Editing Refracting telescope (section)

===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>