Editing Refracting telescope (section)

==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" />