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 was an era in telescope use in the 19th and early 20th century. Great refractors were large refracting telescope

Refracting telescope

A refracting or refractor telescope is a type of optical telescope that uses a lens as its objective to form an image . The refracting telescope design was originally used in spy glasses and astronomical telescopes but is also used for long focus camera lenses...

s using achromatic lens

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 into focus in the same plane....

es (as opposed to mirrors). 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 favored for astronomy.

It was not until the 20th century that they were gradually superseded by large reflecting telescopes for professional astronomy. A great refractor was often the centerpiece of a new 19th century observatory, but was typically used with an entourage of other astronomical instruments such as a Meridian Circle

Meridian circle

The meridian circle, transit circle, or transit telescope is an instrument for observing the time of stars passing the meridian, at the same time measuring its angular distance from the zenith...

, a Heliometer

Heliometer

Heliometer 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 which is capable of much wider use....

, and Astrograph

Astrograph

An astrograph is a telescope designed for the sole purpose of astrophotography. Astrographs are usually used in wide field surveys of the night sky as well as detection of objects such as asteroids, meteors, and comets.-Design:...

, and a smaller refractor such as a Comet Seeker

Comet seeker

A comet seeker is a type of small telescope adapted especially to searching for comets: commonly of short focal length and large aperture, in order to secure the greatest brilliancy of light....

 or Equatorial. Great refractors were often used for observing double stars and equipped with a Filar micrometer

Filar micrometer

A filar micrometer is a device used in astronomical telescopes for astrometry measurements. The word filar derives from Latin filum, a thread...

. Numerous discoveries of minor planets, satellites, the planet Neptune, double stars, and pioneering work on astrophotgraphy was done great refractors.

Era of large refractors

The choice of building large refractors over reflectors was a technological one. The difficulties of fabricating two disks of optical glass required to make a large achromatic lens were formidable. But reflecting telescopes had larger problems. The material their primary mirror

Primary mirror

A primary mirror is the principal light-gathering surface of a reflecting telescope.-Description:The primary mirror of a reflecting telescope is a spherical or parabolic shaped disks of polished reflective metal , or in later telescopes, glass or other material coated with a reflective layer...

 was made of was speculum metal

Speculum metal

Speculum metal is a mixture of around two-thirds copper and one-third tin making a white brittle alloy that can be polished to make a highly reflective surface. It is used primarily to make different kinds of mirrors including early reflecting telescope optical mirrors...

, a substance that only reflected up to 66 percent of the light that hit it and tarnished in months. They had to be removed, polished, and re-figured

Figuring

Figuring is the process of final polishing of an optical surface to remove imperfections or modify the surface curvature to achieve the shape required for a given application.-Types of figuring:...

 to the correct shape. This sometimes proved difficult, with the telescope mirrors sometimes having to be abandoned. Because of this, large refractors seemed to be the better choice.

Although there had been very large (and unwieldy) Non-achromatic

Non-achromatic objective

A non-achromatic objective is an objective lens which is not corrected for chromatic aberration. In telescopes they can a be pre-18th century simple single element objective lenses which were used before the invention of doublet achromatic lenses...

 aerial telescope

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 swivel ball-joint. The observer stood on the ground and held...

s of the late 17th century, and Chester Moore Hall

Chester Moore Hall

Chester Moore Hall was a British lawyer and inventor who produced the first achromatic lenses in 1729 or 1733 ....

 and others had experimented with small achromatic telescope

Achromatic telescope

The achromatic telescope is a refracting telescope that uses an achromatic lens to correct for chromatic aberration.-How it works:When an image passes through a lens, the light is refracted at different angles for different wavelengths. This produces focal lengths that are dependent on the color of...

s in the 18th century, John Dollond

John Dollond

John Dollond was an English optician, known for his successful optics business and his patenting and commercialization of achromatic doublets.-Biography:...

 (1706–1761) invented and created an achromatic object glass and lens which permitted achromatic telescopes up to 3–5 in. (8–13 cm) aperture. The Swiss Pierre Louis Guinand  (1748–1824) discovered and developed a way to make much larger crown and flint glass blanks. He worked with instrument maker Joseph von Fraunhofer

Joseph von Fraunhofer

Joseph von Fraunhofer was a German optician. He is known for the discovery of the dark absorption lines known as Fraunhofer lines in the Sun's spectrum, and for making excellent optical glass and achromatic telescope objectives.-Biography:Fraunhofer was born in Straubing, Bavaria...

  (1787–1826) to use this technology for instruments in the early 19th century.

The era of great refractors started with the first modern, achromatic, refracting telescopes built by Joseph von Fraunhofer

Joseph von Fraunhofer

Joseph von Fraunhofer was a German optician. He is known for the discovery of the dark absorption lines known as Fraunhofer lines in the Sun's spectrum, and for making excellent optical glass and achromatic telescope objectives.-Biography:Fraunhofer was born in Straubing, Bavaria...

 in the early 1820s. The first of these was the Dorpat Great Refractor, also known as the Fraunhofer 9-inch, at what was then Dorpat Observatory in the Governorate of Estonia

Governorate of Estonia

The Governorate of Estonia or Estland, also known as the Government of Estonia or Province of Estonia, was a governorate of the Russian Empire in what is now northern Estonia.-Historical overview:...

 (Estland) (which later became Tartu Observatory

Tartu Observatory

The Tartu Observatory is the largest astronomical observatory in Estonia. It is located on the Tõravere hill, about 20 km south-west of Tartu in Nõo Parish, Tartu County...

 in northern Estonia

Estonia

Estonia , officially the Republic of Estonia , is a state in the Baltic region of Northern Europe. It is bordered to the north by the Gulf of Finland, to the west by the Baltic Sea, to the south by Latvia , and to the east by Lake Peipsi and the Russian Federation . Across the Baltic Sea lies...

). This telescope made by Fraunhofer had a 9 Paris inch

Paris inch

The Paris inch, or pouce, is an old unit of measure, that among other uses, was common for giving the measurement of lenses. The Paris inch could be subdivided into 12 ligne , and 12 Paris inches made a Paris foot. The Paris inch and Paris foot could be abbreviated with " and ' like some other...

 (about 24 cm (9.6″)) aperture achromatic lens and a 4 m (13.4′) focal length. It was also equipped with the first modern equatorial mount

Equatorial mount

An equatorial mount is a mount for instruments that follows the rotation of the sky by having one rotational axis parallel to the Earth's axis of rotation. This type of mount is used for astronomical telescopes and cameras...

 type called a "German equatorial mount" developed by Fraunhofer, a mount that became standard for most large refractors from then on. A Fraunhofer 9-inch (24 cm) at Berlin Observatory

Berlin Observatory

The Berlin Observatory is a series of observatories and related organizations in and around the city of Berlin in Germany, starting from the 18th century...

 was used by Johann Gottfried Galle

Johann Gottfried Galle

Johann Gottfried Galle was a German astronomer at the Berlin Observatory who, on 23 September 1846, with the assistance of student Heinrich Louis d'Arrest, was the first person to view the planet Neptune, and know what he was looking at...

 in the discovery of Neptune

Discovery of Neptune

Neptune was mathematically predicted before it was directly observed. With a prediction by Urbain Le Verrier, telescopic observations confirming the existence of a major planet were made on the night of September 23, 1846, and into the early morning of the 24th, at the Berlin Observatory, by...

.

Refracting telescopes would quadruple in size by the end of the century, culminating with the largest practical refractor ever built, the Yerkes Observatory

Yerkes Observatory

Yerkes Observatory is an astronomical observatory operated by the University of Chicago in Williams Bay, Wisconsin. The observatory, which calls itself "the birthplace of modern astrophysics," was founded in 1897 by George Ellery Hale and financed by Charles T. Yerkes...

 40 inch (1 meter) aperture of 1895. This great refractor pushed the limits of technology of the day; the fabrication of the two element achromatic lens (the largest lens ever made at the time), required 18 attempts and cooperation between Alvan Clark & Sons

Alvan Clark & Sons

Alvan Clark & Sons was an American maker of optics that became famous for crafting lenses for some of the largest refracting telescopes of the 19th and early 20th centuries...

 and Charles Feil of Paris. To achieve its optical aperture it was actually slightly bigger physically, at 41 3/8 inch. Refractors had reached their technological limit; the problems of lens sag

Lens sag

Lens sag is a problem that sometimes afflicts very large refracting telescopes. It is the equivalent of mirror sag in reflecting telescopes. It occurs when the physical weight of the glass causes a distortion in the shape of the lens because the lens can only be supported by the edges...

ging from gravity meant refractors would not exceed around 1 meter. although Alvin G. Clark, who had made the Yerkes 40 inch objective, said a 45 inch (114 cm) would be possible before he passed away. In addition to the lens, the rest of the telescope needed to be a practical and high precision instrument, despite the size. For example, the Yerkes tube alone weighed 75 tons

Short ton

The short ton is a unit of mass equal to . In the United States it is often called simply ton without distinguishing it from the metric ton or the long ton ; rather, the other two are specifically noted. There are, however, some U.S...

, and had to track stars just as accurately as a smaller instrument.

The Great Paris Exhibition Telescope of 1900

Great Paris Exhibition Telescope of 1900

The Great Paris Exhibition Telescope of 1900, with an objective lens of 1.25 m 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 instigated in 1892 by François Deloncle , a member of...

, which was fixed in horizontal position to overcome gravitational distortion on its 1.25 m (49.2 inches) lens, and aimed with a 2 m siderostat, but this demonstration telescope was scrapped after the Exposition Universelle

World's Fair

World's fair, World fair, Universal Exposition, and World Expo are various large public exhibitions held in different parts of the world. The first Expo was held in The Crystal Palace in Hyde Park, London, United Kingdom, in 1851, under the title "Great Exhibition of the Works of Industry of All...

 closed.

End of the era

The era slowly came to end as large reflecting telescopes superseded the great refractors. In 1856-57, Carl August von Steinheil

Carl August von Steinheil

Carl August von Steinheil was a German physicist, inventor, engineer and astronomer.-Biography:Steinheil was born in Ribeauvillé, Alsace. He studied law in Erlangen since 1821, then astronomy in Göttingen and Königsberg. He continued his studies in astronomy and physics when he started living on...

 and Léon Foucault

Léon Foucault

Jean Bernard Léon Foucault was a French physicist best known for the invention of the Foucault pendulum, a device demonstrating the effect of the Earth's rotation...

 introduced a process of depositing a layer of silver on glass telescope mirrors. Silvered glass mirrors were a vast improvement over speculum metal and made reflectors a practical instrument. The era of large reflectors had begun, with telescopes such as the 36 inch (91 cm) Crossley Reflector (1895), 60 inch (150 cm) Mount Wilson Observatory

Mount Wilson Observatory

The Mount Wilson Observatory is an astronomical observatory in Los Angeles County, California, United States. The MWO is located on Mount Wilson, a 5,715 foot peak in the San Gabriel Mountains near Pasadena, northeast of Los Angeles...

 Hale telescope of 1908, and the 100 inch (2.5 m) Mount Wilson Hooker telescope in 1917.
(See also)

Examples

Great refractors were admired for their quality, durability, and usefulness which correlated to features such as lens quality, mount quality, aperture, and also length. Length was important because unlike reflectors (which can be folded and shorted), the focal length

Focal length

The focal length of an optical system is a measure of how strongly the system converges or diverges light. For an optical system in air, it is the distance over which initially collimated rays are brought to a focus...

 of glass lens correlated to the physical length of the telescope and offered some optical and image quality advantages.

Aperture

Approximate historical progression:

The progression of largest refracting telescopes in the 19th century, including some telescopes at Private Observatories that were not really used very much or had problems.
Some of the 2nd largest refractors, or otherwise notable.