Editing Refracting telescope

{{Short description|Type of optical telescope}}
{{Redirect-distinguish|Keplerian telescope|Kepler space telescope}}
{{Use dmy dates|date=September 2020}}
[[File:Zeiss2.jpg|thumb|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 (optics)|lens]] as its [[objective (optics)|objective]] to form an image (also referred to a '''[[dioptrics|dioptric]] [[telescope]]'''). The refracting telescope design was originally used in spyglasses and [[astronomy|astronomical]] telescopes but is also used for [[long-focus lens|long-focus]] [[camera lens]]es. 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were very popular in the second half of the 19th century, for most research purposes, the refracting telescope has been superseded by the [[reflecting telescope]], which allows larger [[aperture]]s. A refractor's [[magnification]] is calculated by dividing the focal length of the objective lens by that of the [[eyepiece]].<ref>{{cite web |url=http://www.northern-stars.com/tel_magnification.htm |title=Telescope Calculations |publisher=Northern Stars |access-date=2013-12-20}}</ref>

Refracting telescopes typically have a lens at the front, then a [[optical train|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 lens]]es/[[telephoto lens]]/[[long-focus lens]].

==Invention==
{{Main|History of the telescope}}
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, Zeeland|Middelburg]] named [[Hans Lippershey]] unsuccessfully tried to patent one.<ref>Albert Van Helden, Sven Dupré, Rob van Gent, The Origins of the Telescope, Amsterdam University Press, 2010, pages 3-4, 15</ref> 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 [[Galilean telescope|a version of his own]], and applied it to making astronomical discoveries.<ref name=":0">{{Cite web|url=https://www.space.com/21950-who-invented-the-telescope.html|title=Who Invented the Telescope?|last1=Science|first1=Lauren Cox 2017-12-21T03:30:00Z|last2=Astronomy|website=Space.com|access-date=2019-10-26}}</ref>

==Refracting telescope designs==<!-- This section is linked from [[Binoculars]] -->
[[Image:Kepschem.png|thumb|400px|right]]
All refracting telescopes use the same principles. The combination of an [[objective (optics)|objective]] [[lens (optics)|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 [[brightness|brighter]], [[wikt:clarity|clearer]], and [[magnification|magnified]] [[virtual image]] '''6'''.

The objective in a refracting telescope [[refraction|refracts]] or bends [[light]]. This refraction causes [[Parallel (geometry)|parallel]] light rays to converge at a [[Focus (optics)|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.<ref>Stephen G. Lipson, Ariel Lipson, Henry Lipson, ''Optical Physics 4th Edition'', Cambridge University Press, {{ISBN|978-0-521-49345-1}}</ref>

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===
<!--- The article Galileo Galilei links here. Please do not change the title of this subsection without making the appropriate amendments to linking articles --->

{{stack|[[Image:Galileantelescope.png|thumb|400px| '''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'''
]]}}
The design [[Galileo Galilei]] used {{Circa|1609}} is commonly called a '''Galilean telescope'''.<ref name=":3">{{Cite web|date=2008|title=Galileo's telescope - The instrument|url=https://brunelleschi.imss.fi.it/esplora/cannocchiale/dswmedia/esplora/eesplora1.html|access-date=2020-09-27|website=Museo Galileo: Institute and Museum of the History of Science}}</ref> It used a convergent (plano-convex) objective lens and a divergent (plano-concave) eyepiece lens (Galileo, 1610).<ref>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}}.</ref> A Galilean telescope, because the design has no intermediary focus, results in a non-inverted (i.e., upright) image.<ref name=":6">{{Cite web|date=2008|title=Galileo's telescope - How it works|url=https://brunelleschi.imss.fi.it/esplora/cannocchiale/dswmedia/esplora/eesplora2.html|access-date=2020-09-27|website=Museo Galileo: Institute and Museum of the History of Science}}</ref>

Galileo's most powerful telescope, with a total length of just under {{convert|1|m|in|spell=us}},<ref name=":3" /> [[Magnification|magnified]] objects about 30 times.<ref name=":6" /> 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.<ref name=":6" /> Despite these flaws, the telescope was still good enough for Galileo to explore the sky. He used it to view [[impact crater|craters]] on the [[Moon]],<ref>{{cite book|last=Edgerton|first=S. Y.|title=The Mirror, the Window, and the Telescope: How Renaissance Linear Perspective Changed Our Vision of the Universe|date=2009|publisher=Cornell University Press|isbn=9780801474804|location=Ithaca|page=159}}</ref> the four [[Galilean moons|largest moons of Jupiter]],<ref>{{cite book|last=Drake|first=S.|title=Galileo at Work|title-link=:IArchive:galileoatwork00stil|date=1978|publisher=University of Chicago Press|isbn=978-0-226-16226-3|location=Chicago|pages=153}}</ref> and the [[phases of Venus]].<ref>{{Cite web|date=2019-06-02|title=Phases of Venus|url=http://intellectualmathematics.com/blog/phases-of-venus/|access-date=2020-09-27|website=Intellectual Mathematics|language=en-US}}</ref>

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.{{fact|date=September 2024}}

The final image ('''y″''') is a virtual image, located at infinity and is the same way up (i.e., non-inverted or upright) as the object.{{fact|date=September 2024}}

===Keplerian telescope===
[[File:Houghton Typ 620.73.451 - Johannes Hevelius, Machinae coelestis, 1673.jpg|thumb|right|Engraved illustration of a {{convert|150|ft|m|abbr=on|order=flip}} focal length Keplerian astronomical refracting telescope built by Johannes Hevelius.<ref>{{cite book|first=Johannes |last=Hevelius |title=Machina Coelestis |volume=First Part |date=1673 |publisher=Auctor}}</ref>]]
The '''Keplerian telescope''', invented by [[Johannes Kepler]] in 1611, is an improvement on Galileo's design.<ref>{{cite book |title= Optics|last= Tunnacliffe|first= AH |author2=Hirst JG |date= 1996|location= Kent, England |isbn= 978-0-900099-15-1|pages= 233–7}}</ref> 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|date=November 2019}} 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 a simple single element objective lens so it needs to have a very high focal ratio to reduce aberrations<ref>{{cite web|title=Galileo's telescope - Chromatic aberration|url=http://brunelleschi.imss.fi.it/esplora/cannocchiale/dswmedia/storia/estoria5_st.html|publisher=Museo Galileo - Istituto e Museo di Storia della Scienza|accessdate=5 March 2012}}</ref> ([[Johannes Hevelius]] built an unwieldy f/225 telescope with a {{convert|8|in|mm|abbr=out|adj=on|order=flip}} objective and a {{convert|150|ft|m|abbr=out|adj=on|order=flip}} [[focal length]],<ref>{{Cite book |last=Bell |first=Louis |url=https://www.gutenberg.org/files/53740/53740-h/53740-h.htm |title=The Telescope |publisher=McGraw-Hill |year=1922 |location=New York |via=The Project Gutenberg}}</ref>{{page needed|date=July 2024}} and even longer tubeless "[[aerial telescope]]s" were constructed). The design also allows for use of a [[Filar micrometer#Prior devices|micrometer]] at the focal plane (to determine the angular size and/or distance between objects observed).

[[Constantijn Huygens Jr.|Huygens]] built an aerial telescope for [[Royal Society of London]] with a 19&nbsp;cm (7.5″)  single-element lens.<ref name="stjarnhimlen.se">{{Cite web|url=http://www.stjarnhimlen.se/bigtel/LargestTelescope.html|title=Largest optical telescopes of the world|website=www.stjarnhimlen.se}}</ref>

===Achromatic refractors===
{{Main|Achromatic telescope}}
{{stack|
[[File:Yerkes Observatory Astro4p6.jpg|thumb|Alvan Clark polishes the big Yerkes achromatic objective lens, over {{convert|1|m|cm|spell=us}} across (1896).]]
[[File:Irving Porter Church Telescope.jpg|thumb|This {{convert|12|in|cm|adj=on}} 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 (optics)|dispersion]], '[[Crown glass (optics)|crown]]' and '[[flint glass]]', to reduce [[chromatic aberration|chromatic]] and [[spherical aberration]]. Each side of each piece is ground and [[polishing|polish]]ed, and then the two pieces are assembled together. Achromatic lenses are corrected to bring two [[wavelength]]s (typically red and blue) into focus in the same plane.{{fact|date=September 2024}}

Chester More Hall is noted as having made the first twin color corrected lens in 1730.<ref>{{cite journal |last1=Tromp |first1=R.M. |title=An adjustable electron achromat for cathode lens microscopy |journal=Ultramicroscopy |date=December 2015 |volume=159 |pages=497–502 |doi=10.1016/j.ultramic.2015.03.001 |pmid=25825026 }}</ref>

Dollond achromats were quite popular in the 18th century.<ref>{{Cite web|url=https://americanhistory.si.edu/collections/search/object/nmah_1250617|title=Dollond Telescope|website=National Museum of American History|access-date=2019-11-19}}</ref><ref name=":4">{{cite book |doi=10.1007/978-1-4419-6403-8_1 |chapter=The Refracting Telescope: A Brief History |title=Choosing and Using a Refracting Telescope |series=Patrick Moore's Practical Astronomy Series |date=2011 |last1=English |first1=Neil |pages=3–20 |isbn=978-1-4419-6402-1 }}</ref> A major appeal was they could be made shorter.<ref name=":4"/> However, problems with glass making meant that the glass objectives were not made more than about {{convert|4|in|cm|spell=in}} in diameter.<ref name=":4"/>

In the late 19th century, the Swiss optician Pierre-Louis Guinand<ref>
*[[:de:Pierre-Louis Guinand|Pierre-Louis Guinand]] was a Swiss who in the late 1700s came up with a breakthrough for making better quality and larger glass, and in time went on to teach [[Joseph von Fraunhofer]] at Utzschinder's (Joseph von Utzschneider (1763-1840<!-- {{cite web |title=Utzschneider und Fraunhofer |url=https://collection.sciencemuseumgroup.org.uk/people/cp26931/utzschneider-und-fraunhofer |website=Science Museum Group Collection |access-date=2023-11-12 |language=en}} -->) glassworks, and eventually started his own optical glass works.
*{{Cite book|url=https://books.google.com/books?id=KAWwzHlDVksC&pg=PA176 |title=The History of the Telescope|last=King|first=Henry C.|date=2003-01-01|publisher=Courier Corporation|isbn=9780486432656|language=en}}</ref> developed a way to make higher quality glass blanks of greater than {{convert|4|in|cm|spell=in}}.<ref name=":4"/> He passed this technology to his apprentice [[Joseph von Fraunhofer]], who further developed this technology and also developed the Fraunhofer doublet lens design.<ref name=":4" /> 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.{{fact|date=September 2024}}

Noted lens makers of the 19th century include:<ref>{{Cite book|url=https://books.google.com/books?id=9jyExgmZxBoC&q=Greenwich+28+inch+refractor&pg=PA520|title=History of Astronomy: An Encyclopedia|last=Lankford|first=John|date=2013-03-07|publisher=Routledge|isbn=9781136508349}}</ref>

{{stack|[[File:The 28-inch Telescope.jpg|thumb|The Greenwich {{convert|28|in|cm|adj=on}} refractor is a popular tourist attraction in 21st century London.]]}}
*[[Alvan Clark & Sons|Alvan Clark]]
*Brashear<ref>{{cite web |title=Brashear House Historical Marker |url=https://explorepahistory.com/hmarker.php?markerId=1-A-38A |website=ExplorePaHistory.com |publisher=WITF, Inc. |access-date=16 November 2021}}</ref>
*[[Chance Brothers]]
*[[Robert-Aglaé Cauchoix|Cauchoix]]<ref>{{Cite web|url=https://www.rauantiques.com/blog/cauchoix-robert-aglae-2/|title=Cauchoix, Robert-Aglae|date=2015-03-31|website=Canvases, Carats and Curiosities|access-date=2019-10-26}}</ref>
*[[Joseph von Fraunhofer|Fraunhofer]]<ref>{{Cite web|url=http://nautil.us/issue/11/light/the-glassmaker-who-sparked-astrophysics|title=The Glassmaker Who Sparked Astrophysics|last=Ferguson|first=Kitty|date=2014-03-20|website=Nautilus|access-date=2019-10-26}}</ref>
* Gautier 
*[[Grubb Parsons|Grubb]] 
*[[Paul Henry and Prosper Henry|Henry Brothers]]
*Lerebours<ref name=":42">{{cite book |doi=10.1007/978-1-4614-5565-3_4 |chapter=The Observatory: At Last! |title=Le Verrier—Magnificent and Detestable Astronomer |series=Astrophysics and Space Science Library |date=2013 |last1=Lequeux |first1=James |volume=397 |pages=77–125 |isbn=978-1-4614-5564-6 }}</ref>
*Tulley<ref>{{cite journal |last1=King |first1=H. C. |title=The optical work of Charles Tulley |journal=Popular Astronomy |date=January 1949 |volume=57 |pages=74 |bibcode=1949PA.....57...74K }}</ref>

Some famous 19th century doublet refractors are the [[James Lick telescope]] (91&nbsp;cm/36 in) and the [[Greenwich 28 inch refractor]] (71&nbsp;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 refractor]]s' in the 19th century saw large achromatic lenses, culminating with the largest achromatic refractor ever built, the [[Great Paris Exhibition Telescope of 1900]].{{fact|date=September 2024}}

In the [[Royal Observatory, Greenwich]] an 1838 instrument named the [[Sheepshanks equatorial|Sheepshanks telescope]] includes an objective by Cauchoix.<ref>{{cite web|url=http://collections.rmg.co.uk/collections/objects/11074.html|title=Sheepshanks telescope|publisher=[[Royal Museums Greenwich]]|location=UK|access-date=2014-02-27}}</ref> The Sheepshanks had a {{convert|6.7|in|cm|adj=on}} wide lens, and was the biggest telescope at Greenwich for about twenty years.<ref>{{cite book |last1=Tombaugh |first1=Clyde W. |last2=Moore |first2=Patrick |title=Out of the Darkness: The Planet Pluto |date=2017 |publisher=Stackpole Books |isbn=978-0-8117-6664-7 |page=56 |url=https://books.google.com/books?id=nP01DwAAQBAJ&q=Sheepshanks+refractor+6.7+inch&pg=PA56 }}</ref>

An 1840 report from the Observatory noted of the then-new Sheepshanks telescope with the Cauchoix doublet:<ref name=":32">{{cite book |title=Astronomical Observations, Made at the Royal Observatory at Greenwich, in the year 1838 |date=1840 |publisher=Clarendon Press |hdl=2027/njp.32101074839562 }}{{page needed|date=July 2024}}</ref>{{cquote|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.<ref name=":2">{{Cite web|url=https://griffithobservatory.org/|title=Griffith Observatory - Southern California's gateway to the cosmos!|website=Griffith Observatory}}</ref> 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.<ref name=":2" />

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]].{{fact|date=September 2024}}

The long achromats, despite having smaller aperture than the larger reflectors, were often favored for "prestige" observatories. In the late 18th century, every few years, a larger and longer refractor would debut.{{fact|date=September 2024}}

For example, the Nice Observatory debuted with {{convert|77|cm|in|abbr=out|adj=on|sigfig=4|spell=us}} refractor, the largest at the time, but was surpassed within only a couple of years.<ref name=list1914>{{cite journal |last1=Hollis |first1=H. P. |title=Large telescopes |journal=The Observatory |date=June 1914 |volume=37 |pages=245–252 |bibcode=1914Obs....37..245H }}</ref>

===Apochromatic refractors===
{{Main|Apochromat}}
[[File:Apochromat.svg|alt=Apochromat lens.svg|thumb|right|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.{{citation needed|date=July 2024}} 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.<ref>{{cite web |url=http://starizona.com/acb/ccd/equipbasicsref.aspx |title=Starizona's Guide to CCD Imaging |publisher=Starizona.com |access-date=17 October 2013 |archive-date=17 October 2013 |archive-url=https://web.archive.org/web/20131017204031/http://starizona.com/acb/ccd/equipbasicsref.aspx |url-status=dead }}</ref> 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.<ref name=":4"/>

One of the famous triplet objectives is the [[Cooke triplet]], noted for being able to correct the Seidal aberrations.<ref>{{Cite book|url=https://books.google.com/books?id=mberzJtkU4MC&q=%22Cooke+triplet%22+-wikipedia&pg=PA199|title=Fundamental Optical Design|last=Kidger|first=Michael J.|date=2002|publisher=SPIE Press|isbn=9780819439154}}</ref> It is recognized as one of the most important [[Objective (optics)|objective]] designs in the field of photography.<ref>{{Cite book|url=https://books.google.com/books?id=czjUBwAAQBAJ&q=%22Cooke+triplet%22+-wikipedia&pg=PA187|title=Classical and Evolutionary Algorithms in the Optimization of Optical Systems|last=Vasiljevic|first=Darko|date=2012-12-06|publisher=Springer Science & Business Media|isbn=9781461510512}}</ref><ref name=":1">{{Citation|last=Vasiljević|first=Darko|chapter=The Cooke triplet optimizations|date=2002|pages=187–211|editor-last=Vasiljević|editor-first=Darko|publisher=Springer US|doi=10.1007/978-1-4615-1051-2_13|isbn=9781461510512|title=Classical and Evolutionary Algorithms in the Optimization of Optical Systems}}</ref> The Cooke triplet can correct, with only three elements, for one wavelength, [[spherical aberration]], [[Coma (optics)|coma]], [[Astigmatism (optical systems)|astigmatism]], [[field curvature]], and [[Distortion (optics)|distortion]].<ref name=":1" />

== Technical considerations ==
[[File:Yerkes Observatory Astro4p7.jpg|thumb|right|The {{convert|102|cm|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 aberration|chromatic]] and [[spherical aberration]]. This affects shorter [[F-number#Focal ratio|focal ratio]]s more than longer ones. An {{f/|6|link=yes}} achromatic refractor is likely to show considerable color fringing (generally a purple halo around bright objects); an {{f/}}16 achromat has much less color fringing.

In very large apertures, there is also a problem of [[lens sag]]ging, 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 {{convert|1|m|in|abbr=out|sp=us}}.<ref>{{cite book|author=Stan Gibilisco|title=Physics Demystified|url=https://archive.org/details/physicsdemystifi0000gibi|url-access=registration|year=2002|publisher=Mcgraw-hill|isbn=978-0-07-138201-4|page=[https://archive.org/details/physicsdemystifi0000gibi/page/532 532]}}</ref>

There is a further problem of glass defects, striae or small [[air bubble]]s trapped within the glass. In addition, glass is [[Opacity (optics)|opaque]] to certain [[wavelength]]s, and even [[optical spectrum|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 telescope]]s, 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]]/[[Voyager 2|2]] used a {{convert|6|cm|in}} lens, launched into space in the late 1970s, an example of the use of refractors in space.<ref name=astrov>{{cite web|url=http://www.astronautix.com/v/voyager.html|archive-url=https://web.archive.org/web/20160911011643/http://www.astronautix.com/v/voyager.html|url-status=dead|archive-date=11 September 2016|title=Voyager|work=astronautix.com}}</ref>

==Applications and achievements==
[[Image:Great Refractor Potsdam.jpg|thumb|right|300px| 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.]]
[[File:Jessica Meir Photography Training.jpg|thumb|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 observable 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). Refractors were often used for positional astronomy, besides from the other uses in photography and terrestrial viewing.
[[File:Spyglass.jpg|thumb|left|Touristic telescope pointed to Matterhorn in Switzerland]]

;Singlets
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.<ref name="Bakich_2000">{{cite book |author=Bakich M. E. |year=2000 |url=https://books.google.com/books?id=PE99nOKjbXAC&q=Amalthea&pg=PA221 |title=The Cambridge Planetary Handbook |publisher=Cambridge University Press |pages=220–221 |isbn=9780521632805}}</ref>

The planet Saturn's moon, [[Titan (moon)|Titan]], was discovered on March 25, 1655, by the Dutch astronomer [[Christiaan Huygens]].<ref>{{cite web|url=http://assets.cambridge.org/052179/3483/sample/0521793483ws.pdf|title=Lifting Titan's Veil|publisher=Cambridge|page=4|archive-url=https://web.archive.org/web/20050222073209/http://assets.cambridge.org/052179/3483/sample/0521793483ws.pdf|archive-date=February 22, 2005}}</ref><ref>{{cite web|url=http://antwrp.gsfc.nasa.gov/apod/ap050325.html|title=Titan|work=Astronomy Picture of the Day|publisher=NASA|archive-url=https://web.archive.org/web/20050327011129/http://antwrp.gsfc.nasa.gov/apod/ap050325.html|archive-date=March 27, 2005}}</ref>

;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 (moon)|Amalthea]].

[[Asaph Hall]] discovered [[Deimos (moon)|Deimos]] on 12 August 1877 at about 07:48 [[Coordinated Universal Time|UTC]] and [[Phobos (moon)|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 day|astronomical convention]] that began the day at noon,<ref>{{cite journal |last1=Campbell |first1=W. W. |title=The Beginning of the Astronomical Day |journal=Publications of the Astronomical Society of the Pacific |date=December 1918 |volume=30 |issue=178 |pages=358 |doi=10.1086/122784 |bibcode=1918PASP...30..358C |doi-access=free }}</ref> give the time of discovery as 11 August 14:40 and 17 August 16:06 [[Washington mean time]] respectively).<ref>{{cite journal |title=Notes |journal=The Observatory |date=September 1877 |volume=1 |pages=181–185 |bibcode=1877Obs.....1..181. }}</ref><ref>{{cite journal |last1=Hall |first1=A. |title=Observations of the Satellites of Mars |journal=Astronomische Nachrichten |date=January 1878 |volume=91 |issue=1 |pages=11–14 |doi=10.1002/asna.18780910103 }}</ref><ref name="Morley1989">{{cite journal |last1=Morley |first1=T. A. |title=A catalogue of ground-based astrometric observations of the Martian satellites, 1877-1982 |journal=Astronomy and Astrophysics Supplement Series |date=February 1989 |volume=77 |issue=2 |pages=209–226 |bibcode=1989A&AS...77..209M }}</ref>

The telescope used for the discovery was the {{convert|26|in|cm|adj=on}} refractor (telescope with a lens) then located at [[Foggy Bottom]].<ref>{{cite web|url=http://amazing-space.stsci.edu/resources/explorations/groundup/lesson/scopes/naval/index.php|title=Telescope: Naval Observatory 26-inch Refractor|website=amazing-space.stsci.edu|access-date=29 October 2018}}</ref> In 1893 the lens was remounted and put in a new dome, where it remains into the 21st century.<ref>{{cite web|url=https://www.usno.navy.mil/USNO/about-us/usnos-telescopes/the-26-inch-refractor|archive-url=https://web.archive.org/web/20180322233721/http://www.usno.navy.mil/USNO/about-us/usnos-telescopes/the-26-inch-refractor|url-status=dead|archive-date=22 March 2018|title=The 26-inch "Great Equatorial" Refractor|website=United States Naval Observatory|access-date=29 October 2018}}</ref>

Jupiter's moon Amalthea was discovered on 9 September 1892, by [[Edward Emerson Barnard]] using the [[James Lick telescope|{{convert|36|in|cm}} refractor telescope]] at [[Lick Observatory]].<ref>{{cite journal| doi = 10.1086/101715| last = Barnard| first = E. E.| date = 12 October 1892| title = Discovery and observations of a fifth satellite to Jupiter| journal = The Astronomical Journal| volume = 12| issue = 11| pages = 81–85| bibcode = 1892AJ.....12...81B}}</ref><ref name="Observatory1894">{{cite book|url=https://books.google.com/books?id=BbA2AQAAIAAJ&pg=RA7-PA27|title=A Brief Account of the Lick Observatory of the University of California|author=Lick Observatory|publisher=The University Press|year=1894|page=7–}}</ref> It was discovered by direct visual observation with the doublet-lens refractor.<ref name="Bakich_2000" />

In 1904, one of the discoveries made using Great Refractor of Potsdam (a double telescope with two doublets) was of the [[interstellar medium]].<ref name=":5">{{Cite book|url=https://books.google.com/books?id=VYvQ_8I_kTwC&q=hartmann+interstellar+matter+potsdam&pg=PA154|title=The Cosmic Connection: How Astronomical Events Impact Life on Earth|last=Kanipe|first=Jeff|date=2011-01-27|publisher=Prometheus Books|isbn=9781591028826}}</ref> The astronomer [[Johannes Franz Hartmann|Professor Hartmann]] determined from observations of the binary star [[Mintaka]] in Orion, that there was the element [[calcium]] in the intervening space.<ref name=":5" />

;Triplets

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.<ref>{{Cite web|url=https://lowell.edu/history/the-pluto-telescope/|title=The Pluto Telescope|website=Lowell Observatory|access-date=2019-11-19}}</ref><ref>{{Cite web|url=https://airandspace.si.edu/multimedia-gallery/6074hjpg|title=Pluto Discovery Plate|website=National Air and Space Museum|date=24 June 2016 |access-date=2019-11-19}}</ref>
{{For timeline|Timeline of discovery of Solar System planets and their moons}}

==List of the largest refracting telescopes==
[[File:Chicago's Great Telescope (3573567148).jpg|thumb|The Yerkes Great refractor mounted at the 1893 World's Fair in Chicago; the tallest, longest, and biggest aperture refractor up to that time.]]
[[File:Refraktor Wien Kerschbaum 1.jpg|thumb|The {{convert|68|cm|abbr=on|lk=in}} refractor at the [[Vienna Observatory|Vienna University Observatory]]]]
Examples of some of the largest achromatic refracting telescopes, over {{convert|60|cm|in|abbr=on}} diameter.
* [[Great Paris Exhibition Telescope of 1900]] ({{convert|1.25|m|in|abbr=on|disp=or}}) – dismantled after exhibition
* [[Yerkes Observatory]] ({{convert|40|in|cm|abbr=on|order=flip|disp=or|sigfig=4}})
* [[Swedish Solar Telescope|Swedish 1-m Solar Telescope]] ({{convert|98|cm|in|abbr=on|disp=or}})
* [[Lick Observatory]] ({{convert|36|in|cm|abbr=on|order=flip|disp=or}})
* [[Paris Observatory]] [[Meudon Great Refractor]] ({{convert|83|cm|in|abbr=on}}, + {{convert|62|cm|in|abbr=on}})
* [[Potsdam Great Refractor]] ({{convert|80|cm|in|abbr=on}}, + {{convert|50|cm|in|abbr=on}})
* [[Nice Observatory]] ({{convert|77|cm|in|abbr=on|disp=or}})
* [[John Wall (inventor)|John Wall]] ({{convert|30|in|cm|abbr=on|order=flip|disp=or|sigfig=4}}) [[Dialyte lens#Dialyte telescopes|dialyte]] refracting telescope - the largest refractor built by an individual, at Hanwell Community Observatory<ref>{{Cite web|url=http://www.hanwellobservatory.org.uk/telescopes/john-wall|title = John Wall refractor &#124; Hanwell Community Observatory}}</ref>
*[[Greenwich 28 inch refractor|28-inch Grubb Refractor]] at [[Royal Greenwich Observatory]], ({{convert|71|cm|in|abbr=on|disp=or}}) aperture lens
*Great Refractor of [[Vienna Observatory]], ({{convert|69|cm|in|abbr=on|disp=or}})
* [[Archenhold Observatory]] – the longest refracting telescope ever built ({{convert|68|cm|in|abbr=on|disp=or}} × {{convert|21|m|ft|abbr=on|disp=or}} focal length)
* United States Naval Observatory refractor, ({{convert|66|cm|in|abbr=on|disp=or}})
* [[Newall Refractor|Newall refractor]] at the [[National Observatory of Athens]] ({{convert|62.5|cm|in|abbr=on|disp=or}})
* [[Lowell Observatory]] ({{convert|24|in|cm|abbr=on|order=flip|disp=or}})

== See also ==
* [[Astrograph]]
* [[Baden-Powell's unilens]]
* [[Catadioptric telescope]]s
* [[List of largest optical refracting telescopes]]
* [[List of largest optical telescopes historically]]
* [[List of telescope types]]
* [[Reflecting telescope]]
* [[Star diagonal]]
* [[Heliometer]]

==References==
{{Reflist}}

==External links==
{{Commons category|Refracting telescopes}}
*[http://www.nasa.gov/audience/foreducators/informal/features/F_Build_a_Telescope_prt.htm nasa.gov – Build a Telescope]
*[http://galileo.rice.edu/lib/student_work/astronomy96/mtelescope.html Making a Galilean Telescope]
*[http://journals.lww.com/optvissci/Fulltext/2007/06000/Angular_and_Linear_Fields_of_View_of_Galilean.16.aspx Angular and Linear Fields of View of Galilean Telescopes and Telemicroscopes]
*[http://www.telescope1609.com/Telescope.htm Introduction to Galileo's Telescope]

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