A refracting telescope (also called a refractor) is a type of optical telescope that uses a lens as its objective to form an image (also referred to a dioptrictelescope). The refracting telescope design was originally used in spyglasses and astronomical telescopes but is also used for long-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were very popular in the second half of the 19th century, for most research purposes, the refracting telescope has been superseded by the reflecting telescope, which allows larger apertures. A refractor's magnification is calculated by dividing the focal length of the objective lens by that of the eyepiece.[1]
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.
Refractors were the earliest type of optical telescope. The first record of a refracting telescope appeared in the Netherlands about 1608, when a spectacle maker from Middelburg named Hans Lippershey unsuccessfully tried to patent one.[2] News of the patent spread fast and Galileo Galilei, happening to be in Venice in the month of May 1609, heard of the invention, constructed a version of his own, and applied it to making astronomical discoveries.[3]
Refracting telescope designs
All refracting telescopes use the same principles. The combination of an objectivelens1 and some type of eyepiece2 is used to gather more light than the human eye is able to collect on its own, focus it 5, and present the viewer with a brighter, clearer, and magnifiedvirtual image6.
The objective in a refracting telescope refracts or bends light. This refraction causes parallel light rays to converge at a focal point; while those not parallel converge upon a focal plane. The telescope converts a bundle of parallel rays to make an angle α, with the optical axis to a second parallel bundle with angle β. The ratio β/α is called the angular magnification. It equals the ratio between the retinal image sizes obtained with and without the telescope.[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 called refracting telescopes or refractors.
Galilean telescope
Optical diagram of Galilean telescopey – Distant object; y′ – Real image from objective; y″ – Magnified virtual image from eyepiece; D – Entrance pupil diameter; d – Virtual exit pupil diameter; L1 – Objective lens; L2 – Eyepiece lens e – Virtual exit pupil – Telescope equals
The design Galileo Galilei used 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 (3ft 3in),[6]magnified objects about 30 times.[8] Galileo had to work with the poor lens technology of the time, and found he had to use aperture stops to reduce the diameter of the objective lens (increase its focal ratio) to limit aberrations, so his telescope produced blurry and distorted images with a narrow field of view.[8] Despite these flaws, the telescope was still good enough for Galileo to explore the sky. He used it to view craters on the Moon,[9] the four largest moons of Jupiter,[10] and the phases of Venus.[11]
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
Engraved illustration of a 46m (150ft) focal length Keplerian astronomical refracting telescope built by Johannes Hevelius.
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–discuss] 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 (8in) objective and a 46-metre (150ft)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).
Alvan Clark polishes the big Yerkes achromatic objective lens, over 1 meter across, in 1896.This 12 inch refractor is mounted in a dome on a mount that matches the Earth's rotation
The next major step in the evolution of refracting telescopes was the invention of the achromatic lens, a lens with multiple elements that helped solve problems with chromatic aberration and allowed shorter focal lengths. It was invented in 1733 by an English barrister named Chester Moore Hall, although it was independently invented and patented by John Dollond around 1758. The design overcame the need for very long focal lengths in refracting telescopes by using an objective made of two pieces of glass with different dispersion, '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]
The Greenwich 28-inch refractor is a popular tourist attraction in 21st century London
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 (17cm) 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 (30.31in) refractor, the largest at the time, but was surpassed within only a couple of years.[32]
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.[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]
The 102 centimetres (40in) refractor, at Yerkes Observatory, the largest achromatic refractor ever put into astronomical use (photo taken on 6 May 1921, as Einstein was visiting)
Refractors suffer from residual chromatic and spherical aberration. This affects shorter focal ratios more than longer ones. A 100mm (4in)f/6 achromatic refractor is likely to show considerable color fringing (generally a purple halo around bright objects). A 100mm (4in)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 (39in).[38]
There is a further problem of glass defects, striae or small air bubbles trapped within the glass. In addition, glass is opaque to certain wavelengths, and even visible light is dimmed by reflection and absorption when it crosses the air-glass interfaces and passes through the glass itself. Most of these problems are avoided or diminished in reflecting telescopes, which can be made in far larger apertures and which have all but replaced refractors for astronomical research.
The ISS-WAC on the Voyager 1/2 used a 6cm (2.36″) lens, launched into space in the late 1970s, an example of the use of refractors in space.[39]
Applications and achievements
The "Große Refraktor" a double telescope with a 80cm (31.5") and 50 cm (19.5") lenses, 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 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.
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 the Moons of Mars and a fifth moon of Jupiter, Amalthea.
The telescope used for the discovery was the 26-inch (66cm) 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]
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]
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 68cm (27in) refractor at the Vienna University Observatory
Examples of some of the largest achromatic refracting telescopes, over 60cm (24in) diameter.
In optics, chromatic aberration (CA), also called chromatic distortion and spherochromatism, is a failure of a lens to focus all colors to the same point. It is caused by dispersion: the refractive index of the lens elements varies with the wavelength of light. The refractive index of most transparent materials decreases with increasing wavelength. Since the focal length of a lens depends on the refractive index, this variation in refractive index affects focusing. Chromatic aberration manifests itself as "fringes" of color along boundaries that separate dark and bright parts of the image.
An achromatic lens or achromat is a lens that is designed to limit the effects of chromatic and spherical aberration. Achromatic lenses are corrected to bring two wavelengths into focus on the same plane. Wavelengths in between these two then have better focus error than could be obtained with a simple lens.
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.
An optical telescope is a telescope that gathers and focuses light mainly from the visible part of the electromagnetic spectrum, to create a magnified image for direct visual inspection, to make a photograph, or to collect data through electronic image sensors.
In optical engineering, an objective is an optical element that gathers light from an object being observed and focuses the light rays from it to produce a real image of the object. Objectives can be a single lens or mirror, or combinations of several optical elements. They are used in microscopes, binoculars, telescopes, cameras, slide projectors, CD players and many other optical instruments. Objectives are also called object lenses, object glasses, or objective glasses.
The Newtonian telescope, also called the Newtonian reflector or just a Newtonian, is a type of reflecting telescope invented by the English scientist Sir Isaac Newton, using a concave primary mirror and a flat diagonal secondary mirror. Newton's first reflecting telescope was completed in 1668 and is the earliest known functional reflecting telescope. The Newtonian telescope's simple design has made it very popular with amateur telescope makers.
An apochromat, or apochromatic lens (apo), is a photographic or other lens that has better correction of chromatic and spherical aberration than the much more common achromat lenses.
An eyepiece, or ocular lens, is a type of lens that is attached to a variety of optical devices such as telescopes and microscopes. It is named because it is usually the lens that is closest to the eye when someone looks through an optical device to observe an object or sample. The objective lens or mirror collects light from an object or sample and brings it to focus creating an image of the object. The eyepiece is placed near the focal point of the objective to magnify this image to the eyes. The amount of magnification depends on the focal length of the eyepiece.
Markree Observatory was an astronomical observatory in County Sligo, Ireland. The asteroid 9 Metis was discovered from this observatory in 1848 by Cooper's assistant Andrew Graham using a comet seeker telescope. The observatory was also home to the largest refractor of the early 1830s, which had a 13.3-inch (340 mm) aperture Cauchoix of Paris lens; the largest in the world at that time. The observatory also housed a number of instruments and was operated to varying degrees throughout the 19th century.
The first reflecting telescope built by Sir Isaac Newton in 1668 is a landmark in the history of telescopes, being the first known successful reflecting telescope. It was the prototype for a design that later came to be called the Newtonian telescope. There were some early prototypes and also modern replicas of this design.
An aerial telescope is a type of very long focal length refracting telescope, built in the second half of the 17th century, that did not use a tube. Instead, the objective was mounted on a pole, tree, tower, building or other structure on a swivel ball-joint. The observer stood on the ground and held the eyepiece, which was connected to the objective by a string or connecting rod. By holding the string tight and maneuvering the eyepiece, the observer could aim the telescope at objects in the sky. The idea for this type of telescope may have originated in the late 17th century with the Dutch mathematician, astronomer and physicist Christiaan Huygens and his brother Constantijn Huygens, Jr., though it is not clear if they actually invented it.
The Galileoscope is a small mass-produced refractor telescope, designed with the intention of increasing public interest in astronomy and science. It was developed for the International Year of Astronomy 2009. It is meant to be an inexpensive means by which millions of people can view the same things seen by Galileo Galilei, such as the craters of Earth's Moon, four of Jupiter's moons, and the Pleiades. The small telescope has an aperture of 50 mm (2.0 in) and a relatively long focal length of 500 mm, for a focal ratio of f/10.
Great refractor refers to a large telescope with a lens, usually the largest refractor at an observatory with an equatorial mount. The preeminence and success of this style in observational astronomy defines an era in modern telescopy in the 19th and early 20th century. Great refractors were large refracting telescopes using achromatic lenses. They were often the largest in the world, or largest in a region. Despite typical designs having smaller apertures than reflectors, great refractors offered a number of advantages and were popular for astronomy. It was also popular to exhibit large refractors at international exhibits, and examples of this include the Trophy Telescope at the 1851 Great Exhibition, and the Yerkes Great Refractor at the 1893 World's Fair in Chicago.
The Craig telescope was a large telescope built in the 1850s, and while much larger than previous refracting telescopes, it had some problems that hampered its use. Its unique design and potential caused a great deal of excitement in its day. The telescope was ready in August 1852 and was visited by William Parsons, 3rd Earl of Rosse, famous for the Leviathan of Parsonstown, a reflecting telescope and the largest telescope of this age with a six foot mirror.
Robert-Aglaé Cauchoix was a French optician and instrument maker, whose lenses played a part in the race of the great refractor telescopes in the first half of the 19th century.
The Greenwich 28-inch refractor is a telescope at the Royal Observatory, Greenwich, where it was first installed in 1893. It is a 28-inch ( 71 cm) aperture objective lens telescope, otherwise known as a refractor, and was made by the telescope maker Sir Howard Grubb. The achromatic lens was made Grubb from Chance Brothers glass. The mounting is older however and dates to the 1850s, having been designed by Royal Observatory director George Airy and the firm Ransomes and Simms. The telescope is noted for its spherical dome which extends beyond the tower, nicknamed the "onion" dome. Another name for this telescope is "The Great Equatorial" which it shares with the building, which housed an older but smaller telescope previously.
Meudon Great Refractor is a double telescope with lenses, in Meudon, France. It is a twin refracting telescope built in 1891, with one visual and one photographic, on a single square-tube together on an equatorial mount, inside a dome. The Refractor was built for the Meudon Observatory, and is the largest double doublet refracting telescope in Europe, but about the same size as several telescopes in this period, when this style of telescope was popular. Other large telescopes of a similar type include the James Lick telescope (91.4), Potsdam Great Refractor (80+50 cm), and the Greenwich 28 inch refractor (71.1 cm).
↑ Sidereus Nuncius or The Sidereal Messenger, 1610, Galileo Galilei et al., 1989, pg. 37, The University of Chicago Press, Albert van Helden tr., (History Dept. Rice University, Houston, TX), ISBN0-226-27903-0.
↑ 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 teach Joseph von Fraunhofer at Utzschinder's (Joseph von Utzschneider (1763-1840) glassworks, and eventually started his own optical glass works.
1 2 Vasiljević, 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. (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.
↑ 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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