Refracting telescope
A refracting telescope (also called a refractor) is a type of
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
Refractors were the earliest type of
Refracting telescope designs
All refracting telescopes use the same principles. The combination of an
The objective in a refracting telescope
Refracting telescopes can come in many different configurations to correct for image orientation and types of aberration. Because the image was formed by the bending of light, or refraction, these telescopes are called refracting telescopes or refractors.
Galilean telescope
The design Galileo Galilei used c. 1609 is commonly called a Galilean telescope.[6] It used a convergent (plano-convex) objective lens and a divergent (plano-concave) eyepiece lens (Galileo, 1610).[7] A Galilean telescope, because the design has no intermediary focus, results in a non-inverted and, with the help of some devices, an upright image.[8]
Galileo's most powerful telescope, with a total length of 980 millimetres (3 ft 3 in),
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.
The final image (y″) is a virtual image, located at infinity and is the same way up as the object.
Keplerian telescope
The Keplerian telescope, invented by Johannes Kepler in 1611, is an improvement on Galileo's design.[13] 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 ] 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[14] (Johannes Hevelius built an unwieldy f/225 telescope with a 200-millimetre (8 in) objective and a 46-metre (150 ft) focal length,[15] 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).
Achromatic refractors
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.
Chester More Hall is noted as having made the first twin color corrected lens in 1730.[17]
Dollond achromats were quite popular in the 18th century.[18][19] A major appeal was they could be made shorter.[19] However, problems with glass making meant that the glass objectives were not made more than about four inches in diameter.[19]
In the late 19th century, the Swiss optician Pierre-Louis Guinand[20] developed a way to make higher quality glass blanks of greater than four inches.[19][21] He passed this technology to his apprentice Joseph von Fraunhofer, who further developed this technology and also developed the Fraunhofer doublet lens design.[19] 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.
Noted lens makers of the 19th century include:[22]
- Alvan Clark
- Brashear[23]
- Chance Brothers
- Cauchoix[24]
- Fraunhofer[25]
- Gautier
- Grubb
- Henry Brothers
- Lerebours[26]
- Tulley[27]
Some famous 19th century doublet refractors are the
In the Royal Observatory, Greenwich an 1838 instrument named the Sheepshanks telescope includes an objective by Cauchoix.[28] The Sheepshanks had a 6.7 inches (17 cm) wide lens, and was the biggest telescope at Greenwich for about twenty years.[29]
An 1840 report from the Observatory noted of the then-new Sheepshanks telescope with the Cauchoix doublet:[30]
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.[31] 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.[31]
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.
The long achromats, despite having smaller aperture than the larger reflectors, were often favoured for "prestige" observatories. In the late 18th century, every few years, a larger and longer refractor would debut.
For example, the Nice Observatory debuted with 77-centimetre (30.31 in) refractor, the largest at the time, but was surpassed within only a couple of years.[32]
Apochromatic refractors
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.[33] 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.[34] 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.[19]
One of the famous triplet objectives is the
Technical considerations
Refractors suffer from residual chromatic and spherical aberration. This affects shorter focal ratios more than longer ones. A 100 mm (4 in) f/6 achromatic refractor is likely to show considerable color fringing (generally a purple halo around bright objects). A 100 mm (4 in) f/16 has little 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 1 meter (39 in).[38]
There is a further problem of glass defects, striae or small
The ISS-WAC on the Voyager 1/2 used a 6 cm (2.36″) lens, launched into space in the late 1970s, an example of the use of refractors in space.[39]
Applications and achievements
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 observeable 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). Refactors were often used for positional astronomy, besides from the other uses in photography and terrestrial viewing.
- Singlets
The Galilean moons and many other moons of the solar system, were discovered with single-element objectives and aerial telescopes.
The planet Saturn's moon, Titan, was discovered on March 25, 1655, by the Dutch astronomer Christiaan Huygens.[41][42]
- 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.
The telescope used for the discovery was the 26-inch (66 cm) refractor (telescope with a lens) then located at Foggy Bottom.[47] In 1893 the lens was remounted and put in a new dome, where it remains into the 21st century.[48]
Jupiter's moon Amalthea was discovered on 9 September 1892, by Edward Emerson Barnard using the 36 inches (91 cm) refractor telescope at Lick Observatory.[49][50] It was discovered by direct visual observation with the doublet-lens refractor.[40]
In 1904, one of the discoveries made using Great Refractor of Potsdam (a double telescope with two doublets) was of the interstellar medium.[51] The astronomer Professor Hartmann determined from observations of the binary star Mintaka in Orion, that there was the element calcium in the intervening space.[51]
- 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.[52][53]
List of the largest refracting telescopes
Examples of some of the largest achromatic refracting telescopes, over 60 cm (24 in) diameter.
- Great Paris Exhibition Telescope of 1900 (1.25 m or 49 in) – dismantled after exhibition
- Yerkes Observatory (101.6 cm or 40 in)
- Swedish 1-m Solar Telescope (98 cm or 39 in)
- Lick Observatory (91 cm or 36 in)
- Paris Observatory Meudon Great Refractor (83 cm (33 in), + 62 cm (24 in))
- Potsdam Great Refractor (80 cm (31 in), + 50 cm (20 in))
- Nice Observatory (77 cm or 30 in)
- John Wall (76.20 cm or 30 in) dialyte refracting telescope - the largest refractor built by an individual, at Hanwell Community Observatory[54]
- Royal Greenwich Observatory, (71 cm or 28 in) aperture lens
- Great Refractor of Vienna Observatory, (69 cm or 27 in)
- Archenhold Observatory – the longest refracting telescope ever built (68 cm or 27 in × 21 m or 69 ft focal length)
- United States Naval Observatory refractor, (66 cm or 26 in)
- Newall refractor at the National Observatory of Athens (62.5 cm or 24.6 in)
- Lowell Observatory (61 cm or 24 in)
See also
- Astrograph
- Baden-Powell's unilens
- Catadioptric telescopes
- List of largest optical refracting telescopes
- List of largest optical telescopes historically
- List of telescope types
- Reflecting telescope
- Star diagonal
- Heliometer
Further reading
References
- ^ "Telescope Calculations". Northern Stars. Retrieved 20 December 2013.
- ^ Albert Van Helden, Sven Dupré, Rob van Gent, The Origins of the Telescope, Amsterdam University Press, 2010, pages 3-4, 15
- ^ Science, Lauren Cox 2017-12-21T03:30:00Z; Astronomy. "Who Invented the Telescope?". Space.com. Retrieved 26 October 2019.
{{cite web}}
: CS1 maint: numeric names: authors list (link) - ISBN 978-0-521-49345-1
- ^ http://upload.wikimedia.org/wikipedia/commons/1/17/Galileantelescope_2.png [bare URL image file]
- ^ a b "Galileo's telescope - The instrument". Museo Galileo: Institute and Museum of the History of Science. 2008. Retrieved 27 September 2020.
- ISBN 0-226-27903-0.
- ^ a b c "Galileo's telescope - How it works". Museo Galileo: Institute and Museum of the History of Science. 2008. Retrieved 27 September 2020.
- ISBN 9780801474804.
- ISBN 978-0-226-16226-3.
- ^ "Phases of Venus". Intellectual Mathematics. 2 June 2019. Retrieved 27 September 2020.
- ^ Hevelius, Johannes (1673). Machina Coelestis. Vol. First Part. Auctor.
- ISBN 978-0-900099-15-1.)
{{cite book}}
: CS1 maint: location missing publisher (link - ^ "Galileo's telescope - Chromatic aberration". Museo Galileo - Istituto e Museo di Storia della Scienza. Retrieved 5 March 2012.
- ^ LOUIS BELL, Ph.D., THE TELESCOPE, McGRAW-HILL BOOK COMPANY - 1922
- ^ Paul Schlyter, Largest optical telescopes of the world
- PMID 25825026.
- ^ "Dollond Telescope". National Museum of American History. Retrieved 19 November 2019.
- ^ ISBN 9781441964038.
- ^
- 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.
- King, Henry C. (1 January 2003). The History of the Telescope. Courier Corporation. ISBN 9780486432656.
- ^ Neil English, Choosing and Using a Refracting Telescope, Springer Science & Business Media - 2010, page 12
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- ^ "Brashear House Historical Marker". ExplorePaHistory.com. WITF, Inc. Retrieved 16 November 2021.
- ^ "Cauchoix, Robert-Aglae". Canvases, Carats and Curiosities. 31 March 2015. Retrieved 26 October 2019.
- ^ Ferguson, Kitty (20 March 2014). "The Glassmaker Who Sparked Astrophysics". Nautilus. Retrieved 26 October 2019.
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- ^ "1949PA.....57...74K Page 75". articles.adsabs.harvard.edu. Retrieved 19 November 2019.
- ^ "Sheepshanks telescope". UK: Royal Museums Greenwich. Retrieved 27 February 2014.
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- ^ Astronomical Observations, Made at the Royal Observatory at Greenwich, ... Clarendon Press. 1840.
- ^ a b "Griffith Observatory - Southern California's gateway to the cosmos!".
- ^ The Observatory, "Large Telescopes", Page 248
- ^ "Apochromat - an overview | ScienceDirect Topics". www.sciencedirect.com. Retrieved 17 October 2022.
- ^ "Starizona's Guide to CCD Imaging". Starizona.com. Archived from the original on 17 October 2013. Retrieved 17 October 2013.
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- ISBN 9781461510512.
- ^ ISBN 9781461510512
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- ^ "Voyager". astronautix.com. Archived from the original on 11 September 2016.
- ^ ISBN 9780521632805.
- ^ "Lifting Titan's Veil" (PDF). Cambridge. p. 4. Archived from the original (PDF) on 22 February 2005.
- ^ "Titan". Astronomy Picture of the Day. NASA. Archived from the original on 27 March 2005.
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- ^ "Notes: The Satellites of Mars". The Observatory, Vol. 1, No. 6. 20 September 1877. pp. 181–185. Retrieved 12 September 2006.
- ^ Hall, A. (17 October 1877). "Observations of the Satellites of Mars" (Signed 21 September 1877). Astronomische Nachrichten, Vol. 91, No. 2161. pp. 11/12–13/14. Retrieved 12 September 2006.
- ^ Morley, T. A.; A Catalogue of Ground-Based Astrometric Observations of the Martian Satellites, 1877-1982, Astronomy and Astrophysics Supplement Series, Vol. 77, No. 2 (February 1989), pp. 209–226 (Table II, p. 220: first observation of Phobos on 1877-08-18.38498)
- ^ "Telescope: Naval Observatory 26-inch Refractor". amazing-space.stsci.edu. Retrieved 29 October 2018.
- ^ "The 26-inch "Great Equatorial" Refractor". United States Naval Observatory. Retrieved 29 October 2018.
- doi:10.1086/101715.
- ^ Lick Observatory (1894). A Brief Account of the Lick Observatory of the University of California. The University Press. p. 7–.
- ^ ISBN 9781591028826.
- ^ "The Pluto Telescope". Lowell Observatory. Retrieved 19 November 2019.
- ^ "Pluto Discovery Plate". National Air and Space Museum. Retrieved 19 November 2019.
- ^ "John Wall refractor | Hanwell Community Observatory".