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Refracting telescope#Galileo's telescope

{{Short description|Type of optical telescope}}

{{Redirect-distinguish|Keplerian telescope|Kepler space telescope}}

{{Use dmy dates|date=September 2020}}

File:Zeiss2.jpg]]

A refracting telescope (also called a refractor) is a type of optical telescope that uses a lens as its objective to form an image (also referred to a dioptric telescope). The refracting telescope design was originally used in spyglasses and astronomical telescopes but is also used for long-focus camera lenses. Although large refracting telescopes were very popular in the second half of the 19th century, for most research purposes, the refracting telescope has been superseded by the reflecting telescope, which allows larger apertures. A refractor's magnification is calculated by dividing the focal length of the objective lens by that of the eyepiece.{{cite web |url=http://www.northern-stars.com/tel_magnification.htm |title=Telescope Calculations |publisher=Northern Stars |access-date=2013-12-20}}

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

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

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 named Hans Lippershey unsuccessfully tried to patent one.Albert Van Helden, Sven Dupré, Rob van Gent, The Origins of the Telescope, Amsterdam University Press, 2010, pages 3-4, 15 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.{{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}}

==Refracting telescope designs==

Image:Kepschem.png

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.Stephen G. Lipson, Ariel Lipson, Henry Lipson, Optical Physics 4th Edition, Cambridge University Press, {{ISBN|978-0-521-49345-1}}

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=

{{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.{{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}} It used a convergent (plano-convex) objective lens and a divergent (plano-concave) eyepiece lens (Galileo, 1610).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}}. A Galilean telescope, because the design has no intermediary focus, results in a non-inverted (i.e., upright) image.{{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}}

Galileo's most powerful telescope, with a total length of just under {{convert|1|m|in|spell=us}}, magnified objects about 30 times. 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. Despite these flaws, the telescope was still good enough for Galileo to explore the sky. He used it to view craters on the Moon,{{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}} the four largest moons of Jupiter,{{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}} and the phases of Venus.{{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}}

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

The Keplerian telescope, invented by Johannes Kepler in 1611, is an improvement on Galileo's design.{{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}} 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 simple single element objective lens so needs to have a very high focal ratio to reduce aberrations{{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}} (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,{{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}}{{page needed|date=July 2024}} 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.{{Cite web|url=http://www.stjarnhimlen.se/bigtel/LargestTelescope.html|title=Largest optical telescopes of the world|website=www.stjarnhimlen.se}}

=Achromatic refractors=

{{Main|Achromatic telescope}}

{{stack|

File:Yerkes Observatory Astro4p6.jpg

File:Irving Porter Church Telescope.jpg

}}

The next major step in the evolution of refracting telescopes was the invention of the achromatic lens, a lens with multiple elements that helped solve problems with chromatic aberration and allowed shorter focal lengths. It was invented in 1733 by an English barrister named Chester Moore Hall, although it was independently invented and patented by John Dollond around 1758. The design overcame the need for very long focal lengths in refracting telescopes by using an objective made of two pieces of glass with different dispersion, 'crown' and 'flint glass', to reduce chromatic and spherical aberration. Each side of each piece is ground and polished, and then the two pieces are assembled together. Achromatic lenses are corrected to bring two wavelengths (typically red and blue) into focus in the same plane.{{fact|date=September 2024}}

Chester More Hall is noted as having made the first twin color corrected lens in 1730.{{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 }}

Dollond achromats were quite popular in the 18th century.{{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}}{{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 }} A major appeal was they could be made shorter. However, problems with glass making meant that the glass objectives were not made more than about {{convert|4|in|cm|spell=in}} in diameter.

In the late 19th century, the Swiss optician 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) 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}} developed a way to make higher quality glass blanks of greater than {{convert|4|in|cm|spell=in}}. He passed this technology to his apprentice Joseph von Fraunhofer, who further developed this technology and also developed the Fraunhofer doublet lens design. 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:{{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}}

{{stack|File:The 28-inch Telescope.jpg}}

  • Alvan Clark
  • Brashear{{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}}
  • Chance Brothers
  • Cauchoix{{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}}
  • Fraunhofer{{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}}
  • Gautier
  • Grubb
  • Henry Brothers
  • Lerebours{{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 }}
  • Tulley{{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 }}

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.{{fact|date=September 2024}}

In the Royal Observatory, Greenwich an 1838 instrument named the Sheepshanks telescope includes an objective by Cauchoix.{{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}} The Sheepshanks had a {{convert|6.7|in|cm|adj=on}} wide lens, and was the biggest telescope at Greenwich for about twenty years.{{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 }}

An 1840 report from the Observatory noted of the then-new Sheepshanks telescope with the Cauchoix doublet:{{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}}{{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.{{Cite web|url=https://griffithobservatory.org/|title=Griffith Observatory - Southern California's gateway to the cosmos!|website=Griffith Observatory}} 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.

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.{{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 }}

=Apochromatic refractors=

{{Main|Apochromat}}

File:Apochromat.svg

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.{{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 }} 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.

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

Technical considerations

File:Yerkes Observatory Astro4p7.jpg, 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. 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 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 {{convert|1|m|in|abbr=out|sp=us}}.{{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]}}

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 {{convert|6|cm|in}} lens, launched into space in the late 1970s, an example of the use of refractors in space.{{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}}

Applications and achievements

Image:Great Refractor Potsdam.jpg

File:Jessica Meir Photography Training.jpg

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

;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.{{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}}

The planet Saturn's moon, Titan, was discovered on March 25, 1655, by the Dutch astronomer Christiaan Huygens.{{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}}{{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}}

;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,{{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 }} give the time of discovery as 11 August 14:40 and 17 August 16:06 Washington mean time respectively).{{cite journal |title=Notes |journal=The Observatory |date=September 1877 |volume=1 |pages=181–185 |bibcode=1877Obs.....1..181. }}{{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 }}{{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 }}

The telescope used for the discovery was the {{convert|26|in|cm|adj=on}} refractor (telescope with a lens) then located at Foggy Bottom.{{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}} In 1893 the lens was remounted and put in a new dome, where it remains into the 21st century.{{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}}

Jupiter's moon Amalthea was discovered on 9 September 1892, by Edward Emerson Barnard using the James Lick telescope at Lick Observatory.{{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}}{{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–}} It was discovered by direct visual observation with the doublet-lens refractor.

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

;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.{{Cite web|url=https://lowell.edu/history/the-pluto-telescope/|title=The Pluto Telescope|website=Lowell Observatory|access-date=2019-11-19}}{{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}}

{{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

File:Refraktor Wien Kerschbaum 1.jpg]]

Examples of some of the largest achromatic refracting telescopes, over {{convert|60|cm|in|abbr=on}} diameter.

See also

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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