![Zeiss2](https://upload.wikimedia.org/wikipedia/commons/7/71/Zeiss2.jpg)
A refracting telescope (also called a refractor) is a type of
optical telescope
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 electro ...
that uses a
lens
A lens is a transmissive optical device which focuses or disperses a light beam by means of refraction. A simple lens consists of a single piece of transparent material, while a compound lens consists of several simple lenses (''elements''), ...
as its
objective
Objective may refer to:
* Objective (optics), an element in a camera or microscope
* ''The Objective'', a 2008 science fiction horror film
* Objective pronoun, a personal pronoun that is used as a grammatical object
* Objective Productions, a Brit ...
to form an image (also referred to a
dioptric
Dioptrics is the branch of optics dealing with refraction, similarly the branch dealing with mirrors is known as catoptrics. Dioptrics is the study of the refraction of light, especially by lenses. Telescopes that create their image with an objec ...
telescope
A telescope is a device used to observe distant objects by their emission, absorption, or reflection of electromagnetic radiation. Originally meaning only an optical instrument using lenses, curved mirrors, or a combination of both to observe ...
). The refracting telescope design was originally used in spyglasses and
astronomical
Astronomy () is a natural science that studies celestial objects and phenomena. It uses mathematics, physics, and chemistry in order to explain their origin and evolution. Objects of interest include planets, moons, stars, nebulae, galaxies ...
telescopes but is also used for
long-focus camera 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were very popular in the second half of the 19th century, for most research purposes, the refracting telescope has been superseded by the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
A reflecting telescope (also called a reflector) is a telescope that uses a single or a combination of curved mirrors that reflect light and form an image. The reflecting telescope was invented in the 17th century by Isaac Newton as an alternati ...
, which allows larger
aperture
In optics, an aperture is a hole or an opening through which light travels. More specifically, the aperture and focal length of an optical system determine the cone angle of a bundle of rays that come to a focus in the image plane.
An opt ...
s. A refractor's
magnification is calculated by dividing the focal length of the objective lens by that of the
eyepiece
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 the device. The ...
.
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 telescope is a technology that has often been applied to other optical devices, such as
binoculars
Binoculars or field glasses are two refracting telescopes mounted side-by-side and aligned to point in the same direction, allowing the viewer to use both eyes (binocular vision) when viewing distant objects. Most binoculars are sized to be held ...
and
zoom lens
A zoom lens is a mechanical assembly of lens elements for which the focal length (and thus angle of view) can be varied, as opposed to a fixed-focal-length (FFL) lens (see prime lens).
A true zoom lens, also called a parfocal lens, is one ...
es/
telephoto lens
A telephoto lens, in photography and cinematography, is a specific type of a long-focus lens in which the physical length of the lens is shorter than the focal length. This is achieved by incorporating a special lens group known as a ''telephoto ...
/
long-focus lens
In photography, a long-focus lens is a camera lens which has a focal length that is longer than the diagonal measure of the film or sensor that receives its image.
It is used to make distant objects appear magnified with magnification increasin ...
.
Invention
Refractors were the earliest type of
optical telescope
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 electro ...
. The first record of a refracting telescope appeared in the
Netherlands
)
, anthem = ( en, "William of Nassau")
, image_map =
, map_caption =
, subdivision_type = Sovereign state
, subdivision_name = Kingdom of the Netherlands
, established_title = Before independence
, established_date = Spanish Netherl ...
about 1608, when a spectacle maker from
Middelburg Middelburg may refer to:
Places and jurisdictions Europe
* Middelburg, Zeeland, the capital city of the province of Zeeland, southwestern Netherlands
** Roman Catholic Diocese of Middelburg, a former Catholic diocese with its see in the Zeeland ...
named
Hans Lippershey
Hans Lipperhey (circa 1570 – buried 29 September 1619), also known as Johann Lippershey or Lippershey, was a German- Dutch spectacle-maker. He is commonly associated with the invention of the telescope, because he was the first one who tried to ...
unsuccessfully tried to patent one. News of the patent spread fast and
Galileo Galilei
Galileo di Vincenzo Bonaiuti de' Galilei (15 February 1564 – 8 January 1642) was an Italian astronomer, physicist and engineer, sometimes described as a polymath. Commonly referred to as Galileo, his name was pronounced (, ). He was ...
, happening to be in
Venice
Venice ( ; it, Venezia ; vec, Venesia or ) is a city in northeastern Italy and the capital of the Veneto Regions of Italy, region. It is built on a group of 118 small islands that are separated by canals and linked by over 400 ...
in the month of May 1609, heard of the invention, constructed a version of his own, and applied it to making astronomical discoveries.
Refracting telescope designs
All refracting telescopes use the same principles. The combination of an
objective
Objective may refer to:
* Objective (optics), an element in a camera or microscope
* ''The Objective'', a 2008 science fiction horror film
* Objective pronoun, a personal pronoun that is used as a grammatical object
* Objective Productions, a Brit ...
lens
A lens is a transmissive optical device which focuses or disperses a light beam by means of refraction. A simple lens consists of a single piece of transparent material, while a compound lens consists of several simple lenses (''elements''), ...
1 and some type of
eyepiece
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 the device. The ...
2 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
magnified virtual image
In optics, an ''image'' is defined as the collection of focus points of light rays coming from an object. A '' real image'' is the collection of focus points made by converging rays, while a virtual image is the collection of focus points ma ...
6.
The objective in a refracting telescope
refracts
In physics, refraction is the redirection of a wave as it passes from one transmission medium, medium to another. The redirection can be caused by the wave's change in speed or by a change in the medium. Refraction of light is the most common ...
or bends
light
Light or visible light is electromagnetic radiation that can be perceived by the human eye. Visible light is usually defined as having wavelengths in the range of 400–700 nanometres (nm), corresponding to frequencies of 750–420 tera ...
. This refraction causes
parallel
Parallel is a geometric term of location which may refer to:
Computing
* Parallel algorithm
* Parallel computing
* Parallel metaheuristic
* Parallel (software), a UNIX utility for running programs in parallel
* Parallel Sysplex, a cluster of ...
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.
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
Galileo di Vincenzo Bonaiuti de' Galilei (15 February 1564 – 8 January 1642) was an Italian astronomer, physicist and engineer, sometimes described as a polymath. Commonly referred to as Galileo, his name was pronounced (, ). He was ...
used is commonly called a Galilean telescope.
It used a convergent (plano-convex) objective lens and a divergent (plano-concave) eyepiece lens (Galileo, 1610). 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.
Galileo's most powerful telescope, with a total length of ,
magnified objects about 30 times.
Because of flaws in its design, such as the shape of the lens and the narrow field of view,
the images were blurry and distorted. Despite these flaws, the telescope was still good enough for Galileo to explore the sky. He used it to view
craters on the
Moon
The Moon is Earth's only natural satellite. It is the fifth largest satellite in the Solar System and the largest and most massive relative to its parent planet, with a diameter about one-quarter that of Earth (comparable to the width of ...
, the four
largest moons of Jupiter, and the
phases of Venus
The phases of Venus are the variations of lighting seen on the planet's surface, similar to lunar phases. The first recorded observations of them are thought to have been telescopic observations by Galileo Galilei in 1610. Although the extreme cr ...
.
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
Johannes Kepler (; ; 27 December 1571 – 15 November 1630) was a German astronomer, mathematician, astrologer, natural philosopher and writer on music. He is a key figure in the 17th-century Scientific Revolution, best known for his laws ...
in 1611, is an improvement on Galileo's design. 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 are converging. This allows for a much wider field of view and greater
eye relief {{Short description, Optical instrument
The eye relief of an optical instrument (such as a telescope, a microscope, or binoculars) is the distance from the last surface of an eyepiece within which the user's eye can obtain the full viewing angle. ...
, but the image for the viewer is inverted. Considerably higher magnifications can be reached with this design, but to overcome aberrations the simple objective lens needs to have a very high
f-ratio (
Johannes Hevelius
Johannes Hevelius
Some sources refer to Hevelius as Polish:
*
*
*
*
*
*
*
Some sources refer to Hevelius as German:
*
*
*
*
*of the Royal Society
* (in German also known as ''Hevel''; pl, Jan Heweliusz; – 28 January 1687) was a councillor ...
built one with a
focal length, and even longer tubeless "
aerial telescope
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 swive ...
s" were constructed). The design also allows for use of a
micrometer Micrometer can mean:
* Micrometer (device), used for accurate measurements by means of a calibrated screw
* American spelling of micrometre
The micrometre ( international spelling as used by the International Bureau of Weights and Measures; ...
at the focal plane (to determine the angular size and/or distance between objects observed).
Huygens built an aerial telescope for
Royal Society of London
The Royal Society, formally The Royal Society of London for Improving Natural Knowledge, is a learned society and the United Kingdom's national academy of sciences. The society fulfils a number of roles: promoting science and its benefits, re ...
with a 19 cm (7.5″) single-element lens.
[Paul Schlyter, ''Largest optical telescopes of the world''](_blank)
/ref>
Achromatic refractors
The next major step in the evolution of refracting telescopes was the invention of the achromatic lens
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 (typically red and blue) into focus on the same plane.
The most comm ...
, 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
Chester Moore Hall (9 December 1703, Leigh, Essex, England – 17 March 1771, Sutton) was a British lawyer and inventor who produced the first achromatic lenses in 1729 or 1733 (accounts differ).
He used the achromatic lens to build the first ach ...
, although it was independently invented and patented by John Dollond
John Dollond FRS (10 June O.S. (21 June N.S.) 170630 November 1761) was an English optician, known for his successful optics business and his patenting and commercialization of achromatic doublets.
Biography
Dollond was the son of a Hugue ...
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
Glass is a non-crystalline, often transparent, amorphous solid that has widespread practical, technological, and decorative use in, for example, window panes, tableware, and optics. Glass is most often formed by rapid cooling (quenching) of ...
with different dispersion
Dispersion may refer to:
Economics and finance
* Dispersion (finance), a measure for the statistical distribution of portfolio returns
* Price dispersion, a variation in prices across sellers of the same item
*Wage dispersion, the amount of variat ...
, 'crown
A crown is a traditional form of head adornment, or hat, worn by monarchs as a symbol of their power and dignity. A crown is often, by extension, a symbol of the monarch's government or items endorsed by it. The word itself is used, partic ...
' and 'flint glass
Flint glass is optical glass that has relatively high refractive index and low Abbe number (high dispersion). Flint glasses are arbitrarily defined as having an Abbe number of 50 to 55 or less. The currently known flint glasses have refractiv ...
', to reduce chromatic and spherical aberration
In optics, spherical aberration (SA) is a type of optical aberration, aberration found in optical systems that have elements with spherical surfaces. Lens (optics), Lenses and curved mirrors are prime examples, because this shape is easier to man ...
. Each side of each piece is ground and polish
Polish may refer to:
* Anything from or related to Poland, a country in Europe
* Polish language
* Poles
Poles,, ; singular masculine: ''Polak'', singular feminine: ''Polka'' or Polish people, are a West Slavic nation and ethnic group, w ...
ed, and then the two pieces are assembled together. Achromatic lenses are corrected to bring two wavelength
In physics, the wavelength is the spatial period of a periodic wave—the distance over which the wave's shape repeats.
It is the distance between consecutive corresponding points of the same phase on the wave, such as two adjacent crests, tro ...
s (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.
Dollond achromats were quite popular in the 18th century. 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 four inches 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 four inches. He passed this technology to his apprentice Joseph von Fraunhofer
Joseph Ritter von Fraunhofer (; ; 6 March 1787 – 7 June 1826) was a German physicist and optical lens manufacturer. He made optical glass, an achromatic telescope, and objective lenses. He also invented the spectroscope and developed diffract ...
, 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:
* Alvan Clark
*Brashear
*Chance Brothers
Chance Brothers and Company was a glassworks originally based in Spon Lane, Smethwick, West Midlands (formerly in Staffordshire), in England. It was a leading glass manufacturer and a pioneer of British glassmaking technology.
The Chance fam ...
* Cauchoix
* Fraunhofer
* Gautier
*Grubb Grubb is a family name and may refer to the following:
* Armstead Otey Grubb (1903–1968), American educator and acting president of Lincoln University
* Catharina Elisabet Grubb (1721–1788), Finnish industrialist
* Curtis Grubb (c. 1730 – ...
* Henry Brothers
*Lerebours
*Tulley
Some famous 19th century doublet refractors are the James Lick telescope
The James Lick Telescope is a refracting telescope built in 1888. It has a lens in diameter—a major achievement in its day. The instrument remains in operation and public viewing is allowed on a limited basis. Also called the "Great Lick Ref ...
(91 cm/36 in) and the Greenwich 28 inch refractor
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 (optics), objective lens telescope, otherwise known as a refracting telesc ...
(71 cm). An example of an older refractor is the Shuckburgh telescope
The Shuckburgh telescope or Shuckburgh equatorial refracting telescope was a diameter aperture telescope on an equatorial mount completed in 1791 for Sir George Shuckburgh (1751–1804) in Warwickshire, England, and built by British instrument ...
(dating to the late 1700s). A famous refractor was the "Trophy Telescope", presented at the 1851 Great Exhibition
The Great Exhibition of the Works of Industry of All Nations, also known as the Great Exhibition or the Crystal Palace Exhibition (in reference to the temporary The Crystal Palace, structure in which it was held), was an International Exhib ...
in London. The era of the 'great refractor
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 19t ...
s' in the 19th century saw large achromatic lenses, culminating with the largest achromatic refractor ever built, the Great Paris Exhibition Telescope of 1900
The Great Paris Exhibition Telescope of 1900, with an objective lens of in diameter, was the largest refracting telescope ever constructed. It was built as the centerpiece of the Paris Universal Exhibition of 1900. Its construction was instigat ...
.
In the Royal Observatory, Greenwich
The Royal Observatory, Greenwich (ROG; known as the Old Royal Observatory from 1957 to 1998, when the working Royal Greenwich Observatory, RGO, temporarily moved south from Greenwich to Herstmonceux) is an observatory situated on a hill in ...
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
Griffith Observatory is an observatory in Los Angeles, California on the south-facing slope of Mount Hollywood in Griffith Park. It commands a view of the Los Angeles Basin including Downtown Los Angeles to the southeast, Hollywood to the sout ...
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
Neptune is the eighth planet from the Sun and the farthest known planet in the Solar System. It is the fourth-largest planet in the Solar System by diameter, the third-most-massive planet, and the densest giant planet. It is 17 times ...
and the Moons of Mars
The two moons of Mars are Phobos (moon), Phobos and Deimos (moon), Deimos. They are irregular in shape. Both were discovered by American astronomer Asaph Hall in August 1877 and are named after the Greek mythology, Greek mythological twin charac ...
.
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 refractor, the largest at the time, but was surpassed within only a couple of years.[''The Observatory'', "Large Telescopes", Page 248]
/ref>
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 down to an order of magnitude less than that of an achromatic lens. Such telescopes contain elements of fluorite
Fluorite (also called fluorspar) is the mineral form of calcium fluoride, CaF2. It belongs to the halide minerals. It crystallizes in isometric cubic habit, although octahedral and more complex isometric forms are not uncommon.
The Mohs sca ...
or special, extra-low dispersion (ED) glass in the objective and produce a very crisp image that is virtually free of chromatic aberration. 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.
One of the famous triplet objectives is the Cooke triplet
The Cooke triplet is a photographic lens designed and patented (patent number GB 22,607) in 1893 by Dennis Taylor who was employed as chief engineer by T. Cooke & Sons of York. It was the first lens system that allowed elimination of most of th ...
, noted for being able to correct the Seidal aberrations. It is recognized as one of the most important objective
Objective may refer to:
* Objective (optics), an element in a camera or microscope
* ''The Objective'', a 2008 science fiction horror film
* Objective pronoun, a personal pronoun that is used as a grammatical object
* Objective Productions, a Brit ...
designs in the field of photography. The Cooke triplet can correct, with only three elements, for one wavelength, spherical aberration
In optics, spherical aberration (SA) is a type of optical aberration, aberration found in optical systems that have elements with spherical surfaces. Lens (optics), Lenses and curved mirrors are prime examples, because this shape is easier to man ...
, coma
A coma is a deep state of prolonged unconsciousness in which a person cannot be awakened, fails to respond normally to painful stimuli, light, or sound, lacks a normal wake-sleep cycle and does not initiate voluntary actions. Coma patients exhi ...
, astigmatism
Astigmatism is a type of refractive error due to rotational asymmetry in the eye's refractive power. This results in distorted or blurred vision at any distance. Other symptoms can include eyestrain, headaches, and trouble driving at n ...
, field curvature
Petzval field curvature, named for Joseph Petzval, describes the optical aberration in which a flat object normal to the optical axis (or a non-flat object past the hyperfocal distance) cannot be brought properly into focus on a flat image pl ...
, and distortion
In signal processing, distortion is the alteration of the original shape (or other characteristic) of a signal. In communications and electronics it means the alteration of the waveform of an information-bearing signal, such as an audio signal ...
.
Technical considerations
Refractors suffer from residual chromatic and spherical aberration
In optics, spherical aberration (SA) is a type of optical aberration, aberration found in optical systems that have elements with spherical surfaces. Lens (optics), Lenses and curved mirrors are prime examples, because this shape is easier to man ...
. This affects shorter focal ratio
In optics, the f-number of an optical system such as a camera lens is the ratio of the system's focal length to the diameter of the entrance pupil ("clear aperture").Smith, Warren ''Modern Optical Engineering'', 4th Ed., 2007 McGraw-Hill Pro ...
s more than longer ones. A achromatic refractor is likely to show considerable color fringing (generally a purple halo around bright objects). A 16 has little color fringing.
In very large apertures, there is also a problem of lens sagging, a result of gravity
In physics, gravity () is a fundamental interaction which causes mutual attraction between all things with mass or energy. Gravity is, by far, the weakest of the four fundamental interactions, approximately 1038 times weaker than the stro ...
deforming glass
Glass is a non-crystalline, often transparent, amorphous solid that has widespread practical, technological, and decorative use in, for example, window panes, tableware, and optics. Glass is most often formed by rapid cooling (quenching) of ...
. 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 air bubble
A bubble is a globule of one substance in another, usually gas in a liquid.
Due to the Marangoni effect, bubbles may remain intact when they reach the surface of the immersive substance.
Common examples
Bubbles are seen in many places in ev ...
s trapped within the glass. In addition, glass is opaque
Opacity or opaque may refer to:
* Impediments to (especially, visible) light:
** Opacities, absorption coefficients
** Opacity (optics), property or degree of blocking the transmission of light
* Metaphors derived from literal optics:
** In lingu ...
to certain wavelength
In physics, the wavelength is the spatial period of a periodic wave—the distance over which the wave's shape repeats.
It is the distance between consecutive corresponding points of the same phase on the wave, such as two adjacent crests, tro ...
s, and even visible light
Light or visible light is electromagnetic radiation that can be perceived by the human eye. Visible light is usually defined as having wavelengths in the range of 400–700 nanometres (nm), corresponding to frequencies of 750–420 te ...
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 telescope
A reflecting telescope (also called a reflector) is a telescope that uses a single or a combination of curved mirrors that reflect light and form an image. The reflecting telescope was invented in the 17th century by Isaac Newton as an alternati ...
s, 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
''Voyager 1'' is a space probe launched by NASA on September 5, 1977, as part of the Voyager program to study the outer Solar System and interstellar space beyond the Sun's heliosphere. Launched 16 days after its twin ''Voyager 2'', ''Voya ...
/ 2 used a 6 cm (2.36″) lens, launched into space in the late 1970s, an example of the use of refractors in space.
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
The Solar SystemCapitalization of the name varies. The International Astronomical Union, the authoritative body regarding astronomical nomenclature, specifies capitalizing the names of all individual astronomical objects but uses mixed "Solar S ...
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.
Galileo Galilei
Galileo di Vincenzo Bonaiuti de' Galilei (15 February 1564 – 8 January 1642) was an Italian astronomer, physicist and engineer, sometimes described as a polymath. Commonly referred to as Galileo, his name was pronounced (, ). He was ...
's discovered the Galilean satellites
The Galilean moons (), or Galilean satellites, are the four largest moons of Jupiter: Io, Europa, Ganymede, and Callisto. They were first seen by Galileo Galilei in December 1609 or January 1610, and recognized by him as satellites of Jupiter ...
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
Christiaan Huygens, Lord of Zeelhem, ( , , ; also spelled Huyghens; la, Hugenius; 14 April 1629 – 8 July 1695) was a Dutch mathematician, physicist, engineer, astronomer, and inventor, who is regarded as one of the greatest scientists of ...
.
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
The two moons of Mars are Phobos (moon), Phobos and Deimos (moon), Deimos. They are irregular in shape. Both were discovered by American astronomer Asaph Hall in August 1877 and are named after the Greek mythology, Greek mythological twin charac ...
and a fifth moon of Jupiter, Amalthea.
Asaph Hall
Asaph Hall III (October 15, 1829 – November 22, 1907) was an American astronomer who is best known for having discovered the two moons of Mars, Deimos and Phobos, in 1877. He determined the orbits of satellites of other planets and of double s ...
discovered Deimos on 12 August 1877 at about 07:48 UTC and Phobos on 18 August 1877, at the US Naval Observatory
United States Naval Observatory (USNO) is a scientific and military facility that produces geopositioning, navigation and timekeeping data for the United States Navy and the United States Department of Defense. Established in 1830 as the Depo ...
in Washington, D.C.
)
, image_skyline =
, image_caption = Clockwise from top left: the Washington Monument and Lincoln Memorial on the National Mall, United States Capitol, Logan Circle, Jefferson Memorial, White House, Adams Morgan, ...
, at about 09:14 GMT
Greenwich Mean Time (GMT) is the mean solar time at the Royal Observatory in Greenwich, London, counted from midnight. At different times in the past, it has been calculated in different ways, including being calculated from noon; as a cons ...
(contemporary sources, using the pre-1925 astronomical convention 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).[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)
The telescope used for the discovery was the refractor (telescope with a lens) then located at Foggy Bottom
Foggy Bottom is one of the oldest late 18th- and 19th-century neighborhoods in Washington, D.C., located west of the White House and downtown Washington, in the Northwest quadrant. It is bounded roughly by 17th Street NW to the east, Rock C ...
. 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
Edward Emerson Barnard (December 16, 1857 – February 6, 1923) was an American astronomer. He was commonly known as E. E. Barnard, and was recognized as a gifted observational astronomer. He is best known for his discovery of the high proper mo ...
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
In astronomy, the interstellar medium is the matter and radiation that exist in the space between the star systems in a galaxy. This matter includes gas in ionic, atomic, and molecular form, as well as dust and cosmic rays. It fills interstella ...
. The astronomer Professor Hartmann determined from observations of the binary star Mintaka
Mintaka , designation Delta Orionis (δ Orionis, abbreviated Delta Ori, δ Ori) and 34 Orionis (34 Ori), is a multiple star system some 1,200 light-years from the Sun in the constellation of Orion. Together with Alnitak (Zeta Or ...
in Orion, that there was the element calcium
Calcium is a chemical element with the symbol Ca and atomic number 20. As an alkaline earth metal, calcium is a reactive metal that forms a dark oxide-nitride layer when exposed to air. Its physical and chemical properties are most similar to ...
in the intervening space.
;Triplets
Planet Pluto
Pluto (minor-planet designation: 134340 Pluto) is a dwarf planet in the Kuiper belt, a ring of trans-Neptunian object, bodies beyond the orbit of Neptune. It is the ninth-largest and tenth-most-massive known object to directly orbit the S ...
was discovered by looking at photographs (i.e. 'plates' in astronomy vernacular) in a blink comparator
A blink comparator is a viewing apparatus formerly used by astronomers to find differences between two photographs of the night sky. It permits rapid switching from viewing one photograph to viewing the other, "blinking" back and forth between th ...
taken with a refracting telescope, an astrograph with a 3 element 13-inch lens.
List of the largest refracting telescopes
Examples of some of the largest achromatic refracting telescopes, over diameter.
* Great Paris Exhibition Telescope of 1900
The Great Paris Exhibition Telescope of 1900, with an objective lens of in diameter, was the largest refracting telescope ever constructed. It was built as the centerpiece of the Paris Universal Exhibition of 1900. Its construction was instigat ...
() – dismantled after exhibition
* Yerkes Observatory
Yerkes Observatory ( ) is an astronomical observatory located in Williams Bay, Wisconsin, United States. The observatory was operated by the University of Chicago Department of Astronomy and Astrophysics from its founding in 1897 to 2018. Owner ...
()
* Swedish 1-m Solar Telescope ()
* Lick Observatory ()
* Paris Observatory
The Paris Observatory (french: Observatoire de Paris ), a research institution of the Paris Sciences et Lettres University, is the foremost astronomical observatory of France, and one of the largest astronomical centers in the world. Its histor ...
Meudon Great Refractor
Meudon Great Refractor (also known as the ''Grande Lunette'') is a double telescope with lenses (83 cm + 62 cm), in Meudon, France. It is a twin refracting telescope built in 1891, with one visual and one photographic, on a single squar ...
(, + )
* Potsdam Great Refractor (, + )
* Nice Observatory
The Nice Observatory (french: Observatoire de Nice) is an astronomical observatory located in Nice, France on the summit of Mount Gros. The observatory was founded in 1879, by the banker Raphaël Bischoffsheim. The architect was Charles Garni ...
()
* John Wall
Johnathan Hildred Wall Jr. (born September 6, 1990) is an American professional basketball player for the Los Angeles Clippers of the National Basketball Association (NBA). A Raleigh, North Carolina native, Wall was chosen with the first overa ...
() dialyte refracting telescope - the largest refractor built by an individual, at Hanwell Community Observatory
* 28-inch Grubb Refractor at Royal Greenwich Observatory, () aperture lens
*Great Refractor of Vienna Observatory
The Vienna Observatory (german: Universitätssternwarte Wien) is an astronomical observatory in Vienna, Austria. It is part of the University of Vienna. The first observatory was built in 1753–1754 on the roof of one of the university buildings ...
, ()
* Archenhold Observatory
The Archenhold Observatory () was named in honour of Friedrich Simon Archenhold, is an observatory in Berlin-Treptow. It houses the ''Großer Refraktor'' (Great Refractor), which is the longest pointable telescope in the world. It is also called th ...
– the longest refracting telescope ever built ( × focal length)
* United States Naval Observatory refractor, ()
* Newall refractor at the National Observatory of Athens
The National Observatory of Athens (NOA; el, Εθνικό Αστεροσκοπείο Αθηνών) is a research institute in Athens, Greece. Founded in 1842, it is the oldest research foundation in Greece, as it was the first scientific rese ...
()
* Lowell Observatory ()
See also
* Astrograph
An astrograph (or astrographic camera) is a telescope designed for the sole purpose of astrophotography. Astrographs are mostly used in wide-field astronomical surveys of the sky and for detection of objects such as asteroids, meteors, a ...
* Baden-Powell's unilens
The unilens monocular is a simple refracting telescope for field use, designed by Robert Baden-Powell, 1st Baron Baden-Powell, Robert Baden-Powell. Consisting of only one Lens (optics), lens, it is the simplest of all telescopes, and while occupyin ...
* Catadioptric telescope
A catadioptric optical system is one where refraction and reflection are combined in an optical system, usually via lenses ( dioptrics) and curved mirrors ( catoptrics). Catadioptric combinations are used in focusing systems such as searchlights ...
s
* List of largest optical refracting telescopes
Refracting telescopes use a lens to focus light. The largest refracting telescope in the world is the Yerkes Observatory 40 inch (102 cm) refractor, used for astronomical and scientific observation for over a century. The Swedish 1-m S ...
* List of largest optical telescopes historically
Telescopes have grown in size since they first appeared around 1608. The following tables list the increase in size over the years. Different technologies can and have been used to build telescopes, which are used to magnify distant views especia ...
* List of telescope types
The following are lists of devices categorized as types of telescopes or devices associated with telescopes. They are broken into major classifications with many variations due to professional, amateur, and commercial sub-types. Telescopes can be ...
* Reflecting telescope
A reflecting telescope (also called a reflector) is a telescope that uses a single or a combination of curved mirrors that reflect light and form an image. The reflecting telescope was invented in the 17th century by Isaac Newton as an alternati ...
* Star diagonal
* Heliometer
A heliometer (from Greek ἥλιος ''hḗlios'' "sun" and ''measure'') is an instrument originally designed for measuring the variation of the sun's diameter at different seasons of the year, but applied now to the modern form of the instrument ...
Further reading
The optical work of Charles Tulley
References
External links
Angular and Linear Fields of View of Galilean Telescopes and Telemicroscopes
Refracting telescopes
{{DEFAULTSORT:Refracting Telescope
Telescopes
Telescope types
Dutch inventions
Science and technology in the Dutch Republic