the Moon and four moons orbiting Jupiter.

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

Keplerian telescope

Houghton Typ 620.73.451 - Johannes Hevelius, Machinae coelestis, 1673
Woodcut illustration of a 46 m (150 ft) focal length Keplerian astronomical refracting telescope built by Johannes Hevelius.[7]

The Keplerian telescope, invented by Johannes Kepler in 1611, is an improvement on Galileo's design.[8] 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 to overcome aberrations the simple objective lens needs to have a very high f-ratio (Johannes Hevelius built one with a 46-metre (150 ft) focal length, and even longer tubeless "aerial telescopes" were constructed). The design also allows for use of a micrometer at the focal plane (used to determine the angular size and/or distance between objects observed).

Achromatic refractors

The achromatic refracting lens 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 limit the effects of chromatic and spherical aberration. Each side of each piece 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. 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.

Apochromatic refractors

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 up to 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 the objective and produce a very crisp image that is virtually free of chromatic aberration.[9] Due to the special materials needed in the fabrication, apochromatic refractors are usually more expensive than telescopes of other types with a comparable aperture.

Technical considerations

Yerkes Observatory Astro4p7
The 1.0-meter (40 in) refractor, at Yerkes Observatory, the largest achromatic refractor ever put into astronomical use

Refractors suffer from residual chromatic and spherical aberration. This affects shorter focal ratios more than longer ones. A 100 mm (4 in) f/6 achromatic refractor is likely to show considerable color fringing (generally a purple halo around bright objects). A 100 mm (4 in) f/16 has little 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 a refracting telescope is around 1 meter (39 in).[10]

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.

List of the largest refracting telescopes

A list of the largest refracting telescopes over 60 cm (24 in) diameter.

Potsdam Great Refractor

The Great Refractor at the Astrophysical Institute Potsdam, Germany

Grande Lunette Nice

The 76 cm (30 in) refractor at Nice Observatory


The Great Refractor at the Archenhold Observatory in Berlin

Refraktor Wien Kerschbaum 1

The 68 cm (27 in) refractor at the Vienna University Observatory


51-centimeter (20 in) refractor at the Observatories at Chabot Space & Science Center in Oakland, California


20-centimeter (8 in) refractor at the Observatories at Chabot Space & Science Center in Oakland, California

Aachen Refraktor

Refractor at the Observatory in Aachen, Germany

Lowell Observatory - Clark telescope

The Alvan Clark Refractor Telescope at Lowell Observatory in Flagstaff, AZ

Irving Porter Church Telescope

The 30 cm (12 in) Irving Porter Church Memorial Refractor at Fuertes Observatory in Ithaca, NY


Diagram of a commercial refractor


Apochromatic refractor


Coin-operated telescope in Germany

See also


  1. ^ "Telescope Calculations". Northern Stars. Retrieved 2013-12-20.
  2. ^ "The First Telescopes". American Institute of Physics. Retrieved 15 July 2015.
  3. ^ "Physics for Scientists and Engineers, Refractors were the earliest type of optical telescope". Raymond A. Serway. Retrieved 15 July 2015.
  4. ^ Stephen G. Lipson, Ariel Lipson, Henry Lipson, Optical Physics 4th Edition, Cambridge University Press, ISBN 978-0-521-49345-1
  5. ^
  6. ^ Sidereus Nuncius or The Sidereal Messenger, 1610, Galileo Galilei et al., 1989, pg. 37, The University of Chicago Press, Albert van Helden tr., (History Dept. Rice University, Houston, TX), ISBN 0-226-27903-0.
  7. ^ Hevelius, Johannes (1673). Machina Coelestis. First Part. Auctor.
  8. ^ Tunnacliffe, AH; Hirst JG (1996). Optics. Kent, England. pp. 233–7. ISBN 0-900099-15-1.
  9. ^ "Starizona's Guide to CCD Imaging". Retrieved 17 October 2013.
  10. ^ "Physics Demystified" By Stan Gibilisco, ISBN 0-07-138201-1, page 515
  11. ^ "John Wall refractor - Hanwell Community Observatory".

External links

Achromatic telescope

The achromatic telescope is a refracting telescope that uses an achromatic lens to correct for chromatic aberration.

Alvan Graham Clark

Alvan Graham Clark (July 10, 1832 – June 9, 1897) was an American astronomer and telescope-maker.

Born in Fall River, Massachusetts, he was the son of Alvan Clark, founder of Alvan Clark & Sons.

On January 31, 1862, while testing a new 18.5-inch (470 mm) aperture great refractor telescope in Cambridgeport, Massachusetts, Clark made the first ever observation of a white dwarf star. This discovery of Sirius B, or affectionately "the Pup", proved an earlier hypotheses (Friedrich Bessel in 1844) that Sirius, the brightest star in the night sky with an apparent magnitude of −1.46, had an unseen companion disturbing its motion. Clark used the largest refracting telescope lens in existence at the time, and the largest telescope in the United States, to observe the magnitude 8 companion.

Clark's 18½ inch refracting telescope was then delivered to his customer, the landmark Dearborn Observatory of Northwestern University in Evanston, Illinois, where it is still being used today.

Anna L. Nickel telescope

The Anna L. Nickel telescope is a 1-meter reflecting telescope located at Lick Observatory in the U.S. state of California.

The smaller dome on the main building at Lick had originally held the secondhand 12-inch Clark refracting telescope, the first telescope to be used at Lick. In 1979 it was replaced with the Anna L. Nickel telescope, a 1-meter reflecting telescope. The telescope is named for Anna L. Nickel, a San Francisco native who donated $50,000, a large portion of her estate, to the Observatory.

The Nickel telescope was built entirely by UC Santa Cruz personnel and utilized many spare parts, including a replacement mirror for the Crossley Reflector. It eased demand for time on the 120-inch C. Donald Shane telescope by taking on research programs that do not require the Shane's greater light-gathering power. Due to its optical design, it can use the same instruments as the Shane so instruments can be tested on it.

Beam expander

Beam expanders are optical devices that take a collimated beam of light and expand its size (or, used in reverse, reduce its size).

In laser physics they are used either as intracavity or extracavity elements. They can be telescopic in nature or prismatic. Generally prismatic beam expanders use several prisms and are known as multiple-prism beam expanders.

Telescopic beam expanders include refracting and reflective telescopes. A refracting telescope commonly used is the Galilean telescope which can function as a simple beam expander for collimated light. The main advantage of the Galilean design is that it never focuses a collimated beam to a point, so effects associated with high power density such as dielectric breakdown are more avoidable than with focusing designs such as the Keplerian telescope. When used as intracavity beam expanders, in laser resonators, these telescopes provide two-dimensional beam expansion in the 20–50 range.In tunable laser resonators intracavity beam expansion usually illuminates the whole width of a diffraction grating. Thus beam expansion reduces the beam divergence and enables the emission of very narrow linewidths which is a desired feature for many analytical applications including laser spectroscopy.

Brera Astronomical Observatory

The Brera Observatory (Italian: Osservatorio Astronomico di Brera) is an astronomical observatory in the Brera district of Milan, Italy. It was built in the historic Palazzo Brera in 1764 by the Jesuit astronomer Ruggero Boscovich. Following the suppression of the Jesuits by Clement XIV on 21 July 1773, the palace and the observatory passed to the then rulers of northern Italy, the Austrian Habsburg dynasty. The observatory has since remained under state control.

In 1862, The Government of Italy funded the purchase of a 218mm Merz Equatorial Refracting Telescope that was ordered to the German constructor Georg Merz, in 1862.

Chester Moore Hall

Chester Moore Hall (9 December 1703, Leigh, Essex, England – 17 March 1771, Sutton, Surrey) was a British lawyer and inventor who produced the first achromatic lenses in 1729 or 1733 (accounts differ).

He used the achromatic lens to build the first refracting telescope free from chromatic aberration (colour distortion).He lived at New Hall, Sutton, Surrey.

His name was also spelled Chester Moor Hall and Chester More Hall.

Cincinnati Observatory

The Cincinnati Observatory is located in Cincinnati, Ohio (United States) on top of Mount Lookout. It consists of two observatory buildings housing an 11-inch (28 cm) and 16 inch (41 cm) aperture refracting telescope. It is the oldest professional observatory in the United States. It was a key facility for astronomical research and education at the University of Cincinnati and currently operates as a 19th-century observatory. There are regular viewings through both historical telescopes as well as tours and additional programs. The observatory also has an extensive outreach program, providing astronomical education for the Ohio/Kentucky/Indiana region.The Cincinnati Observatory is a contributing property to the Observatory Historic District.

Craig telescope

The Craig telescope was a large telescope built in the 1850s, and while much larger than previous refracting telescopes, it had some problems that hampered its use. Its unique design and potential caused a great deal of excitement in its day. The telescope was ready in August 1852 and was visited by William Parsons (he was famous for the Leviathan of Parsonstown, the largest telescope overall of this age). It is known to been used to observe the planet Saturn.

Dominion Observatory

The Dominion Observatory was an astronomical observatory in Ottawa, Ontario that operated from 1902 to 1970. The Observatory was also an institution within the Canadian Federal Government. The observatory grew out of the Department of the Interior's need for the precise coordinates and timekeeping that at that time could only come from an observatory. For several years they had used a small observatory on the Ottawa River for this purpose. In 1902, it was decided that Canada needed a larger national observatory similar to the Royal Greenwich Observatory in Britain.

Chief Dominion Architect David Ewart designed the Dominion Observatory in 1902.The new building was then erected near Dow's Lake on the Agriculture Department's Central Experimental Farm land. This Romanesque Revival building was completed in 1905. Its main instrument was a 15-inch refracting telescope, the largest refracting telescope ever installed in Canada. While the building and institution were primarily dedicated to astronomical timekeeping in support of surveying, a number of other activities took place here. The Dominion Observatory was Canada's leading institution in Geophysics for many decades, which included the operation of Canada's national seismometer network. The facility did important work but with this bridgehead into the world of astronomy and the growth of the field of astrophysics Canadian astronomers quickly demanded a facility designed for the new scientific age. In 1917, the Dominion Astrophysical Observatory was opened in Victoria, B.C. and it supplanted the Dominion Observatory as Canada's foremost astronomical observatory. For many years the Dominion Observatory was best known to Canadians as the source of Canada's official time signal.

The observatory continued in operation until 1970 at which time Canada's science institutions were reorganized. The national time-keeping and astronomical activities were transferred to the National Research Council of Canada, while the geophysics, surveying and mapping were transferred to the Department of Energy Mines and Resources. The Geophysics work was later merged into the Geological Survey of Canada, now part of Natural Resources Canada. Astronomical timekeeping observations at the Dominion Observatory had ceased many years prior to this, when crystal oscillator clocks and later atomics clocks were found to be superior to astronomical timekeeping. The building became home to NRCan offices. The telescope had been open for public viewing from 1905 until 1970. In 1974, the telescope was moved from the Dominion Observatory to the Helen Sawyer Hogg Observatory at the Canada Science and Technology Museum where it remains to this day.

In the mid-1990s, the rumour within Natural Resources was that consideration was given to demolish the building to save money in a time of budgetary cutbacks. However, these plans did not come to fruition. As of 2008, the building is the home to the Office of Energy Efficiency, a part of the Energy Branch, Natural Resources Canada.


Dunsink (Irish: Dún Sinche, meaning "Fort of Sinneach") is a townland in the civil parish of Castleknock, Dublin, in Ireland. It is most famous for Dunsink Observatory where William Rowan Hamilton and Hermann Brück were directors. It is the oldest scientific institution in Ireland. Built in 1783-1785, the observatory houses a 12-inch (30 cm) refracting telescope which was built by Grubbs of Dublin using a French manufactured lens.

The 1880 Definition of Time Act set the official time in Ireland to be Dublin mean time. This was regarded to mean the time at Dunsink, which was about GMT-25m21. In 1916 another Act moved Ireland to Greenwich Mean Time. Dunsink time is mentioned five times in James Joyce's novel, Ulysses.

The observatory is situated on a hill 8 km northwest of Dublin's city centre, where the Astronomy section of the School of Cosmic Physics in the Dublin Institute for Advanced Studies is located.

Dunsink is also the site of a major unofficial Traveller encampment, the subject of considerable media coverage in 2007.

George Bassett Clark

George Bassett Clark (February 14, 1827 – December 20, 1891) was an American instrument maker and astronomer.

Born in Lowell, Massachusetts and educated at Phillips Academy, Andover, he was the son of Alvan Clark, part of a family of refracting telescope makers in the 19th century. In 1846, George Bassett Clark joined his father and brother at the family's telescope works in Cambridge, Massachusetts. The firm, Alvan Clark & Sons, made many of the record-breaking refracting instruments, including the still-largest refracting telescope at the Yerkes Observatory, gaining "worldwide fame and distribution", wrote one author on astronomy in 1899. Clark was elected a Fellow of the American Academy of Arts and Sciences in 1878.

James Lick telescope

The James Lick Telescope is a refracting telescope built in 1888. It has a lens 36 inches (91 cm) in diameter- a major achievement in its day. The instrument remains in operation and public viewing is allowed on a limited basis. Also called the "Great Lick Refractor" or simply "Lick Refractor", it was the largest refracting telescope in the world until 1897 and now ranks third, after the 40-inch unit at the Yerkes Observatory and the Swedish 1-m Solar Telescope. The telescope is located at the University of California's Lick Observatory atop Mount Hamilton at an elevation of 4,209 feet (1,283 m) above sea level. The instrument is housed inside a dome that is powered by hydraulic systems that raise and lower the floor, rotate the dome and drive the clock mechanism to track the Earth's rotation. The original hydraulic arrangement still operates today, with the exception that the original wind-powered pumps that once filled the reservoirs have been replaced with electric pumps. James Lick is entombed below the floor of the observing room of the telescope.

Here are some excerpts from an 1894 book describing the telescope:

The height of the marble floor of the main building above mean sea level is 4209 feet. On a closely connected peak half a mile to the east of the Observatory, and 50 feet higher, are the reservoirs from which water for household and photographic purposes is distributed. A spring about 350 feet below and one mile to the northeast of the Observatory supplies excellent water. Another peak seven-eighths of a mile to the east is the summit of Mount Hamilton; it is 180 feet higher than the Observatory, and supports the reservoirs supplying power for moving the dome, raising the movable floor, and winding the driving clock of the great telescope. This system receives its supply from the winter rains falling on the roofs; the water being pumped to the reservoirs on the higher peak by means of windmills.The movable floor in the dome is the first of the kind to be constructed. It is 60 feet (18 m) in diameter, and can be raised or lowered through a distance of 16 1⁄2 feet (5.0 m), its purpose being to bring the observer within convenient reach of the eye end of the telescope.

Jewett Observatory

The James Richard Jewett Observatory is an astronomical observatory owned and operated by Washington State University. It is located in Pullman, Washington (US). It houses the largest refracting telescope in the state of Washington. The 12-inch lens was originally ground and polished between 1887 and 1889 by Alvan Clark & Sons for an amateur astronomer, who died before the telescope could be assembled. The lens was put into storage, and was purchased by the university when it came up for auction in the 1950s. Its present dome was dedicated in 1953 and it is named after the father of a supporter of the observatory, Mr. George Jewett of Spokane.

The observatory is not used for research purposes; it is primarily used for undergraduate student labs and training, and houses twelve portable telescopes for this purpose. Additionally, the telescope is open for use to any students who have been instructed in its use, as well as to the public on monthly star party nights.

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 alternative to the refracting telescope which, at that time, was a design that suffered from severe chromatic aberration. Although reflecting telescopes produce other types of optical aberrations, it is a design that allows for very large diameter objectives. Almost all of the major telescopes used in astronomy research are reflectors. Reflecting telescopes come in many design variations and may employ extra optical elements to improve image quality or place the image in a mechanically advantageous position. Since reflecting telescopes use mirrors, the design is sometimes referred to as a "catoptric" telescope.

Shuckburgh telescope

The Shuckburgh telescope or Shuckburgh equatorial refracting telescope was a 4.1 inch diameter aperture telescope on an equatorial mount completed in 1791 for Sir George Shuckburgh (1751–1804) in Warwickshire, England, and built by British instrument maker Jesse Ramsden (1735–1800).

It was transferred to the Royal Observatory, Greenwich in 1811 and the London Science Museum in 1929. Even though it was not particularly successful, its design was influential. It was one of the larger achromatic doublet telescopes at the time, and one of the largest to have an equatorial mount.It was pictured in the Rees Cyclopedia of the early 1800s. It was early pictured in Philosophical Transactions, published in 1793.


Telescopes are optical instruments that make distant objects appear magnified by using an arrangement of lenses or curved mirrors and lenses, or various devices used to observe distant objects by their emission, absorption, or reflection of electromagnetic radiation. The first known practical telescopes were refracting telescopes invented in the Netherlands at the beginning of the 17th century, by using glass lenses. They found use in both terrestrial applications and astronomy.

The reflecting telescope, which uses mirrors to collect and focus light, was invented within a few decades of the first refracting telescope. In the 20th century, many new types of telescopes were invented, including radio telescopes in the 1930s and infrared telescopes in the 1960s. The word telescope now refers to a wide range of instruments capable of detecting different regions of the electromagnetic spectrum, and in some cases other types of detectors.

University of Alabama Observatory

The University of Alabama Observatory is an astronomical observatory owned and operated by the University of Alabama in Tuscaloosa, Alabama. The new domed observatory was built atop Gallalee Hall, completed in 1949. It replaced the Old Observatory, which had been in use from 1849 until the 1890s. Initially equipped with a 10-inch (0.25 m) refracting telescope, this was the university's primary telescope from 1950 until 2004. The old telescope was removed and then sold to an antique telescope collector to make way for the new instrument. A new 16-inch (0.41 m) Ritchey-Chrétien reflector, manufactured by DFM Engineering, was installed in 2005.

Vienna Observatory

The Vienna Observatory (German: Universitätssternwarte Wien) is an astronomical observatory in Vienna, Austria. It is part of the University of Vienna. The first observatory was built in 1753–1754 on the roof of one of the university buildings.

A new observatory was built between 1874 and 1879, and was finally inaugurated by Emperor Franz Joseph I of Austria in 1883. The main dome houses a refractor with a diameter of 68 centimetres (27 in) and a focal length of 10.5 metres (34 ft) built by the Grubb Telescope Company. At that time, it was the world's largest refracting telescope.

Yerkes Observatory

Yerkes Observatory is an astronomical observatory in Williams Bay, Wisconsin operated by the University of Chicago Department of Astronomy and Astrophysics. It closed public operations in 2018.

The observatory, which called itself "the birthplace of modern astrophysics", was founded in 1897 by astronomer George Ellery Hale and financed by businessman Charles T. Yerkes.

It represented a shift in the thinking about observatories, from their being mere housing for telescopes and observers, to the early-20th-century concept of observation equipment integrated with laboratory space for physics and chemistry.

The observatory houses a 40-inch (102-cm) diameter doublet lens refracting telescope, the largest ever successfully used for astronomy, and a collection of over 170,000 photographic plates.

Notable astronomers who conducted research at Yerkes include Edwin Hubble (who did his graduate work at Yerkes and for whom the Hubble Space Telescope was named), Subrahmanyan Chandrasekhar (for whom the Chandra Space Telescope was named), Russian-American astronomer Otto Struve, Dutch-American astronomer Gerard Kuiper,

and the twentieth-century popularizer of astronomy Carl Sagan.

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