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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 dioptrics telescope). The refracting telescope design was originally used in spyglasses and astronomy telescopes but is also used for long-focus lens . 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were very popular in the second half of the 19th century, for most research purposes, the refracting telescope has been superseded by the reflecting telescope, which allows larger . A refractor's magnification is calculated by dividing the focal length of the objective lens by that of the eyepiece.
Refracting telescopes typically have a lens at the front, then a optical train, 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 /telephoto lens/long-focus lens.
The objective in a refracting telescope refraction or bends light. This refraction causes parallel light rays to converge at a focal point; while those not parallel converge upon a focal plane. The telescope converts a bundle of parallel rays to make an angle α, with the optical axis to a second parallel bundle with angle β. The ratio β/α is called the angular magnification. It equals the ratio between the retinal image sizes obtained with and without the telescope.Stephen G. Lipson, Ariel Lipson, Henry Lipson, Optical Physics 4th Edition, Cambridge University Press,
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.
Galileo's most powerful telescope, with a total length of just under , Magnification objects about 30 times. Galileo had to work with the poor lens technology of the time, and found he had to use aperture stops to reduce the diameter of the objective lens (increase its focal ratio) to limit aberrations, so his telescope produced blurry and distorted images with a narrow field of view. Despite these flaws, the telescope was still good enough for Galileo to explore the sky. He used it to view impact crater on the Moon, the four Galilean moons, and the phases of Venus.
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 (i.e., non-inverted or upright) as the object.
Huygens built an aerial telescope for Royal Society of London with a 19 cm (7.5″) single-element lens.
Chester More Hall is noted as having made the first twin color corrected lens in 1730.
Dollond achromats were quite popular in the 18th century. (2025). 9781441964021 ISBN 9781441964021 A major appeal was they could be made shorter. However, problems with glass making meant that the glass objectives were not made more than about in diameter.
In the late 19th century, the Swiss optician Pierre-Louis Guinand
developed a way to make higher quality glass blanks of greater than . He passed this technology to his apprentice Joseph von Fraunhofer, who further developed this technology and also developed the Fraunhofer doublet lens design. The breakthrough in glass making techniques led to the great refractors of the 19th century, that became progressively larger through the decade, eventually reaching over 1 meter by the end of that century before being superseded by silvered-glass reflecting telescopes in astronomy.
Noted lens makers of the 19th century include:
Some famous 19th century doublet refractors are the James Lick telescope (91 cm/36 in) and the Greenwich 28 inch refractor (71 cm). An example of an older refractor is the Shuckburgh telescope (dating to the late 1700s). A famous refractor was the "Trophy Telescope", presented at the 1851 Great Exhibition in London. The era of the '' in the 19th century saw large achromatic lenses, culminating with the largest achromatic refractor ever built, the Great Paris Exhibition Telescope of 1900.
In the Royal Observatory, Greenwich an 1838 instrument named the Sheepshanks telescope includes an objective by Cauchoix. The Sheepshanks had a wide lens, and was the biggest telescope at Greenwich for about twenty years.
An 1840 report from the Observatory noted of the then-new Sheepshanks telescope with the Cauchoix doublet:In the 1900s a noted optics maker was Zeiss. An example of prime achievements of refractors, over 7 million people have been able to view through the 12-inch Zeiss refractor at Griffith Observatory since its opening in 1935; this is the most people to have viewed through any telescope.
Achromats were popular in astronomy for making star catalogs, and they required less maintenance than metal mirrors. Some famous discoveries using achromats are the planet Neptune and the Moons of Mars.
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.
For example, the Nice Observatory debuted with refractor, the largest at the time, but was surpassed within only a couple of years.
In the 18th century, Dollond, a popular maker of doublet telescopes, also made a triplet, although they were not really as popular as the two element telescopes.
One of the famous triplet objectives is the Cooke triplet, noted for being able to correct the Seidal aberrations. It is recognized as one of the most important objective designs in the field of photography. The Cooke triplet can correct, with only three elements, for one wavelength, spherical aberration, coma, astigmatism, field curvature, and distortion.
In very large apertures, there is also a problem of , 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 .
There is a further problem of glass defects, striae or small trapped within the glass. In addition, glass is opaque to certain , and even optical spectrum 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 lens, launched into space in the late 1970s, an example of the use of refractors in space.
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. 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.
Galileo Galilei's discovered the Galilean satellites of Jupiter in 1610 with a refracting telescope.
The planet Saturn's moon, Titan, was discovered on March 25, 1655, by the Dutch astronomer Christiaan Huygens.
By the 18th century refractors began to have major competition from reflectors, which could be made quite large and did not normally suffer from the same inherent problem with chromatic aberration. Nevertheless, the astronomical community continued to use doublet refractors of modest aperture in comparison to modern instruments. Noted discoveries include the moons of Mars and a fifth moon of Jupiter, Amalthea.
Asaph Hall discovered Deimos on 12 August 1877 at about 07:48 UTC and Phobos on 18 August 1877, at the US Naval Observatory in Washington, D.C., at about 09:14 GMT (contemporary sources, using the pre-1925 Astronomical day that began the day at noon, give the time of discovery as 11 August 14:40 and 17 August 16:06 Washington mean time respectively).
The telescope used for the discovery was the refractor (telescope with a lens) then located at Foggy Bottom. In 1893 the lens was remounted and put in a new dome, where it remains into the 21st century.
Jupiter's moon Amalthea was discovered on 9 September 1892, by Edward Emerson Barnard using the refractor telescope at Lick Observatory. It was discovered by direct visual observation with the doublet-lens refractor.
In 1904, one of the discoveries made using Great Refractor of Potsdam (a double telescope with two doublets) was of the interstellar medium. The astronomer Professor Hartmann determined from observations of the binary star Mintaka in Orion, that there was the element calcium in the intervening space.
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.
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