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Refracting telescope

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A 200 mm diameter refracting telescope at the Poznań Observatory

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were very popular in the second half of the 19th century, for most research purposes, the refracting telescope has been superseded by the reflecting telescope, which allows larger apertures.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

Refracting telescopes typically have a lens at the front, then a long tube, then an eyepiece or instrumentation at the rear, where the telescope view comes to focus. Originally, telescopes had an objective of one element, but a century later, two and even three element lenses were made.

Refracting telescopes use technology that has often been applied to other optical devices, such as binoculars and zoom lenses/telephoto lens/long-focus lens.


Refractors were the earliest type of optical telescope. The first record of a refracting telescope appeared in the Netherlands about 1608, when a spectacle maker from Middelburg named Hans Lippershey unsuccessfully tried to patent one.[2] News of the patent spread fast and Galileo Galilei, happening to be in Venice in the month of May 1609, heard of the invention, constructed a version of his own, and applied it to making astronomical discoveries.[3]

Refracting telescope designs[edit]

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.

Galilean telescope[edit]

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 c. 1609 is commonly called a Galilean telescope.[6] It used a convergent (plano-convex) objective lens and a divergent (plano-concave) eyepiece lens (Galileo, 1610).[7] A Galilean telescope, because the design has no intermediary focus, results in a non-inverted, i.e, an upright image.[8]

Galileo's most powerful telescope, with a total length of 980 millimetres (3 ft 3 in),[6] magnified objects about 30 times.[8] Galileo had to work with the poor lens technology of the time, and found he had to use aperture stops to reduce the diameter of the objective lens (increase its focal ratio) to limit aberrations, so his telescope produced blurry and distorted images with a narrow field of view.[8] Despite these flaws, the telescope was still good enough for Galileo to explore the sky. He used it to view craters on the Moon,[9] the four largest moons of Jupiter,[10] and the phases of Venus.[11]

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[edit]

Engraved illustration of a 46 m (150 ft) focal length Keplerian astronomical refracting telescope built by Johannes Hevelius.[12]

The Keplerian telescope, invented by Johannes Kepler in 1611, is an improvement on Galileo's design.[13] 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[dubiousdiscuss] are converging. This allows for a much wider field of view and greater eye relief, but the image for the viewer is inverted. Considerably higher magnifications can be reached with this design, but, like the Galilean telescope, it still uses simple single element objective lens so needs to have a very high focal ratio to reduce aberrations[14] (Johannes Hevelius built an unwieldy f/225 telescope with a 200-millimetre (8 in) objective and a 46-metre (150 ft) focal length,[15] and even longer tubeless "aerial telescopes" were constructed). The design also allows for use of a micrometer at the focal plane (to determine the angular size and/or distance between objects observed).

Huygens built an aerial telescope for Royal Society of London with a 19 cm (7.5″) single-element lens.[16]

Achromatic refractors[edit]

Alvan Clark polishes the big Yerkes achromatic objective lens, over 1 meter across, in 1896.
This 12 inch refractor is mounted in a dome on a mount that matches the Earth's rotation

The next major step in the evolution of refracting telescopes was the invention of the achromatic lens, a lens with multiple elements that helped solve problems with chromatic aberration and allowed shorter focal lengths. It was invented in 1733 by an English barrister named Chester Moore Hall, although it was independently invented and patented by John Dollond around 1758. The design overcame the need for very long focal lengths in refracting telescopes by using an objective made of two pieces of glass with different dispersion, 'crown' and 'flint glass', to reduce chromatic and spherical aberration. Each side of each piece is ground and polished, and then the two pieces are assembled together. Achromatic lenses are corrected to bring two wavelengths (typically red and blue) into focus in the same plane.

Chester More Hall is noted as having made the first twin color corrected lens in 1730.[17]

Dollond achromats were quite popular in the 18th century.[18][19] A major appeal was they could be made shorter.[19] However, problems with glass making meant that the glass objectives were not made more than about four inches in diameter.[19]

In the late 19th century, the Swiss optician Pierre-Louis Guinand[20] developed a way to make higher quality glass blanks of greater than four inches.[19][21] He passed this technology to his apprentice Joseph von Fraunhofer, who further developed this technology and also developed the Fraunhofer doublet lens design.[19] The breakthrough in glass making techniques led to the great refractors of the 19th century, that became progressively larger through the decade, eventually reaching over 1 meter by the end of that century before being superseded by silvered-glass reflecting telescopes in astronomy.

Noted lens makers of the 19th century include:[22]

The Greenwich 28-inch refractor is a popular tourist attraction in 21st century London

Some famous 19th century doublet refractors are the James Lick telescope (91 cm/36 in) and the Greenwich 28 inch refractor (71 cm). An example of an older refractor is the Shuckburgh telescope (dating to the late 1700s). A famous refractor was the "Trophy Telescope", presented at the 1851 Great Exhibition in London. The era of the 'great refractors' in the 19th century saw large achromatic lenses, culminating with the largest achromatic refractor ever built, the Great Paris Exhibition Telescope of 1900.

In the Royal Observatory, Greenwich an 1838 instrument named the Sheepshanks telescope includes an objective by Cauchoix.[28] The Sheepshanks had a 6.7 inches (17 cm) wide lens, and was the biggest telescope at Greenwich for about twenty years.[29]

An 1840 report from the Observatory noted of the then-new Sheepshanks telescope with the Cauchoix doublet:[30]

The power and general goodness of this telescope make it a most welcome addition to the instruments of the observatory

In the 1900s a noted optics maker was Zeiss.[31] An example of prime achievements of refractors, over 7 million people have been able to view through the 12-inch Zeiss refractor at Griffith Observatory since its opening in 1935; this is the most people to have viewed through any telescope.[31]

Achromats were popular in astronomy for making star catalogs, and they required less maintenance than metal mirrors. Some famous discoveries using achromats are the planet Neptune and the Moons of Mars.

The long achromats, despite having smaller aperture than the larger reflectors, were often favoured for "prestige" observatories. In the late 18th century, every few years, a larger and longer refractor would debut.

For example, the Nice Observatory debuted with 77-centimetre (30.31 in) refractor, the largest at the time, but was surpassed within only a couple of years.[32]

Apochromatic refractors[edit]

Apochromat lens.svg
The Apochromatic lens usually comprises three elements that bring light of three different frequencies to a common focus

Apochromatic refractors have objectives built with special, extra-low dispersion materials. They are designed to bring three wavelengths (typically red, green, and blue) into focus in the same plane. The residual color error (tertiary spectrum) can be an order of magnitude less than that of an achromatic lens.[33] 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.[34] Due to the special materials needed in the fabrication, apochromatic refractors are usually more expensive than telescopes of other types with a comparable aperture.

In the 18th century, Dollond, a popular maker of doublet telescopes, also made a triplet, although they were not really as popular as the two element telescopes.[19]

One of the famous triplet objectives is the Cooke triplet, noted for being able to correct the Seidal aberrations.[35] It is recognized as one of the most important objective designs in the field of photography.[36][37] The Cooke triplet can correct, with only three elements, for one wavelength, spherical aberration, coma, astigmatism, field curvature, and distortion.[37]

Technical considerations[edit]

The 102 centimetres (40 in) refractor, at Yerkes Observatory, the largest achromatic refractor ever put into astronomical use (photo taken on 6 May 1921, as Einstein was visiting)

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).[38]

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

The ISS-WAC on the Voyager 1/2 used a 6 cm (2.36″) lens, launched into space in the late 1970s, an example of the use of refractors in space.[39]

Applications and achievements[edit]

The "Große Refraktor" a double telescope with a 80cm (31.5") and 50 cm (19.5") lenses, was used to discover calcium as an interstellar medium in 1904.
Astronaut trains with camera with large lens

Refracting telescopes were noted for their use in astronomy as well as for terrestrial viewing. Many early discoveries of the Solar System were made with singlet refractors.

The use of refracting telescopic optics are ubiquitous in photography, and are also used in Earth orbit.

One of the more famous applications of the refracting telescope was when Galileo used it to discover the four largest moons of Jupiter in 1609. Furthermore, early refractors were also used several decades later to discover Titan, the largest moon of Saturn, along with three more of Saturn's moons.

In the 19th century, refracting telescopes were used for pioneering work on astrophotography and spectroscopy, and the related instrument, the heliometer, was used to calculate the distance to another star for the first time. Their modest apertures did not lead to as many discoveries and typically so small in aperture that many astronomical objects were simply not observeable until the advent of long-exposure photography, by which time the reputation and quirks of reflecting telescopes were beginning to exceed those of the refractors. Despite this, some discoveries include the Moons of Mars, a fifth Moon of Jupiter, and many double star discoveries including Sirius (the Dog star). Refactors were often used for positional astronomy, besides from the other uses in photography and terrestrial viewing.

Touristic telescope pointed to Matterhorn in Switzerland

The Galilean moons and many other moons of the solar system, were discovered with single-element objectives and aerial telescopes.

Galileo Galilei's discovered the Galilean satellites of Jupiter in 1610 with a refracting telescope.[40]

The planet Saturn's moon, Titan, was discovered on March 25, 1655, by the Dutch astronomer Christiaan Huygens.[41][42]


In 1861, the brightest star in the night sky, Sirius, was found to have smaller stellar companion using the 18 and half-inch Dearborn refracting telescope.

By the 18th century refractors began to have major competition from reflectors, which could be made quite large and did not normally suffer from the same inherent problem with chromatic aberration. Nevertheless, the astronomical community continued to use doublet refractors of modest aperture in comparison to modern instruments. Noted discoveries include the Moons of Mars and a fifth moon of Jupiter, Amalthea.

Asaph Hall discovered Deimos on 12 August 1877 at about 07:48 UTC and Phobos on 18 August 1877, at the US Naval Observatory in Washington, D.C., at about 09:14 GMT (contemporary sources, using the pre-1925 astronomical convention that began the day at noon,[43] give the time of discovery as 11 August 14:40 and 17 August 16:06 Washington mean time respectively).[44][45][46]

The telescope used for the discovery was the 26-inch (66 cm) refractor (telescope with a lens) then located at Foggy Bottom.[47] In 1893 the lens was remounted and put in a new dome, where it remains into the 21st century.[48]

Jupiter's moon Amalthea was discovered on 9 September 1892, by Edward Emerson Barnard using the 36 inches (91 cm) refractor telescope at Lick Observatory.[49][50] It was discovered by direct visual observation with the doublet-lens refractor.[40]

In 1904, one of the discoveries made using Great Refractor of Potsdam (a double telescope with two doublets) was of the interstellar medium.[51] The astronomer Professor Hartmann determined from observations of the binary star Mintaka in Orion, that there was the element calcium in the intervening space.[51]


Planet Pluto was discovered by looking at photographs (i.e. 'plates' in astronomy vernacular) in a blink comparator taken with a refracting telescope, an astrograph with a 3 element 13-inch lens.[52][53]

List of the largest refracting telescopes[edit]

The Yerkes Great refractor mounted at the 1893 World's Fair in Chicago; the tallest, longest, and biggest aperture refractor up to that time.
The 68 cm (27 in) refractor at the Vienna University Observatory

Examples of some of the largest achromatic refracting telescopes, over 60 cm (24 in) diameter.

See also[edit]

Further reading[edit]


  1. ^ "Telescope Calculations". Northern Stars. Retrieved 20 December 2013.
  2. ^ Albert Van Helden, Sven Dupré, Rob van Gent, The Origins of the Telescope, Amsterdam University Press, 2010, pages 3-4, 15
  3. ^ Science, Lauren Cox 2017-12-21T03:30:00Z; Astronomy. "Who Invented the Telescope?". Space.com. Retrieved 26 October 2019.{{cite web}}: CS1 maint: numeric names: authors list (link)
  4. ^ Stephen G. Lipson, Ariel Lipson, Henry Lipson, Optical Physics 4th Edition, Cambridge University Press, ISBN 978-0-521-49345-1
  5. ^ http://upload.wikimedia.org/wikipedia/commons/1/17/Galileantelescope_2.png [bare URL image file]
  6. ^ a b "Galileo's telescope - The instrument". Museo Galileo: Institute and Museum of the History of Science. 2008. Retrieved 27 September 2020.
  7. ^ 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.
  8. ^ a b c "Galileo's telescope - How it works". Museo Galileo: Institute and Museum of the History of Science. 2008. Retrieved 27 September 2020.
  9. ^ Edgerton, S. Y. (2009). The Mirror, the Window, and the Telescope: How Renaissance Linear Perspective Changed Our Vision of the Universe. Ithaca: Cornell University Press. p. 159. ISBN 9780801474804.
  10. ^ Drake, S. (1978). Galileo at Work. Chicago: University of Chicago Press. p. 153. ISBN 978-0-226-16226-3.
  11. ^ "Phases of Venus". Intellectual Mathematics. 2 June 2019. Retrieved 27 September 2020.
  12. ^ Hevelius, Johannes (1673). Machina Coelestis. Vol. First Part. Auctor.
  13. ^ Tunnacliffe, AH; Hirst JG (1996). Optics. Kent, England. pp. 233–7. ISBN 978-0-900099-15-1.{{cite book}}: CS1 maint: location missing publisher (link)
  14. ^ "Galileo's telescope - Chromatic aberration". Museo Galileo - Istituto e Museo di Storia della Scienza. Retrieved 5 March 2012.
  15. ^ "The Project Gutenberg eBook of The Telescope, by Louis Bell, Ph.D." www.gutenberg.org.
  16. ^ "Largest optical telescopes of the world". www.stjarnhimlen.se.
  17. ^ Tromp, R. M. (December 2015). "An adjustable electron achromat for cathode lens microscopy". Ultramicroscopy. 159 Pt 3: 497–502. doi:10.1016/j.ultramic.2015.03.001. ISSN 1879-2723. PMID 25825026.
  18. ^ "Dollond Telescope". National Museum of American History. Retrieved 19 November 2019.
  19. ^ a b c d e f English, Neil (28 September 2010). Choosing and Using a Refracting Telescope. Springer Science & Business Media. ISBN 9781441964038.
  20. ^
  21. ^ Neil English, Choosing and Using a Refracting Telescope, Springer Science & Business Media - 2010, page 12
  22. ^ Lankford, John (7 March 2013). History of Astronomy: An Encyclopedia. Routledge. ISBN 9781136508349.
  23. ^ "Brashear House Historical Marker". ExplorePaHistory.com. WITF, Inc. Retrieved 16 November 2021.
  24. ^ "Cauchoix, Robert-Aglae". Canvases, Carats and Curiosities. 31 March 2015. Retrieved 26 October 2019.
  25. ^ Ferguson, Kitty (20 March 2014). "The Glassmaker Who Sparked Astrophysics". Nautilus. Retrieved 26 October 2019.
  26. ^ Lequeux, James (15 March 2013). Le Verrier—Magnificent and Detestable Astronomer. Springer Science & Business Media. ISBN 978-1-4614-5565-3.
  27. ^ "1949PA.....57...74K Page 75". articles.adsabs.harvard.edu. Retrieved 19 November 2019.
  28. ^ "Sheepshanks telescope". UK: Royal Museums Greenwich. Retrieved 27 February 2014.
  29. ^ Tombaugh, Clyde W.; Moore, Patrick (15 September 2017). Out of the Darkness: The Planet Pluto. Stackpole Books. ISBN 9780811766647.
  30. ^ Astronomical Observations, Made at the Royal Observatory at Greenwich, ... Clarendon Press. 1840.
  31. ^ a b "Griffith Observatory - Southern California's gateway to the cosmos!". Griffith Observatory.
  32. ^ "1914Obs....37..245H Page 248". articles.adsabs.harvard.edu.
  33. ^ "Apochromat - an overview | ScienceDirect Topics". www.sciencedirect.com. Retrieved 17 October 2022.
  34. ^ "Starizona's Guide to CCD Imaging". Starizona.com. Archived from the original on 17 October 2013. Retrieved 17 October 2013.
  35. ^ Kidger, Michael J. (2002). Fundamental Optical Design. SPIE Press. ISBN 9780819439154.
  36. ^ Vasiljevic, Darko (6 December 2012). Classical and Evolutionary Algorithms in the Optimization of Optical Systems. Springer Science & Business Media. ISBN 9781461510512.
  37. ^ a b Vasiljević, Darko (2002), "The Cooke triplet optimizations", in Vasiljević, Darko (ed.), Classical and Evolutionary Algorithms in the Optimization of Optical Systems, Springer US, pp. 187–211, doi:10.1007/978-1-4615-1051-2_13, ISBN 9781461510512
  38. ^ Stan Gibilisco (2002). Physics Demystified. Mcgraw-hill. p. 532. ISBN 978-0-07-138201-4.
  39. ^ "Voyager". astronautix.com. Archived from the original on 11 September 2016.
  40. ^ a b Bakich M. E. (2000). The Cambridge Planetary Handbook. Cambridge University Press. pp. 220–221. ISBN 9780521632805.
  41. ^ "Lifting Titan's Veil" (PDF). Cambridge. p. 4. Archived from the original (PDF) on 22 February 2005.
  42. ^ "Titan". Astronomy Picture of the Day. NASA. Archived from the original on 27 March 2005.
  43. ^ Campbell, W.W. (1918). "The Beginning of the Astronomical Day". Publications of the Astronomical Society of the Pacific. 30 (178): 358. Bibcode:1918PASP...30..358C. doi:10.1086/122784.
  44. ^ "Notes: The Satellites of Mars". The Observatory, Vol. 1, No. 6. 20 September 1877. pp. 181–185. Retrieved 12 September 2006.
  45. ^ Hall, A. (17 October 1877). "Observations of the Satellites of Mars" (Signed 21 September 1877). Astronomische Nachrichten, Vol. 91, No. 2161. pp. 11/12–13/14. Retrieved 12 September 2006.
  46. ^ Morley, T. A.; A Catalogue of Ground-Based Astrometric Observations of the Martian Satellites, 1877-1982, Astronomy and Astrophysics Supplement Series, Vol. 77, No. 2 (February 1989), pp. 209–226 (Table II, p. 220: first observation of Phobos on 1877-08-18.38498)
  47. ^ "Telescope: Naval Observatory 26-inch Refractor". amazing-space.stsci.edu. Retrieved 29 October 2018.
  48. ^ "The 26-inch "Great Equatorial" Refractor". United States Naval Observatory. Retrieved 29 October 2018.
  49. ^ Barnard, E. E. (12 October 1892). "Discovery and observations of a fifth satellite to Jupiter". The Astronomical Journal. 12 (11): 81–85. Bibcode:1892AJ.....12...81B. doi:10.1086/101715.
  50. ^ Lick Observatory (1894). A Brief Account of the Lick Observatory of the University of California. The University Press. p. 7–.
  51. ^ a b Kanipe, Jeff (27 January 2011). The Cosmic Connection: How Astronomical Events Impact Life on Earth. Prometheus Books. ISBN 9781591028826.
  52. ^ "The Pluto Telescope". Lowell Observatory. Retrieved 19 November 2019.
  53. ^ "Pluto Discovery Plate". National Air and Space Museum. Retrieved 19 November 2019.
  54. ^ "John Wall refractor | Hanwell Community Observatory".

External links[edit]