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Telepscope Dollond London Vintage Bronze & Leather Expanding Telescope with Wooden Box Dimensions of Telescope starts at 150mm and expands to 410mm The wooden box is 170mm x 70 mm x 50 mm and has an gold anchor emblem The box and Telescope weights 450 grams It pulls open to expand and pushes closed The Lens Cap has the Makers name Dollond, Location London and Logo and the year 1920 In Very Good Condition Sorry about the poor quality photos. They don’t do the item justice. A lot of my buyers tell me the coin looks better in real life than in my photos Comes from a pet and smoke free home Like all my Auctions Bidding starts a a penny with no reserve…if your the only bidder you win it for 1p…Grab a Bargain! Click Here to Check out my Other Antique Items & Coins Bid with Confidence – Check My 100% Positive Feedback from over 1,000 Satisfied CustomersI have over 10 years of Ebay Selling Experience – So Why Not Treat Yourself? 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Philadelphia, Lahore, Kinshasa, Miami, Ho Chi Minh City, Madrid, Tianjin, Kuala Lumpur, Toronto, Milan, Shenyang, Dallas, Fort Worth, Boston, Belo Horizonte, Khartoum, Riyadh, Singapore, Washington, Detroit, Barcelona,, Houston, Athens, Berlin, Sydney, Atlanta, Guadalajara, San Francisco, Oakland, Montreal, Monterey, Melbourne, Ankara, Recife, Phoenix/Mesa, Durban, Porto Alegre, Dalian, Jeddah, Seattle, Cape Town, San Diego, Fortaleza, Curitiba, Rome, Naples, Minneapolis, St. Paul, Tel Aviv, Birmingham, Frankfurt, Lisbon, Manchester, San Juan, Katowice, Tashkent, Fukuoka, Baku, Sumqayit, St. Louis, Baltimore, Sapporo, Tampa, St. Petersburg, Taichung, Warsaw, Denver, Cologne, Bonn, Hamburg, Dubai, Pretoria, Vancouver, Beirut, Budapest, Cleveland, Pittsburgh, Campinas, Harare, Brasilia, Kuwait, Munich, Portland, Brussels, Vienna, San Jose, Damman , Copenhagen, Brisbane, Riverside, San Bernardino, Cincinnati and Accra Telescope Radio and submilimeterInfraredVisible lightUltravioletX-rayGamma ray The 100-inch (2.54 m) Hooker reflecting telescope at Mount Wilson Observatory near Los Angeles, USA, used by Edwin Hubble to measure galaxy redshifts and discover the general expansion of the universe.A telescope is a device used to observe distant objects by their emission, absorption, or reflection of electromagnetic radiation.[1] Originally meaning only an optical instrument using lenses, curved mirrors, or a combination of both to observe distant objects, the word telescope now refers to a wide range of instruments capable of detecting different regions of the electromagnetic spectrum, and in some cases other types of detectors. The first known practical telescopes were refracting telescopes with glass lenses and were invented in the Netherlands at the beginning of the 17th century. They were used for both terrestrial applications and astronomy. The reflecting telescope, which uses mirrors to collect and focus light, was invented within a few decades of the first refracting telescope. In the 20th century, many new types of telescopes were invented, including radio telescopes in the 1930s and infrared telescopes in the 1960s. EtymologyThe word telescope was coined in 1611 by the Greek mathematician Giovanni Demisiani for one of Galileo Galilei’s instruments presented at a banquet at the Accademia dei Lincei.[2][3] In the Starry Messenger, Galileo had used the Latin term perspicillum. The root of the word is from the Ancient Greek τῆλε, romanized tele ‘far’ and σκοπεῖν, skopein ‘to look or see’; τηλεσκόπος, teleskopos ‘far-seeing’.[4] HistoryMain article: History of the telescope 17th century telescopeThe earliest existing record of a telescope was a 1608 patent submitted to the government in the Netherlands by Middelburg spectacle maker Hans Lipperhey for a refracting telescope.[5] The actual inventor is unknown but word of it spread through Europe. Galileo heard about it and, in 1609, built his own version, and made his telescopic observations of celestial objects.[6][7] The idea that the objective, or light-gathering element, could be a mirror instead of a lens was being investigated soon after the invention of the refracting telescope.[8] The potential advantages of using parabolic mirrors—reduction of spherical aberration and no chromatic aberration—led to many proposed designs and several attempts to build reflecting telescopes.[9] In 1668, Isaac Newton built the first practical reflecting telescope, of a design which now bears his name, the Newtonian reflector.[10] The invention of the achromatic lens in 1733 partially corrected color aberrations present in the simple lens[11] and enabled the construction of shorter, more functional refracting telescopes.[citation needed] Reflecting telescopes, though not limited by the color problems seen in refractors, were hampered by the use of fast tarnishing speculum metal mirrors employed during the 18th and early 19th century—a problem alleviated by the introduction of silver coated glass mirrors in 1857, and aluminized mirrors in 1932.[12] The maximum physical size limit for refracting telescopes is about 1 meter (39 inches), dictating that the vast majority of large optical researching telescopes built since the turn of the 20th century have been reflectors. The largest reflecting telescopes currently have objectives larger than 10 meters (33 feet), and work is underway on several 30-40m designs.[13] The 20th century also saw the development of telescopes that worked in a wide range of wavelengths from radio to gamma-rays. The first purpose-built radio telescope went into operation in 1937. Since then, a large variety of complex astronomical instruments have been developed. In spaceMain article: Space telescopeSince the atmosphere is opaque for most of the electromagnetic spectrum, only a few bands can be observed from the Earth’s surface. These bands are visible – near-infrared and a portion of the radio-wave part of the spectrum.[14] For this reason there are no X-ray or far-infrared ground-based telescopes as these have to be observed from orbit. Even if a wavelength is observable from the ground, it might still be advantageous to place a telescope on a satellite due to issues such as clouds, astronomical seeing and light pollution.[15] The disadvantages of launching a space telescope include cost, size, maintainability and upgradability.[16] By electromagnetic spectrumRadio, infrared, visible, ultraviolet, x-ray and gamma raySix views of the Crab Nebula at different wavelengths of lightThe name “telescope” covers a wide range of instruments. Most detect electromagnetic radiation, but there are major differences in how astronomers must go about collecting light (electromagnetic radiation) in different frequency bands. As wavelengths become longer, it becomes easier to use antenna technology to interact with electromagnetic radiation (although it is possible to make very tiny antenna). The near-infrared can be collected much like visible light, however in the far-infrared and submillimetre range, telescopes can operate more like a radio telescope. For example, the James Clerk Maxwell Telescope observes from wavelengths from 3 μm (0.003 mm) to 2000 μm (2 mm), but uses a parabolic aluminum antenna.[17] On the other hand, the Spitzer Space Telescope, observing from about 3 μm (0.003 mm) to 180 μm (0.18 mm) uses a mirror (reflecting optics). Also using reflecting optics, the Hubble Space Telescope with Wide Field Camera 3 can observe in the frequency range from about 0.2 μm (0.0002 mm) to 1.7 μm (0.0017 mm) (from ultra-violet to infrared light).[18] With photons of the shorter wavelengths, with the higher frequencies, glancing-incident optics, rather than fully reflecting optics are used. Telescopes such as TRACE and SOHO use special mirrors to reflect extreme ultraviolet, producing higher resolution and brighter images than are otherwise possible. A larger aperture does not just mean that more light is collected, it also enables a finer angular resolution. Telescopes may also be classified by location: ground telescope, space telescope, or flying telescope. They may also be classified by whether they are operated by professional astronomers or amateur astronomers. A vehicle or permanent campus containing one or more telescopes or other instruments is called an observatory. Radio and submilimeterMain articles: Radio telescope, Radio astronomy, and Submillimetre astronomysee captionThree radio telescopes belonging to the Atacama Large Millimeter ArrayRadio telescopes are directional radio antennas that typically employ a large dish to collect radio waves. The dishes are sometimes constructed of a conductive wire mesh whose openings are smaller than the wavelength being observed. Unlike an optical telescope, which produces a magnified image of the patch of sky being observed, a traditional radio telescope dish contains a single receiver and records a single time-varying signal characteristic of the observed region; this signal may be sampled at various frequencies. In some newer radio telescope designs, a single dish contains an array of several receivers; this is known as a focal-plane array. By collecting and correlating signals simultaneously received by several dishes, high-resolution images can be computed. Such multi-dish arrays are known as astronomical interferometers and the technique is called aperture synthesis. The ‘virtual’ apertures of these arrays are similar in size to the distance between the telescopes. As of 2005, the record array size is many times the diameter of the Earth – using space-based very-long-baseline-interferometry (VLBI) telescopes such as the Japanese HALCA (Highly Advanced Laboratory for Communications and Astronomy) VSOP (VLBI Space Observatory Program) satellite.[19] Aperture synthesis is now also being applied to optical telescopes using optical interferometers (arrays of optical telescopes) and aperture masking interferometry at single reflecting telescopes. Radio telescopes are also used to collect microwave radiation, which has the advantage of being able to pass through the atmosphere and interstellar gas and dust clouds. Some radio telescopes such as the Allen Telescope Array are used by programs such as SETI[20] and the Arecibo Observatory to search for extraterrestrial life.[21][22] InfraredMain articles: Infrared telescope and Infrared astronomyVisible lightMain articles: Optical telescope and Visible-light astronomyDome-like telescope with extruding mirror mountOne of four auxiliary telescopes belong to the Very Large Telescope arrayAn optical telescope gathers and focuses light mainly from the visible part of the electromagnetic spectrum.[23] Optical telescopes increase the apparent angular size of distant objects as well as their apparent brightness. For the image to be observed, photographed, studied, and sent to a computer, telescopes work by employing one or more curved optical elements, usually made from glass lenses and/or mirrors, to gather light and other electromagnetic radiation to bring that light or radiation to a focal point. Optical telescopes are used for astronomy and in many non-astronomical instruments, including: theodolites (including transits), spotting scopes, monoculars, binoculars, camera lenses, and spyglasses. There are three main optical types: The refracting telescope which uses lenses to form an image.[24]The reflecting telescope which uses an arrangement of mirrors to form an image.[25]The catadioptric telescope which uses mirrors combined with lenses to form an image.A Fresnel imager is a proposed ultra-lightweight design for a space telescope that uses a Fresnel lens to focus light.[26][27] Beyond these basic optical types there are many sub-types of varying optical design classified by the task they perform such as astrographs,[28] comet seekers[29] and solar telescopes.[30] UltravioletMain article: Ultraviolet astronomyMost ultraviolet light is absorbed by the Earth’s atmosphere, so observations at these wavelengths must be performed from the upper atmosphere or from space.[31][32] X-rayMain articles: X-ray telescope and X-ray astronomysee captionHitomi telescope’s X-ray focusing mirror, consisting of over two hundred concentric aluminium shellsX-rays are much harder to collect and focus than electromagnetic radiation of longer wavelengths. X-ray telescopes can use X-ray optics, such as Wolter telescopes composed of ring-shaped ‘glancing’ mirrors made of heavy metals that are able to reflect the rays just a few degrees. The mirrors are usually a section of a rotated parabola and a hyperbola, or ellipse. In 1952, Hans Wolter outlined 3 ways a telescope could be built using only this kind of mirror.[33][34] Examples of space observatories using this type of telescope are the Einstein Observatory,[35] ROSAT,[36] and the Chandra X-ray Observatory.[37][38] In 2012 the NuSTAR X-ray Telescope was launched which uses Wolter telescope design optics at the end of a long deployable mast to enable photon energies of 79 keV.[39][40] Gamma rayMain article: Gamma-ray astronomy The Compton Gamma Ray Observatory released into orbit by the Space Shuttle in 1991Higher energy X-ray and gamma ray telescopes refrain from focusing completely and use coded aperture masks: the patterns of the shadow the mask creates can be reconstructed to form an image. X-ray and Gamma-ray telescopes are usually installed on high-flying balloons[41][42] or Earth-orbiting satellites since the Earth’s atmosphere is opaque to this part of the electromagnetic spectrum. An example of this type of telescope is the Fermi Gamma-ray Space Telescope which was launched in June 2008.[43][44] The detection of very high energy gamma rays, with shorter wavelength and higher frequency than regular gamma rays, requires further specialization. An example of this type of observatory is the ground based telescope VERITAS.[45][46] A discovery in 2012 may allow focusing gamma-ray telescopes.[47] At photon energies greater than 700 keV, the index of refraction starts to increase again.[47] Lists of telescopesList of optical telescopesList of largest optical reflecting telescopesList of largest optical refracting telescopesList of largest optical telescopes historicallyList of radio telescopesList of solar telescopesList of space observatoriesList of telescope parts and constructionList of telescope typesSee alsoAirmassAmateur telescope makingAngular resolutionASCOM open standards for computer control of telescopesBahtinov maskBioptic telescopeCarey maskDew shieldDynameterf-numberFirst lightHartmann maskKeyhole problemMicroscopePlanetariumsRemote Telescope Markup LanguageRobotic telescopeTimeline of telescope technologyTimeline of telescopes, observatories, and observing technologyReferences Company, Houghton Mifflin Harcourt Publishing. “The American Heritage Dictionary entry: TELESCOPE”. www.ahdictionary.com. Archived from the original on 11 March 2020. Retrieved 12 July 2018. Sobel (2000, p.43), Drake (1978, p.196) Rosen, Edward, The Naming of the Telescope (1947) Jack, Albert (2015). They Laughed at Galileo: How the Great Inventors Proved Their Critics Wrong. ISBN 978-1629147581. galileo.rice.edu The Galileo Project > Science > The Telescope by Al Van Helden: The Hague discussed the patent applications first of Hans Lipperhey of Middelburg, and then of Archived 23 June 2004 at the Wayback MachineJacob Metius of Alkmaar… another citizen of Middelburg, Zacharias Janssen is sometimes associated with the invention “NASA – Telescope History”. www.nasa.gov. Archived from the original on 14 February 2021. Retrieved 11 July 2017. Loker, Aleck (20 November 2017). Profiles in Colonial History. Aleck Loker. ISBN 978-1-928874-16-4. Archived from the original on 27 May 2016. Retrieved 12 December 2015 – via Google Books. Watson, Fred (20 November 2017). Stargazer: The Life and Times of the Telescope. Allen & Unwin. ISBN 978-1-74176-392-8. Archived from the original on 2 March 2021. Retrieved 21 November 2020 – via Google Books. Attempts by Niccolò Zucchi and James Gregory and theoretical designs by Bonaventura Cavalieri, Marin Mersenne, and Gregory among others Hall, A. Rupert (1992). Isaac Newton: Adventurer in Thought. Cambridge University Press. p. 67. ISBN 9780521566698. “Chester Moor Hall”. Encyclopædia Britannica. Retrieved 25 May 2016. Bakich, Michael E. (10 July 2003). “Chapter Two: Equipment”. The Cambridge Encyclopedia of Amateur Astronomy (PDF). Cambridge University Press. p. 33. ISBN 9780521812986. Archived from the original (PDF) on 10 September 2009. Tate, Karl (30 August 2013). “World’s Largest Reflecting Telescopes Explained (Infographic)”. Space.com. Stierwalt, Everyday Einstein Sabrina. “Why Do We Put Telescopes in Space?”. Scientific American. Retrieved 20 August 2022. Siegel, Ethan. “5 Reasons Why Astronomy Is Better From The Ground Than In Space”. Forbes. Retrieved 20 August 2022. Siegel, Ethan. “This Is Why We Can’t Just Do All Of Our Astronomy From Space”. Forbes. Retrieved 20 August 2022. ASTROLab du parc national du Mont-Mégantic (January 2016). “The James-Clerk-Maxwell Observatory”. Canada under the stars. Archived from the original on 5 February 2011. Retrieved 16 April 2017. “Hubble’s Instruments: WFC3 – Wide Field Camera 3”. www.spacetelescope.org. Archived from the original on 12 November 2020. Retrieved 16 April 2017. “Observatories Across the Electromagnetic Spectrum”. imagine.gsfc.nasa.gov. Retrieved 20 August 2022. Dalton, Rex (1 August 2000). “Microsoft moguls back search for ET intelligence”. Nature. 406 (6796): 551. doi:10.1038/35020722. ISSN 1476-4687. PMID 10949267. S2CID 4415108. Tarter, Jill (September 2001). “The Search for Extraterrestrial Intelligence (SETI)”. Annual Review of Astronomy and Astrophysics. 39 (1): 511–548. Bibcode:2001ARA&A..39..511T. doi:10.1146/annurev.astro.39.1.511. ISSN 0066-4146. Archived from the original on 20 August 2022. Retrieved 20 August 2022. Nola Taylor Tillman (2 August 2016). “SETI & the Search for Extraterrestrial Life”. Space.com. Retrieved 20 August 2022. Jones, Barrie W. (2 September 2008). The Search for Life Continued: Planets Around Other Stars. Springer Science & Business Media. ISBN 978-0-387-76559-4. Archived from the original on 8 March 2020. Retrieved 12 December 2015. Lauren Cox (26 October 2021). “Who Invented the Telescope?”. Space.com. Retrieved 20 August 2022. Rupert, Charles G. (1918). “1918PA…..26..525R Page 525”. Popular Astronomy. 26: 525. Bibcode:1918PA…..26..525R. Retrieved 20 August 2022. “Telescope could focus light without a mirror or lens”. New Scientist. Retrieved 20 August 2022. Koechlin, L.; Serre, D.; Duchon, P. (1 November 2005). “High resolution imaging with Fresnel interferometric arrays: suitability for exoplanet detection”. Astronomy & Astrophysics. 443 (2): 709–720. arXiv:astro-ph/0510383. Bibcode:2005A&A…443..709K. doi:10.1051/0004-6361:20052880. ISSN 0004-6361. S2CID 119423063. “Celestron Rowe-Ackermann Schmidt Astrograph – Astronomy Now”. Retrieved 20 August 2022. “Telescope (Comet Seeker)”. Smithsonian Institution. Retrieved 20 August 2022. Stenflo, J. O. (1 January 2001). “Limitations and Opportunities for the Diagnostics of Solar and Stellar Magnetic Fields”. Magnetic Fields Across the Hertzsprung-Russell Diagram. 248: 639. Bibcode:2001ASPC..248..639S. Allen, C. W. (2000). Allen’s astrophysical quantities. Arthur N. Cox (4th ed.). New York: AIP Press. ISBN 0-387-98746-0. OCLC 40473741. Ortiz, Roberto; Guerrero, Martín A. (28 June 2016). “Ultraviolet emission from main-sequence companions of AGB stars”. Monthly Notices of the Royal Astronomical Society. 461 (3): 3036–3046. doi:10.1093/mnras/stw1547. ISSN 0035-8711. Wolter, H. (1952), “Glancing Incidence Mirror Systems as Imaging Optics for X-rays”, Annalen der Physik, 10 (1): 94–114, Bibcode:1952AnP…445…94W, doi:10.1002/andp.19524450108. Wolter, H. (1952), “Verallgemeinerte Schwarzschildsche Spiegelsysteme streifender Reflexion als Optiken für Röntgenstrahlen”, Annalen der Physik, 10 (4–5): 286–295, Bibcode:1952AnP…445..286W, doi:10.1002/andp.19524450410. Giacconi, R.; Branduardi, G.; Briel, U.; Epstein, A.; Fabricant, D.; Feigelson, E.; Forman, W.; Gorenstein, P.; Grindlay, J.; Gursky, H.; Harnden, F. R.; Henry, J. P.; Jones, C.; Kellogg, E.; Koch, D. (June 1979). “The Einstein /HEAO 2/ X-ray Observatory”. The Astrophysical Journal. 230: 540. Bibcode:1979ApJ…230..540G. doi:10.1086/157110. ISSN 0004-637X. S2CID 120943949. “DLR – About the ROSAT mission”. DLRARTICLE DLR Portal. Retrieved 20 August 2022. Schwartz, Daniel A. (1 August 2004). “The development and scientific impact of the chandra x-ray observatory”. International Journal of Modern Physics D. 13 (7): 1239–1247. arXiv:astro-ph/0402275. Bibcode:2004IJMPD..13.1239S. doi:10.1142/S0218271804005377. ISSN 0218-2718. S2CID 858689. Madejski, Greg (2006). “Recent and Future Observations in the X‐ray and Gamma‐ray Bands: Chandra, Suzaku, GLAST, and NuSTAR”. AIP Conference Proceedings. 801 (1): 21–30. arXiv:astro-ph/0512012. Bibcode:2005AIPC..801…21M. doi:10.1063/1.2141828. ISSN 0094-243X. S2CID 14601312. “NuStar: Instrumentation: Optics”. Archived from the original on 1 November 2010. Hailey, Charles J.; An, HongJun; Blaedel, Kenneth L.; Brejnholt, Nicolai F.; Christensen, Finn E.; Craig, William W.; Decker, Todd A.; Doll, Melanie; Gum, Jeff; Koglin, Jason E.; Jensen, Carsten P.; Hale, Layton; Mori, Kaya; Pivovaroff, Michael J.; Sharpe, Marton (29 July 2010). Arnaud, Monique; Murray, Stephen S; Takahashi, Tadayuki (eds.). “The Nuclear Spectroscopic Telescope Array (NuSTAR): optics overview and current status”. Space Telescopes and Instrumentation 2010: Ultraviolet to Gamma Ray. SPIE. 7732: 197–209. Bibcode:2010SPIE.7732E..0TH. doi:10.1117/12.857654. S2CID 121831705. Braga, João; D’Amico, Flavio; Avila, Manuel A. C.; Penacchioni, Ana V.; Sacahui, J. Rodrigo; Santiago, Valdivino A. de; Mattiello-Francisco, Fátima; Strauss, Cesar; Fialho, Márcio A. A. (1 August 2015). “The protoMIRAX hard X-ray imaging balloon experiment”. Astronomy & Astrophysics. 580: A108. arXiv:1505.06631. Bibcode:2015A&A…580A.108B. doi:10.1051/0004-6361/201526343. ISSN 0004-6361. S2CID 119222297. Brett Tingley (13 July 2022). “Balloon-borne telescope lifts off to study black holes and neutron stars”. Space.com. Retrieved 20 August 2022. Atwood, W. B.; Abdo, A. A.; Ackermann, M.; Althouse, W.; Anderson, B.; Axelsson, M.; Baldini, L.; Ballet, J.; Band, D. L.; Barbiellini, G.; Bartelt, J.; Bastieri, D.; Baughman, B. M.; Bechtol, K.; Bédérède, D. (1 June 2009). “The Large Area Telescope on Thefermi Gamma-Ray Space Telescopemission”. The Astrophysical Journal. 697 (2): 1071–1102. arXiv:0902.1089. Bibcode:2009ApJ…697.1071A. doi:10.1088/0004-637X/697/2/1071. ISSN 0004-637X. S2CID 26361978. Ackermann, M.; Ajello, M.; Baldini, L.; Ballet, J.; Barbiellini, G.; Bastieri, D.; Bellazzini, R.; Bissaldi, E.; Bloom, E. D.; Bonino, R.; Bottacini, E.; Brandt, T. J.; Bregeon, J.; Bruel, P.; Buehler, R. (13 July 2017). “Search for Extended Sources in the Galactic Plane Using Six Years ofFermi-Large Area Telescope Pass 8 Data above 10 GeV”. The Astrophysical Journal. 843 (2): 139. arXiv:1702.00476. Bibcode:2017ApJ…843..139A. doi:10.3847/1538-4357/aa775a. ISSN 1538-4357. S2CID 119187437. Krennrich, F.; Bond, I. H.; Boyle, P. J.; Bradbury, S. M.; Buckley, J. H.; Carter-Lewis, D.; Celik, O.; Cui, W.; Daniel, M.; D’Vali, M.; de la Calle Perez, I.; Duke, C.; Falcone, A.; Fegan, D. J.; Fegan, S. J. (1 April 2004). “VERITAS: the Very Energetic Radiation Imaging Telescope Array System”. New Astronomy Reviews. 2nd VERITAS Symposium on the Astrophysics of Extragalactic Sources. 48 (5): 345–349. Bibcode:2004NewAR..48..345K. doi:10.1016/j.newar.2003.12.050. hdl:10379/9414. ISSN 1387-6473. Weekes, T. C.; Cawley, M. F.; Fegan, D. J.; Gibbs, K. G.; Hillas, A. M.; Kowk, P. W.; Lamb, R. C.; Lewis, D. A.; Macomb, D.; Porter, N. A.; Reynolds, P. T.; Vacanti, G. (1 July 1989). “Observation of TeV Gamma Rays from the Crab Nebula Using the Atmospheric Cerenkov Imaging Technique”. The Astrophysical Journal. 342: 379. Bibcode:1989ApJ…342..379W. doi:10.1086/167599. ISSN 0004-637X. S2CID 119424766. “Silicon ‘prism’ bends gamma rays – Physics World”. 9 May 2012. Archived from the original on 12 May 2013. Retrieved 15 May 2012.Further readingElliott, Robert S. (1966), Electromagnetics, McGraw-HillKing, Henry C. (1979). The history of the telescope. H. Spencer Jones. New York: Dover Publications. ISBN 0-486-23893-8. OCLC 6025190.Pasachoff, Jay M. (1981). Contemporary astronomy (2nd ed.). Philadelphia: Saunders College Pub. ISBN 0-03-057861-2. OCLC 7734917.Rashed, Roshdi; Morelon, Régis (1996), Encyclopedia of the History of Arabic Science, vol. 1 & 3, Routledge, ISBN 978-0-415-12410-2Sabra, A.I.; Hogendijk, J.P. (2003). The Enterprise of Science in Islam: New Perspectives. MIT Press. pp. 85–118. ISBN 978-0-262-19482-2.Wade, Nicholas J.; Finger, Stanley (2001), “The eye as an optical instrument: from camera obscura to Helmholtz’s perspective”, Perception, 30 (10): 1157–1177, doi:10.1068/p3210, PMID 11721819, S2CID 8185797Watson, Fred (2007). Stargazer : the life and times of the telescope. Crows Nest, NSW: Allen & Unwin. ISBN 978-1-74176-392-8. OCLC 173996168.External links Wikiquote has quotations related to Telescope. Wikimedia Commons has media related to Telescope.Galileo to Gamma Cephei – The History of the TelescopeThe Galileo Project – The Telescope by Al Van Helden”The First Telescopes”. Part of an exhibit from Cosmic Journey: A History of Scientific Cosmology Archived 9 April 2008 at the Wayback Machine by the American Institute of PhysicsTaylor, Harold Dennis; Gill, David (1911). “Telescope” . Encyclopædia Britannica. Vol. 26 (11th ed.). pp. 557–573.Outside the Optical: Other Kinds of TelescopesGray, Meghan; Merrifield, Michael (2009). “Telescope Diameter”. Sixty Symbols. Brady Haran for the University of Nottingham.vteAstronomyOutlineHistory TimelineAstronomerAstronomical symbolsAstronomical objectGlossaryAstronomy byMannerAmateurObservationalSidewalkSpace telescopeCelestial subjectGalactic / ExtragalacticLocal system SolarEM methodsRadioSubmillimetreInfrared (Far-infrared)Visible-light (optical)UltravioletX-rayGamma-rayOther methodsNeutrinoCosmic raysGravitational radiationHigh-energyRadarSphericalMulti-messengerCultureAustralian AboriginalBabylonianChechen (Nakh)ChineseEgyptianGreekHebrewIndianInuitMayaMedieval IslamicPersianSerbian folkTibetanOpticaltelescopesListCategoryExtremely large telescopeGran Telescopio CanariasHubble Space TelescopeKeck ObservatoryLarge Binocular TelescopeSouthern African Large TelescopeVery Large TelescopeRelatedArchaeoastronomyAstrobiologyAstrochemistryAstrophysicsAstrology and astronomyAstrometryAstroparticle physicsBinocularsPhotometryPlanetariumPlanetary geologyPhysical cosmologyQuantum cosmologyList of astronomers FrenchMedieval IslamicRussianWomenTelescope historylistsZodiac Optical telescopes Other wavelengths Interferometric telescopesHistory of the telescope Early depiction of a “Dutch telescope” from 1624.The history of the telescope can be traced to before the invention of the earliest known telescope, which appeared in 1608 in the Netherlands, when a patent was submitted by Hans Lippershey, an eyeglass maker. Although Lippershey did not receive his patent, news of the invention soon spread across Europe. The design of these early refracting telescopes consisted of a convex objective lens and a concave eyepiece. Galileo improved on this design the following year and applied it to astronomy. In 1611, Johannes Kepler described how a far more useful telescope could be made with a convex objective lens and a convex eyepiece lens. By 1655, astronomers such as Christiaan Huygens were building powerful but unwieldy Keplerian telescopes with compound eyepieces.[1] Isaac Newton is credited with building the first reflector in 1668 with a design that incorporated a small flat diagonal mirror to reflect the light to an eyepiece mounted on the side of the telescope. Laurent Cassegrain in 1672 described the design of a reflector with a small convex secondary mirror to reflect light through a central hole in the main mirror. The achromatic lens, which greatly reduced color aberrations in objective lenses and allowed for shorter and more functional telescopes, first appeared in a 1733 telescope made by Chester Moore Hall, who did not publicize it. John Dollond learned of Hall’s invention[2][3] and began producing telescopes using it in commercial quantities, starting in 1758. Important developments in reflecting telescopes were John Hadley’s production of larger paraboloidal mirrors in 1721; the process of silvering glass mirrors introduced by Léon Foucault in 1857;[4] and the adoption of long-lasting aluminized coatings on reflector mirrors in 1932.[5] The Ritchey-Chretien variant of Cassegrain reflector was invented around 1910, but not widely adopted until after 1950; many modern telescopes including the Hubble Space Telescope use this design, which gives a wider field of view than a classic Cassegrain. During the period 1850–1900, reflectors suffered from problems with speculum metal mirrors, and a considerable number of “Great Refractors” were built from 60 cm to 1 metre aperture, culminating in the Yerkes Observatory refractor in 1897; however, starting from the early 1900s a series of ever-larger reflectors with glass mirrors were built, including the Mount Wilson 60-inch (1.5 metre), the 100-inch (2.5 metre) Hooker Telescope (1917) and the 200-inch (5 metre) Hale telescope (1948); essentially all major research telescopes since 1900 have been reflectors. A number of 4-metre class (160 inch) telescopes were built on superior higher altitude sites including Hawaii and the Chilean desert in the 1975–1985 era. The development of the computer-controlled alt-azimuth mount in the 1970s and active optics in the 1980s enabled a new generation of even larger telescopes, starting with the 10-metre (400 inch) Keck telescopes in 1993/1996, and a number of 8-metre telescopes including the ESO Very Large Telescope, Gemini Observatory and Subaru Telescope. The era of radio telescopes (along with radio astronomy) was born with Karl Guthe Jansky’s serendipitous discovery of an astronomical radio source in 1931. Many types of telescopes were developed in the 20th century for a wide range of wavelengths from radio to gamma-rays. The development of space observatories after 1960 allowed access to several bands impossible to observe from the ground, including X-rays and longer wavelength infrared bands. Optical telescopesOptical foundationsSee also: History of optics Optical diagram showing light being refracted by a spherical glass container full of water, from Roger Bacon, De multiplicatione specierumFurther information: Lens (optics) § HistoryObjects resembling lenses date back 4000 years although it is unknown if they were used for their optical properties or just as decoration.[6] Greek accounts of the optical properties of water-filled spheres (5th century BC) were followed by many centuries of writings on optics, including Ptolemy (2nd century) in his Optics, who wrote about the properties of light including reflection, refraction, and color, followed by Ibn Sahl (10th century) and Ibn Al-Haytham (11th century).[7][unreliable source?] Actual use of lenses dates back to the widespread manufacture and use of eyeglasses in Northern Italy beginning in the late 13th century.[8][6][9][10][11] The invention of the use of concave lenses to correct near-sightedness is ascribed to Nicholas of Cusa in 1451. Invention Notes on Hans Lippershey’s unsuccessful telescope patent in 1608The first record of a telescope comes from the Netherlands in 1608. It is in a patent filed by Middelburg spectacle-maker Hans Lippershey with the States General of the Netherlands on 2 October 1608 for his instrument “for seeing things far away as if they were nearby”.[12] A few weeks later another Dutch instrument-maker, Jacob Metius also applied for a patent. The States General did not award a patent since the knowledge of the device already seemed to be ubiquitous[13][14] but the Dutch government awarded Lippershey with a contract for copies of his design. The original Dutch telescopes were composed of a convex and a concave lens—telescopes that are constructed this way do not invert the image. Lippershey’s original design had only 3x magnification. Telescopes seem to have been made in the Netherlands in considerable numbers soon after this date of “invention”, and rapidly found their way all over Europe.[citation needed] Claims of prior invention Reproduction of one of the four optical devices that Zacharias Snijder in 1841 claimed were early telescopes built by Zacharias Janssen. Its actual function and creator has been disputed over the years.[15][16]In 1655 Dutch diplomat William de Boreel tried to solve the mystery of who invented the telescope. He had a local magistrate in Middelburg follow up on Boreel’s childhood and early adult recollections of a spectacle maker named “Hans” who he remembered as the inventor of the telescope. The magistrate was contacted by a then unknown claimant, Middelburg spectacle maker Johannes Zachariassen, who testified that his father, Zacharias Janssen invented the telescope and the microscope as early as 1590. This testimony seemed convincing to Boreel, who now recollected that Zacharias and his father, Hans Martens, must have been who he remembered.[17] Boreel’s conclusion that Zacharias Janssen invented the telescope a little ahead of another spectacle maker, Hans Lippershey, was adopted by Pierre Borel in his 1656 book De vero telescopii inventore.[18][19] Discrepancies in Boreel’s investigation and Zachariassen’s testimony (including Zachariassen misrepresenting his date of birth and role in the invention) has led some historians to consider this claim dubious.[20] The “Janssen” claim would continue over the years and be added on to with Zacharias Snijder in 1841 presenting 4 iron tubes with lenses in them claimed to be 1590 examples of Janssen’s telescope[16] and historian Cornelis de Waard’s 1906 claim that the man who tried to sell a broken telescope to astronomer Simon Marius at the 1608 Frankfurt Book Fair must have been Janssen.[21] In 1682,[22] the minutes of the Royal Society in London Robert Hooke noted Thomas Digges’ 1571 Pantometria, (a book on measurement, partially based on his father Leonard Digges’ notes and observations) seemed to support an English claim to the invention of the telescope, describing Leonard as having a fare seeing glass in the mid 1500s based on an idea by Roger Bacon.[23][24] Thomas described it as “by proportional Glasses duly situate in convenient angles, not only discovered things far off, read letters, numbered pieces of money with the very coin and superscription thereof, cast by some of his friends of purpose upon downs in open fields, but also seven miles off declared what hath been done at that instant in private places.” Comments on the use of proportional or “perspective glass” are also made in the writings of John Dee (1575) and William Bourne (1585).[25] Bourne was asked in 1580 to investigate the Diggs device by Queen Elizabeth I’s chief advisor Lord Burghley. Bourne’s is the best description of it, and from his writing it seemed to consist of peering into a large curved mirror that reflected the image produced by a large lens.[26] The idea of an “Elizabethan Telescope” has been expanded over the years, including astronomer and historian Colin Ronan concluding in the 1990s that this reflecting/refracting telescope was built by Leonard Digges between 1540 and 1559.[27][28][29] This “backwards” reflecting telescope would have been unwieldy, it needed very large mirrors and lens to work, the observer had to stand backwards to look at an upside down view, and Bourne noted it had a very narrow field of view making it unsuitable for military purposes.[26] The optical performance required to see the details of coins lying about in fields, or private activities seven miles away, seems to be far beyond the technology of the time[30] and it could be the “perspective glass” being described was a far simpler idea, originating with Bacon, of using a single lens held in front of the eye to magnify a distant view.[31] Translations of the notebooks of Leonardo da Vinci and Girolamo Fracastoro shows both using water filled crystals or a combination of lenses to magnify the Moon, although the descriptions are too sketchy to determine if they were arranged like a telescope.[32][33][34] A 1959 research paper by Simon de Guilleuma claimed that evidence he had uncovered pointed to the French born spectacle maker Juan Roget (died before 1624) as another possible builder of an early telescope that predated Hans Lippershey’s patent application.[35] Spread of the inventionLippershey’s application for a patent was mentioned at the end of a diplomatic report on an embassy to Holland from the Kingdom of Siam sent by the Siamese king Ekathotsarot: Ambassades du Roy de Siam envoyé à l’Excellence du Prince Maurice, arrivé à La Haye le 10 Septemb. 1608 (Embassy of the King of Siam sent to his Excellency Prince Maurice, arrived at The Hague on 10 September 1608). This report was issued in October 1608 and distributed across Europe, leading to experiments by other scientists, such as the Italian Paolo Sarpi, who received the report in November, and the English mathematician and astronomer Thomas Harriot, who used a six-powered telescope by the summer of 1609 to observe features on the moon.[36] 19th-century painting depicting Galileo Galilei displaying his telescope to Leonardo Donato in 1609.The Italian polymath Galileo Galilei was in Venice in June 1609[37] and there heard of the “Dutch perspective glass”, a military spyglass,[38] by means of which distant objects appeared nearer and larger. Galileo states that he solved the problem of the construction of a telescope the first night after his return to Padua from Venice and made his first telescope the next day by using a convex objective lens in one extremity of a leaden tube and a concave eyepiece lens in the other end, an arrangement that came to be called a Galilean telescope.[39] A few days afterwards, having succeeded in making a better telescope than the first, he took it to Venice where he communicated the details of his invention to the public and presented the instrument itself to the doge Leonardo Donato, who was sitting in full council. The senate in return settled him for life in his lectureship at Padua and doubled his salary. In 1610 Galileo Galilei discovered with his telescope that Venus showed phases, despite remaining near the Sun in Earth’s sky (first image). This proved that it orbits the Sun and not Earth, as predicted by Copernicus’s heliocentric model and disproved the then conventional geocentric model (second image).Galileo set himself to improving the telescope, producing telescopes of increased power. His first telescope had a 3x magnification, but he soon made instruments which magnified 8x and finally, one nearly a meter long with a 37mm objective (which he would stop down to 16mm or 12mm) and a 23x magnification.[40] With this last instrument he began a series of astronomical observations in October or November 1609, observing the satellites of Jupiter, hills and valleys on the Moon, the phases of Venus[41] and spots on the sun (using the projection method rather than direct observation). Galileo noted that the revolution of the satellites of Jupiter, the phases of Venus, rotation of the Sun and the tilted path its spots followed for part of the year pointed to the validity of the sun-centered Copernican system over other Earth-centered systems such as the one proposed by Ptolemy. Galileo’s instrument was the first to be given the name “telescope”. The name was invented by the Greek poet/theologian Giovanni Demisiani at a banquet held on April 14, 1611 by Prince Federico Cesi to make Galileo Galilei a member of the Accademia dei Lincei.[42] The word was created from the Greek tele = ‘far’ and skopein = ‘to look or see’; teleskopos = ‘far-seeing’. By 1626 knowledge of the telescope had spread to China when German Jesuit and astronomer Johann Adam Schall von Bell published Yuan jing shuo, (Explanation of the Telescope) in Chinese and Latin.[43] Further refinementsRefracting telescopesJohannes Kepler first explained the theory and some of the practical advantages of a telescope constructed of two convex lenses in his Catoptrics (1611). The first person who actually constructed a telescope of this form was the Jesuit Christoph Scheiner who gives a description of it in his Rosa Ursina (1630).[39] William Gascoigne was the first who commanded a chief advantage of the form of telescope suggested by Kepler: that a small material object could be placed at the common focal plane of the objective and the eyepiece. This led to his invention of the micrometer, and his application of telescopic sights to precision astronomical instruments. It was not till about the middle of the 17th century that Kepler’s telescope came into general use: not so much because of the advantages pointed out by Gascoigne, but because its field of view was much larger than in the Galilean telescope.[39] The first powerful telescopes of Keplerian construction were made by Christiaan Huygens after much labor—in which his brother assisted him. With one of these: an objective diameter of 2.24 inches (57 mm) and a 12 ft (3.7 m) focal length,[44] he discovered the brightest of Saturn’s satellites (Titan) in 1655; in 1659, he published his “Systema Saturnium” which, for the first time, gave a true explanation of Saturn’s ring—founded on observations made with the same instrument.[39] Long focal length refractors Engraved illustration of a 45 m (148 ft) focal length Keplerian astronomical refracting telescope built by Johannes Hevelius. From his book, “Machina coelestis” (first part), published in 1673.The sharpness of the image in Kepler’s telescope was limited by the chromatic aberration introduced by the non-uniform refractive properties of the objective lens. The only way to overcome this limitation at high magnifying powers was to create objectives with very long focal lengths. Giovanni Cassini discovered Saturn’s fifth satellite (Rhea) in 1672 with a telescope 35 feet (11 m) long. Astronomers such as Johannes Hevelius were constructing telescopes with focal lengths as long as 150 feet (46 m). Besides having really long tubes these telescopes needed scaffolding or long masts and cranes to hold them up. Their value as research tools was minimal since the telescope’s frame “tube” flexed and vibrated in the slightest breeze and sometimes collapsed altogether.[45][46] Aerial telescopesMain article: Aerial telescopeIn some of the very long refracting telescopes constructed after 1675, no tube was employed at all. The objective was mounted on a swiveling ball-joint on top of a pole, tree, or any available tall structure and aimed by means of string or connecting rod. The eyepiece was handheld or mounted on a stand at the focus, and the image was found by trial and error. These were consequently termed aerial telescopes.[47] and have been attributed to Christiaan Huygens and his brother Constantijn Huygens, Jr.[45][48] although it is not clear that they invented it.[49] Christiaan Huygens and his brother made objectives up to 8.5 inches (220 mm) diameter[44] and 210 ft (64 m) focal length and others such as Adrien Auzout made telescopes with focal lengths up to 600 ft (180 m). Telescopes of such great length were naturally difficult to use and must have taxed to the utmost the skill and patience of the observers.[39] Aerial telescopes were employed by several other astronomers. Cassini discovered Saturn’s third and fourth satellites in 1684 with aerial telescope objectives made by Giuseppe Campani that were 100 and 136 ft (30 and 41 m) in focal length. Reflecting telescopesSee also: Reflecting telescopeThe ability of a curved mirror to form an image may have been known since the time of Euclid[50] and had been extensively studied by Alhazen in the 11th century. Galileo, Giovanni Francesco Sagredo, and others, spurred on by their knowledge that curved mirrors had similar properties to lenses, discussed the idea of building a telescope using a mirror as the image forming objective.[51] Niccolò Zucchi, an Italian Jesuit astronomer and physicist, wrote in his book Optica philosophia of 1652 that he tried replacing the lens of a refracting telescope with a bronze concave mirror in 1616. Zucchi tried looking into the mirror with a hand held concave lens but did not get a satisfactory image, possibly due to the poor quality of the mirror, the angle it was tilted at, or the fact that his head partially obstructed the image.[52] Light path in a Gregorian telescope.In 1636 Marin Mersenne proposed a telescope consisting of a paraboloidal primary mirror and a paraboloidal secondary mirror bouncing the image through a hole in the primary, solving the problem of viewing the image.[53] James Gregory went into further detail in his book Optica Promota (1663), pointing out that a reflecting telescope with a mirror that was shaped like the part of a conic section, would correct spherical aberration as well as the chromatic aberration seen in refractors. The design he came up with bears his name: the “Gregorian telescope”; but according to his own confession, Gregory had no practical skill and he could find no optician capable of realizing his ideas and after some fruitless attempts, was obliged to abandon all hope of bringing his telescope into practical use. Light path in a Newtonian telescope. A replica of Newton’s second reflecting telescope which was presented to the Royal Society in 1672.[54]In 1666 Isaac Newton, based on his theories of refraction and color, perceived that the faults of the refracting telescope were due more to a lens’s varying refraction of light of different colors than to a lens’s imperfect shape. He concluded that light could not be refracted through a lens without causing chromatic aberrations, although he incorrectly concluded from some rough experiments[55] that all refracting substances would diverge the prismatic colors in a constant proportion to their mean refraction. From these experiments Newton concluded that no improvement could be made in the refracting telescope.[56] Newton’s experiments with mirrors showed that they did not suffer from the chromatic errors of lenses, for all colors of light the angle of incidence reflected in a mirror was equal to the angle of reflection, so as a proof to his theories Newton set out to build a reflecting telescope.[57] Newton completed his first telescope in 1668 and it is the earliest known functional reflecting telescope.[58] After much experiment, he chose an alloy (speculum metal) of tin and copper as the most suitable material for his objective mirror. He later devised means for grinding and polishing them, but chose a spherical shape for his mirror instead of a parabola to simplify construction. He added to his reflector what is the hallmark of the design of a “Newtonian telescope”, a secondary “diagonal” mirror near the primary mirror’s focus to reflect the image at 90° angle to an eyepiece mounted on the side of the telescope. This unique addition allowed the image to be viewed with minimal obstruction of the objective mirror. He also made all the tube, mount, and fittings. Newton’s first compact reflecting telescope had a mirror diameter of 1.3 inches and a focal ratio of f/5.[59] With it he found that he could see the four Galilean moons of Jupiter and the crescent phase of the planet Venus. Encouraged by this success, he made a second telescope with a magnifying power of 38x which he presented to the Royal Society of London in December 1672. This type of telescope is still called a Newtonian telescope. Light path in a Cassegrain telescope.A third form of reflecting telescope, the “Cassegrain reflector” was devised in 1672 by Laurent Cassegrain. The telescope had a small convex hyperboloidal secondary mirror placed near the prime focus to reflect light through a central hole in the main mirror. No further practical advance appears to have been made in the design or construction of the reflecting telescopes for another 50 years until John Hadley (best known as the inventor of the octant) developed ways to make precision aspheric and parabolic speculum metal mirrors. In 1721 he showed the first parabolic Newtonian reflector to the Royal Society.[60] It had a 6-inch (15 cm) diameter, 62+3⁄4-inch (159 cm) focal length speculum metal objective mirror. The instrument was examined by James Pound and James Bradley.[61] After remarking that Newton’s telescope had lain neglected for fifty years, they stated that Hadley had sufficiently shown that the invention did not consist in bare theory. They compared its performance with that of a 7.5 inches (190 mm) diameter aerial telescope originally presented to the Royal Society by Constantijn Huygens, Jr. and found that Hadley’s reflector, “will bear such a charge as to make it magnify the object as many times as the latter with its due charge”, and that it represents objects as distinct, though not altogether so clear and bright. Bradley and Samuel Molyneux, having been instructed by Hadley in his methods of polishing speculum metal, succeeded in producing large reflecting telescopes of their own, one of which had a focal length of 8 ft (2.4 m). These methods of fabricating mirrors were passed on by Molyneux to two London opticians —Scarlet and Hearn— who started a business manufacturing telescopes.[62] The British mathematician, optician James Short began experimenting with building telescopes based on Gregory’s designs in the 1730s. He first tried making his mirrors out of glass as suggested by Gregory, but he later switched to speculum metal mirrors creating Gregorian telescopes with original designers parabolic and elliptic figures. Short then adopted telescope-making as his profession which he practised first in Edinburgh, and afterward in London. All Short’s telescopes were of the Gregorian form. Short died in London in 1768, having made a considerable fortune selling telescopes. Since speculum metal mirror secondaries or diagonal mirrors greatly reduced the light that reached the eyepiece, several reflecting telescope designers tried to do away with them. In 1762 Mikhail Lomonosov presented a reflecting telescope before the Russian Academy of Sciences forum. It had its primary mirror tilted at four degrees to telescope’s axis so the image could be viewed via an eyepiece mounted at the front of the telescope tube without the observer’s head blocking the incoming light. This innovation was not published until 1827, so this type came to be called the Herschelian telescope after a similar design by William Herschel.[63] William Herschel’s 49-inch (1,200 mm) “40-foot” telescope of 1789. Illustration from Encyclopædia Britannica Third Edition published in 1797.About the year 1774 William Herschel (then a teacher of music in Bath, England) began to occupy his leisure hours with the construction of reflector telescope mirrors, finally devoted himself entirely to their construction and use in astronomical research. In 1778, he selected a 6+1⁄4-inch (16 cm) reflector mirror (the best of some 400 telescope mirrors which he had made) and with it, built a 7-foot (2.1 m) focal length telescope. Using this telescope, he made his early brilliant astronomical discoveries. In 1783, Herschel completed a reflector of approximately 18 inches (46 cm) in diameter and 20 ft (6.1 m) focal length. He observed the heavens with this telescope for some twenty years, replacing the mirror several times. In 1789 Herschel finished building his largest reflecting telescope with a mirror of 49 inches (120 cm) and a focal length of 40 ft (12 m), (commonly known as his 40-foot telescope) at his new home, at Observatory House in Slough, England. To cut down on the light loss from the poor reflectivity of the speculum mirrors of that day, Herschel eliminated the small diagonal mirror from his design and tilted his primary mirror so he could view the formed image directly. This design has come to be called the Herschelian telescope. He discovered Saturn’s sixth known moon, Enceladus, the first night he used it (August 28, 1789), and on September 17, its seventh known moon, Mimas. This telescope was world’s largest telescope for over 50 years. However, this large scope was difficult to handle and thus less used than his favorite 18.7-inch reflector. In 1845 William Parsons, 3rd Earl of Rosse built his 72-inch (180 cm) Newtonian reflector called the “Leviathan of Parsonstown” with which he discovered the spiral form of galaxies. All of these larger reflectors suffered from the poor reflectivity and fast tarnishing nature of their speculum metal mirrors. This meant they need more than one mirror per telescope since mirrors had to be frequently removed and re-polished. This was time-consuming since the polishing process could change the curve of the mirror, so it usually had to be “re-figured” to the correct shape. Achromatic refracting telescopesSee also: Achromatic lens Light path through an achromatic lens.From the time of the invention of the first refracting telescopes it was generally supposed that chromatic errors seen in lenses simply arose from errors in the spherical figure of their surfaces. Opticians tried to construct lenses of varying forms of curvature to correct these errors.[39] Isaac Newton discovered in 1666 that chromatic colors actually arose from the un-even refraction of light as it passed through the glass medium. This led opticians to experiment with lenses constructed of more than one type of glass in an attempt to canceling the errors produced by each type of glass. It was hoped that this would create an “achromatic lens”; a lens that would focus all colors to a single point, and produce instruments of much shorter focal length. The first person who succeeded in making a practical achromatic refracting telescope was Chester Moore Hall from Essex, England.[citation needed] He argued that the different humours of the human eye refract rays of light to produce an image on the retina which is free from color, and he reasonably argued that it might be possible to produce a like result by combining lenses composed of different refracting media. After devoting some time to the inquiry he found that by combining two lenses formed of different kinds of glass, he could make an achromatic lens where the effects of the unequal refractions of two colors of light (red and blue) was corrected. In 1733, he succeeded in constructing telescope lenses which exhibited much reduced chromatic aberration. One of his instruments had an objective measuring 2+1⁄2 inches (6.4 cm) with a relatively short focal length of 20 inches (51 cm). Hall was a man of independent means and seems to have been careless of fame; at least he took no trouble to communicate his invention to the world. At a trial in Westminster Hall about the patent rights granted to John Dollond (Watkin v. Dollond), Hall was admitted to be the first inventor of the achromatic telescope. However, it was ruled by Lord Mansfield that it was not the original inventor who ought to profit from such invention, but the one who brought it forth for the benefit of mankind. In 1747, Leonhard Euler sent to the Prussian Academy of Sciences a paper in which he tried to prove the possibility of correcting both the chromatic and the spherical aberration of a lens. Like Gregory and Hall, he argued that since the various humours of the human eye were so combined as to produce a perfect image, it should be possible by suitable combinations of lenses of different refracting media to construct a perfect telescope objective. Adopting a hypothetical law of the dispersion of differently colored rays of light, he proved analytically the possibility of constructing an achromatic objective composed of lenses of glass and water. All of Euler’s efforts to produce an actual objective of this construction were fruitless—a failure which he attributed solely to the difficulty of procuring lenses that worked precisely to the requisite curves.[64] John Dollond agreed with the accuracy of Euler’s analysis, but disputed his hypothesis on the grounds that it was purely a theoretical assumption: that the theory was opposed to the results of Newton’s experiments on the refraction of light, and that it was impossible to determine a physical law from analytical reasoning alone.[65] In 1754, Euler sent to the Berlin Academy a further paper in which starting from the hypothesis that light consists of vibrations excited in an elastic fluid by luminous bodies—and that the difference of color of light is due to the greater or lesser frequency of these vibrations in a given time— he deduced his previous results. He did not doubt the accuracy of Newton’s experiments quoted by Dollond. Dollond did not reply to this, but soon afterwards he received an abstract of a paper by the Swedish mathematician and astronomer, Samuel Klingenstierna, which led him to doubt the accuracy of the results deduced by Newton on the dispersion of refracted light. Klingenstierna showed from purely geometrical considerations (fully appreciated by Dollond) that the results of Newton’s experiments could not be brought into harmony with other universally accepted facts of refraction. Dollond telescope.As a practical man, Dollond at once put his doubts to the test of experiment: he confirmed the conclusions of Klingenstierna, discovered a difference far beyond his hopes in the refractive qualities of different kinds of glass with respect to the divergence of colors, and was thus rapidly led to the construction of lenses in which first the chromatic aberration—and afterwards—the spherical aberration were corrected.[66] Dollond was aware of the conditions necessary for the attainment of achromatism in refracting telescopes, but relied on the accuracy of experiments made by Newton. His writings show that with the exception of his bravado, he would have arrived sooner at a discovery for which his mind was fully prepared. Dollond’s paper[66] recounts the successive steps by which he arrived at his discovery independently of Hall’s earlier invention—and the logical processes by which these steps were suggested to his mind. In 1765 Peter Dollond (son of John Dollond) introduced the triple objective, which consisted of a combination of two convex lenses of crown glass with a concave flint lens between them. He made many telescopes of this kind.[citation needed] The difficulty of procuring disks of glass (especially of flint glass) of suitable purity and homogeneity limited the diameter and light gathering power of the lenses found in the achromatic telescope. It was in vain that the French Academy of Sciences offered prizes for large perfect disks of optical flint glass. The difficulties with the impractical metal mirrors of reflecting telescopes led to the construction of large refracting telescopes. By 1866 refracting telescopes had reached 18 inches (46 cm) in aperture with many larger “Great refractors” being built in the mid to late 19th century. In 1897, the refractor reached its maximum practical limit in a research telescope with the construction of the Yerkes Observatorys’ 40-inch (100 cm) refractor (although a larger refractor Great Paris Exhibition Telescope of 1900 with an objective of 49.2 inches (1.25 m) diameter was temporarily exhibited at the Paris 1900 Exposition). No larger refractors could be built because of gravity’s effect on the lens. Since a lens can only be held in place by its edge, the center of a large lens will sag due to gravity, distorting the image it produces.[67] Large reflecting telescopesSee also: List of largest optical telescopes historically The 200-inch (5.1 m) Hale telescope at Mount PalomarIn 1856–57, Karl August von Steinheil and Léon Foucault introduced a process of depositing a layer of silver on glass telescope mirrors. Th
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