Refracting telescope

Last updated
A 200 mm refracting telescope at the Poznan Observatory Zeiss2.jpg
A 200 mm 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 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were very popular in the second half of the 19th century, for most research purposes the refracting telescope has been superseded by the reflecting telescope which allows larger apertures.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]


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 telescope is a 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


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

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 Galileantelescope.png
Optical diagram of Galilean telescopey – 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

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

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

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[ dubious ] 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 (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. [9]

Achromatic refractors

Alvan Clark polishes the big Yerkes achromatic objective lens, over 1 meter across, in 1896. Yerkes Observatory Astro4p6.jpg
Alvan Clark polishes the big Yerkes achromatic objective lens, over 1 meter across, in 1896.
This 12 inch refractor is mounted in dome and a mount the rotates with the turn of the Earth Irving Porter Church Telescope.jpg
This 12 inch refractor is mounted in dome and a mount the rotates with the turn of the Earth

The next major step in the evolution of refracting telescopes was the invention of the achromatic lens , a lens with multiple elements to 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. [10]

Dollond achromats were quite popular in the 18th century. [11] [12] A major appeal was they could be made shorter. [12] However, problems with glass making meant that the glass objectives were not made more than about four inches in diameter. [12]

In the late 19th Century the Glass maker Guinand developed a way to make larger, higher quality glass blanks greater than four inches. [12] He had also passed this technology to his apprentice Fraunhofer, whom further developed this technology and also developed the Fraunhofer doublet lens design. [12] The breakthrough in glass making techniques lead 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: [13]

The Greenwich 28-inch refractor is a popular tourist attraction in the 21st century in London The 28-inch Telescope.jpg
The Greenwich 28-inch refractor is a popular tourist attraction in the 21st century in 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. [19] The Sheepshanks had a 6.7 inch (17 cm) wide lens, and was the biggest telescope at Greenwich for about twenty years. [20]

An 1840 report from the Observatory noted of the then-new Sheepshanks telescope with the Cauchoix doublet: [21]

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. [22] 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 opening in 1935; this is the most people to have viewed through any telescope. [22]

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 then 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 years. [23]

Apochromatic refractors

The Apochromatic lens usually comprises three elements that bring light of three different frequencies to a common focus Apochromat.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 down to an order of magnitude less than that of an achromatic lens.[ citation needed ] 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. [24] 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. [12]

One of the famous triplet objectives, is the Cooke triplet, noted for being able to correct the Seidal aberrations. [25] It is recognized as one of the most important objective designs in the field of photography. [26] [27] The Cooke triplet can correct, with only three elements, for one wavelength, spherical aberration, coma, astigmatism, field curvature, and distortion. [27]

Technical considerations

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) Yerkes Observatory Astro4p7.jpg
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). [28]

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. [29]

Applications & Achievements

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. Great Refractor Potsdam.jpg
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 Jessica Meir Photography Training.jpg
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 really famous applications of refracting telescope, was when Galileo used it for astronomy; he discovered the four largest moons of Jupiter in 1609. Furthermore, early refractors were also used to discover the largest moon of Saturn, Titan, several decades later as well as 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. There modest apertures did not lead to as many discoveries and typically so small in aperture many astronomical objects were simply not observeable until the advent of long-exposure photograph, by which time the reputation and quirks of reflecting telescopes was 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.


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. [30]

The planet Saturn's moon, Titan, was discovered on March 25, 1655, by the Dutch astronomer Christiaan Huygens. [31] [32]

Doublets 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, [33] give the time of discovery as 11 August 14:40 and 17 August 16:06 Washington mean time respectively). [34] [35] [36]

The telescope used for the discovery was the 26-inch (66 cm) refractor (telescope with a lens) then located at Foggy Bottom. [37] In 1893 the lens was remounted and put in a new dome, where it remains into the 21st century. [38]

Jupiter's moon Amalthea was discovered on 9 September 1892, by Edward Emerson Barnard using the 36 inch (91 cm) refractor telescope at Lick Observatory. [39] [40] It was discovered by direct visual observation with the doublet-lens refractor. [30]

In 1904, one of the discoveries made using Great Refractor of Potsdam (a double telescope with two doublets) was of the interstellar medium. [41] The astronomer Professor Hartmann determined from observations of the binary star Mintaka in Orion, that there was the element calcium in the intervening space. [41]


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. [42] [43]

List of the largest refracting telescopes

The Yerkes Great refractor mounted at the 1893 World's Fair in Chicago; the tallest, longest, and biggest aperture refactor up to that time. Chicago's Great Telescope (3573567148).jpg
The Yerkes Great refractor mounted at the 1893 World's Fair in Chicago; the tallest, longest, and biggest aperture refactor up to that time.
The 68 cm (27 in) refractor at the Vienna University Observatory Refraktor Wien Kerschbaum 1.jpg
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

Further reading

Related Research Articles

Chromatic aberration Failure of a lens to focus all colors on the same point

In optics, chromatic aberration (CA), also called chromatic distortion and spherochromatism, is a failure of a lens to focus all colors to the same point. It is caused by dispersion: the refractive index of the lens elements varies with the wavelength of light. The refractive index of most transparent materials decreases with increasing wavelength. Since the focal length of a lens depends on the refractive index, this variation in refractive index affects focusing. Chromatic aberration manifests itself as "fringes" of color along boundaries that separate dark and bright parts of the image.

Achromatic lens optical instrument corrected for dispersion at 2 wavelengths

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 into focus on the same plane.

Yerkes Observatory Astronomical observatory in Wisconsin

Yerkes Observatory is an astronomical observatory located in Williams Bay, Wisconsin, U.S.A. It was operated by the University of Chicago Department of Astronomy and Astrophysics. 1897-2020. Ownership was transferred to the non-profit Yerkes Future Foundation (YFF) in May, 2020.

History of the telescope aspect of history

The history of the telescope can be traced to before the invention of the earliest known telescope, which appeared in 1608 in the Netherlands, when a patent was submitted by Hans Lippershey, an eyeglass maker. Although Lippershey did not receive his patent, news of the invention soon spread across Europe. The design of these early refracting telescopes consisted of a convex objective lens and a concave eyepiece. Galileo improved on this design the following year and applied it to astronomy. In 1611, Johannes Kepler described how a far more useful telescope could be made with a convex objective lens and a convex eyepiece lens. By 1655, astronomers such as Christiaan Huygens were building powerful but unwieldy Keplerian telescopes with compound eyepieces.

Optical telescope Telescope for observations with visible light

An optical telescope is a telescope that gathers and focuses light, mainly from the visible part of the electromagnetic spectrum, to create a magnified image for direct view, or to make a photograph, or to collect data through electronic image sensors.

Newtonian telescope

The Newtonian telescope, also called the Newtonian reflector or just the Newtonian, is a type of reflecting telescope invented by the English scientist Sir Isaac Newton (1642–1727), using a concave primary mirror and a flat diagonal secondary mirror. Newton's first reflecting telescope was completed in 1668 and is the earliest known functional reflecting telescope. The Newtonian telescope's simple design has made it very popular with amateur telescope makers.

Apochromat type of photographic or other lens

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.

Eyepiece Type of lens attached to a variety of optical devices such as telescopes and microscopes

An eyepiece, or ocular lens, is a type of lens that is attached to a variety of optical devices such as telescopes and microscopes. It is so named because it is usually the lens that is closest to the eye when someone looks through the device. The objective lens or mirror collects light and brings it to focus creating an image. The eyepiece is placed near the focal point of the objective to magnify this image. The amount of magnification depends on the focal length of the eyepiece.

Markree Observatory

Markree Observatory was an astronomical observatory in County Sligo, Ireland. The asteroid 9 Metis was discovered from this observatory in 1848 by Cooper's assistant Andrew Graham using a comet seeker telecope. The observatory was also home to the largest refractor of the early 1830s— an over 13-inch aperture Cauchoix of Paris lens; the largest in the world at that time. The observatory also housed a number of instruments and was operated to varying degrees throughout the 19th century.

Telescope Optical instrument that makes distant objects appear magnified

A telescope is an optical instrument that makes 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 were used for both terrestrial applications and astronomy.

Newtons reflector

The first reflecting telescope built by Sir Isaac Newton in 1668 is a landmark in the history of telescopes, being the first known successful reflecting telescope. It was the prototype for a design that later came to be called the Newtonian telescope. There were some early prototypes and also modern replicas of this design.

Aerial telescope very long focal length refracting telescope without a tube, built in the 17th century; the objective was mounted on a pole, tree, etc. on a swivel ball-joint; the observer stood on the ground and maneuvered the eyepiece to aim at celestial objects

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 swivel ball-joint. The observer stood on the ground and held the eyepiece, which was connected to the objective by a string or connecting rod. By holding the string tight and maneuvering the eyepiece, the observer could aim the telescope at objects in the sky. The idea for this type of telescope may have originated in the late 17th century with the Dutch mathematician, astronomer and physicist Christiaan Huygens and his brother Constantijn Huygens, Jr., though it is not clear if they actually invented it.


The Galileoscope is a small mass-produced refractor telescope, designed with the intention of increasing public interest in astronomy and science. It was developed for the International Year of Astronomy 2009. The small telescope has an aperture of 50 mm and a relatively long focal length of 500 mm, for a focal ratio of f/10.

Great refractor Wikimedia disambiguation page

Great refractor refers to a large telescope with a lens, usually the largest refractor at an observatory with an equatorial mount. 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 telescopes using achromatic lenses. 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.

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, 3rd Earl of Rosse, famous for the Leviathan of Parsonstown, a reflecting telescope and the largest telescope of this age with a six foot mirror.

Robert-Aglaé Cauchoix Scientific instrument and telescope maker

Robert-Aglaé Cauchoix was a French optician and instrument maker, whose lenses played a part in the race of the great refractor telescopes in the first half of the 19th century.

Meudon Great Refractor

Meudon Great Refractor is a double telescope with lenses, in Meudon, France. It is a twin refracting telescope built in 1891, with one visual and one photographic, on a single square-tube together on an equatorial mount, inside a dome. The Refractor was built for the Meudon Observatory, and is the largest double doublet refracting telescope in Europe, but about the same size as several telescopes in this period, when this style of telescope was popular. Other large telescopes of a similar type include the James Lick telescope (91.4), Potsdam Great Refractor (80+50 cm), and the Greenwich 28 inch refractor (71.1 cm).


  1. "Telescope Calculations". Northern Stars. Retrieved 2013-12-20.
  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?". Retrieved 2019-10-26.
  4. Stephen G. Lipson, Ariel Lipson, Henry Lipson, Optical Physics 4th Edition, Cambridge University Press, ISBN   978-0-521-49345-1
  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   978-0-900099-15-1.
  9. Paul Schlyter, Largest optical telescopes of the world
  10. 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.
  11. "Dollond Telescope". National Museum of American History. Retrieved 2019-11-19.
  12. 1 2 3 4 5 6 English, Neil (2010-09-28). Choosing and Using a Refracting Telescope. Springer Science & Business Media. ISBN   9781441964038.
  13. Lankford, John (2013-03-07). History of Astronomy: An Encyclopedia. Routledge. ISBN   9781136508349.
  14. "Cauchoix, Robert-Aglae". Canvases, Carats and Curiosities. 2015-03-31. Retrieved 2019-10-26.
  15. Ferguson, Kitty (2014-03-20). "The Glassmaker Who Sparked Astrophysics". Nautilus. Retrieved 2019-10-26.
  16. Lequeux, James (2013-03-15). Le Verrier—Magnificent and Detestable Astronomer. Springer Science & Business Media. ISBN   978-1-4614-5565-3.
  17. "1949PA.....57...74K Page 75". Retrieved 2019-11-19.
  18. "Sheepshanks telescope". UK: Royal Museums Greenwich . Retrieved 27 February 2014.
  19. Tombaugh, Clyde W.; Moore, Patrick (2017-09-15). Out of the Darkness: The Planet Pluto. Stackpole Books. ISBN   9780811766647.
  20. Astronomical Observations, Made at the Royal Observatory at Greenwich, ... Clarendon Press. 1840.
  21. 1 2
  22. The Observatory, "Large Telescopes", Page 248
  23. "Starizona's Guide to CCD Imaging". Retrieved 17 October 2013.
  24. Kidger, Michael J. (2002). Fundamental Optical Design. SPIE Press. ISBN   9780819439154.
  25. Vasiljevic, Darko (2012-12-06). Classical and Evolutionary Algorithms in the Optimization of Optical Systems. Springer Science & Business Media. ISBN   9781461510512.
  26. 1 2 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
  27. Stan Gibilisco (2002). Physics Demystified . Mcgraw-hill. p.  532. ISBN   978-0-07-138201-4.
  28. "Voyager".
  29. 1 2 Bakich M. E. (2000). The Cambridge Planetary Handbook. Cambridge University Press. pp. 220–221. ISBN   9780521632805.
  30. "Lifting Titan's Veil" (PDF). Cambridge. p. 4. Archived from the original (PDF) on February 22, 2005.
  31. "Titan". Astronomy Picture of the Day. NASA. Archived from the original on March 27, 2005.
  32. 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 .
  33. "Notes: The Satellites of Mars". The Observatory, Vol. 1, No. 6. 20 September 1877. pp. 181–185. Retrieved 12 September 2006.
  34. 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.
  35. 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)
  36. "Telescope: Naval Observatory 26-inch Refractor". Retrieved 29 October 2018.
  37. "The 26-inch "Great Equatorial" Refractor". United States Naval Observatory. Retrieved 29 October 2018.
  38. Barnard 1892.
  39. Lick Observatory (1894). A Brief Account of the Lick Observatory of the University of California. The University Press. p. 7–.
  40. 1 2 Kanipe, Jeff (2011-01-27). The Cosmic Connection: How Astronomical Events Impact Life on Earth. Prometheus Books. ISBN   9781591028826.
  41. "The Pluto Telescope". Lowell Observatory. Retrieved 2019-11-19.
  42. "Pluto Discovery Plate". National Air and Space Museum. Retrieved 2019-11-19.