Arefracting telescope (also called arefractor) is a type ofoptical telescope that uses alens as itsobjective to form an image (also referred to adioptrictelescope). The refracting telescope design was originally used in spyglasses andastronomical telescopes but is also used forlong-focuscamera 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were very popular in the second half of the 19th century, for most research purposes, the refracting telescope has been superseded by thereflecting telescope, which allows largerapertures. A refractor'smagnification is calculated by dividing the focal length of the objective lens by that of theeyepiece.[1]
Refracting telescopes typically have a lens at the front, then along 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.
Refractors were the earliest type ofoptical telescope. The first record of a refracting telescope appeared in theNetherlands about 1608, when a spectacle maker fromMiddelburg namedHans Lippershey unsuccessfully tried to patent one.[2] News of the patent spread fast andGalileo Galilei, happening to be inVenice in the month of May 1609, heard of the invention, constructeda version of his own, and applied it to making astronomical discoveries.[3]
All refracting telescopes use the same principles. The combination of anobjectivelens1 and some type ofeyepiece2 is used to gather more light than the human eye is able to collect on its own, focus it5, and present the viewer with abrighter,clearer, andmagnifiedvirtual image6.
The objective in a refracting telescoperefracts or bendslight. This refraction causesparallel light rays to converge at afocal point; while those not parallel converge upon afocal 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.[4]
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 calledrefracting telescopes orrefractors.
The designGalileo Galilei usedc. 1609 is commonly called aGalilean telescope.[5] It used a convergent (plano-convex) objective lens and a divergent (plano-concave) eyepiece lens (Galileo, 1610).[6] A Galilean telescope, because the design has no intermediary focus, results in a non-inverted (i.e., upright) image.[7]
Galileo's most powerful telescope, with a total length of just under 1 meter (39 in),[5]magnified objects about 30 times.[7] 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 itsfocal ratio) to limit aberrations, so his telescope produced blurry and distorted images with a narrow field of view.[7] Despite these flaws, the telescope was still good enough for Galileo to explore the sky. He used it to viewcraters on theMoon,[8] the fourlargest moons of Jupiter,[9] and thephases of Venus.[10]
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 angleα1 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.[citation needed]
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.[citation needed]
Engraved illustration of a 46 m (150 ft) focal length Keplerian astronomical refracting telescope built by Johannes Hevelius.[11]
TheKeplerian telescope, invented byJohannes Kepler in 1611, is an improvement on Galileo's design.[12] 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 –discuss] are converging. This allows for a much wider field of view and greatereye relief, but the image for the viewer is inverted. Considerably higher magnifications can be reached with this design, but, like the Galilean telescope, it still uses a simple single element objective lens so it needs to have a very high focal ratio to reduce aberrations[13] (Johannes Hevelius built an unwieldy f/225 telescope with a 200-millimetre (8 in) objective and a 46-metre (150 ft)focal length,[14][page needed] and even longer tubeless "aerial telescopes" were constructed). The design also allows for use of amicrometer at the focal plane (to determine the angular size and/or distance between objects observed).
Alvan Clark polishes the big Yerkes achromatic objective lens, over 1 meter (100 cm) across (1896).This 12-inch (30 cm) refractor is mounted in a dome on a mount that matches the Earth's rotation.
The next major step in the evolution of refracting telescopes was the invention of theachromatic 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 namedChester Moore Hall, although it was independently invented and patented byJohn Dollond around 1758. The design overcame the need for very long focal lengths in refracting telescopes by using an objective made of two pieces ofglass with differentdispersion, 'crown' and 'flint glass', to reducechromatic andspherical aberration. Each side of each piece is ground andpolished, and then the two pieces are assembled together. Achromatic lenses are corrected to bring twowavelengths (typically red and blue) into focus in the same plane.[citation needed]
Chester More Hall is noted as having made the first twin color corrected lens in 1730.[16]
Dollond achromats were quite popular in the 18th century.[17][18] A major appeal was they could be made shorter.[18] However, problems with glass making meant that the glass objectives were not made more than about four inches (10 cm) in diameter.[18]
In the late 19th century, the Swiss optician Pierre-Louis Guinand[19] developed a way to make higher quality glass blanks of greater than four inches (10 cm).[18] He passed this technology to his apprenticeJoseph von Fraunhofer, who further developed this technology and also developed the Fraunhofer doublet lens design.[18] 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.[citation needed]
Noted lens makers of the 19th century include:[20]
The Greenwich 28-inch (71 cm) refractor is a popular tourist attraction in 21st century London.
In theRoyal Observatory, Greenwich an 1838 instrument named theSheepshanks telescope includes an objective by Cauchoix.[26] The Sheepshanks had a 6.7-inch (17 cm) wide lens, and was the biggest telescope at Greenwich for about twenty years.[27]
An 1840 report from the Observatory noted of the then-new Sheepshanks telescope with the Cauchoix doublet:[28]
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.[29] An example of prime achievements of refractors, over 7 million people have been able to view through the 12-inch Zeiss refractor atGriffith Observatory since its opening in 1935; this is the most people to have viewed through any telescope.[29]
Achromats were popular in astronomy for making star catalogs, and they required less maintenance than metal mirrors. Some famous discoveries using achromats are the planetNeptune and theMoons of Mars.[citation needed]
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.[citation needed]
For example, the Nice Observatory debuted with 77-centimeter (30.31 in) refractor, the largest at the time, but was surpassed within only a couple of years.[30]
The Apochromatic lens usually comprises three elements that bring light of three different frequencies to a common focus
Apochromatic refractors have objectives built with special, extra-low dispersion materials. They are designed to bring three wavelengths (typically red, green, and blue) into focus in the same plane. The residual color error (tertiary spectrum) can be an order of magnitude less than that of an achromatic lens.[citation needed] Such telescopes contain elements offluorite or special, extra-low dispersion (ED) glass in the objective and produce a very crisp image that is virtually free of chromatic aberration.[31] 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.[18]
The 102 centimetres (40 in) refractor, atYerkes Observatory, the largest achromatic refractor ever put into astronomical use (photo taken on 6 May 1921, as Einstein was visiting)
Refractors suffer from residualchromatic andspherical aberration. This affects shorterfocal ratios more than longer ones. Anf/6 achromatic refractor is likely to show considerable color fringing (generally a purple halo around bright objects); anf/16 achromat has much less color fringing.
In very large apertures, there is also a problem oflens sagging, a result ofgravity deformingglass. 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).[35]
There is a further problem of glass defects, striae or smallair bubbles trapped within the glass. In addition, glass isopaque to certainwavelengths, and evenvisible 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 inreflecting telescopes, which can be made in far larger apertures and which have all but replaced refractors for astronomical research.
The ISS-WAC on theVoyager 1/2 used a 6 centimetres (2.4 in) lens, launched into space in the late 1970s, an example of the use of refractors in space.[36]
The "Große Refraktor", a double telescope with a 80cm (31.5") and 50 cm (19.5") lens, was used to discover calcium as an interstellar medium in 1904.Astronaut trains with camera with large lens
Refracting telescopes were noted for their use in astronomy as well as for terrestrial viewing. Many early discoveries of theSolar 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.[contradictory] 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.
Touristic telescope pointed to Matterhorn in Switzerland
Singlets
The Galilean moons and many other moons of the solar system, were discovered with single-element objectives and aerial telescopes.
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 themoons 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 atFoggy Bottom.[44] In 1893 the lens was remounted and put in a new dome, where it remains into the 21st century.[45]
In 1904, one of the discoveries made using Great Refractor of Potsdam (a double telescope with two doublets) was of theinterstellar medium.[48] The astronomerProfessor Hartmann determined from observations of the binary starMintaka in Orion, that there was the elementcalcium in the intervening space.[48]
Triplets
PlanetPluto was discovered by looking at photographs (i.e. 'plates' in astronomy vernacular) in ablink comparator taken with a refracting telescope, an astrograph with a 3 element 13-inch lens.[49][50]
The Yerkes Great refractor mounted at the 1893 World's Fair in Chicago; the tallest, longest, and biggest aperture refractor up to that time.The 68 cm (27 in) refractor at theVienna University Observatory
Examples of some of the largest achromatic refracting telescopes, over 60 cm (24 in) diameter.
^Edgerton, S. Y. (2009).The Mirror, the Window, and the Telescope: How Renaissance Linear Perspective Changed Our Vision of the Universe. Ithaca: Cornell University Press. p. 159.ISBN9780801474804.
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 teachJoseph von Fraunhofer at Utzschinder's (Joseph von Utzschneider (1763-1840) glassworks, and eventually started his own optical glass works.
^Lequeux, James (2013). "The Observatory: At Last!".Le Verrier—Magnificent and Detestable Astronomer. Astrophysics and Space Science Library. Vol. 397. pp. 77–125.doi:10.1007/978-1-4614-5565-3_4.ISBN978-1-4614-5564-6.
^King, H. C. (January 1949). "The optical work of Charles Tulley".Popular Astronomy.57: 74.Bibcode:1949PA.....57...74K.
^Royal Observatory, Greenwich (1840).Astronomical Observations, Made at the Royal Observatory at Greenwich, in the year 1838. Clarendon Press.hdl:2027/njp.32101074839562.{{cite book}}:|work= ignored (help)[page needed]
^abVasiljević, Darko (2002), "The Cooke triplet optimizations", in Vasiljević, Darko (ed.),Classical and Evolutionary Algorithms in the Optimization of Optical Systems, Springer US, pp. 187–211,doi:10.1007/978-1-4615-1051-2_13,ISBN9781461510512
^Hall, A. (January 1878). "Observations of the Satellites of Mars".Astronomische Nachrichten.91 (1):11–14.doi:10.1002/asna.18780910103.
^Morley, T. A. (February 1989). "A catalogue of ground-based astrometric observations of the Martian satellites, 1877-1982".Astronomy and Astrophysics Supplement Series.77 (2):209–226.Bibcode:1989A&AS...77..209M.
^Barnard, E. E. (12 October 1892). "Discovery and observations of a fifth satellite to Jupiter".The Astronomical Journal.12 (11):81–85.Bibcode:1892AJ.....12...81B.doi:10.1086/101715.
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