40-foot telescope
Encyclopedia
William Herschel
's 40-foot telescope, also known as the Great Forty-Foot telescope, was a reflecting telescope
constructed between 1785 and 1789 at Observatory House
in Slough
, England
. It used a 120 cm (48-inch) diameter primary mirror
with a 12 m (40 foot) long focal length
(hence its name "Forty-Foot"). It was the largest telescope in the world for 50 years. It may have been used to discover Enceladus
and Mimas
, the 6th and 7th moons of Saturn. It was dismantled in 1840; today the original mirror and a 10 feet (3 m) section of the tube remain.
, with the assistance of Caroline Herschel
, between 1785 and 1789 in Slough
, with components made in Clay Hall near Windsor
. The 40 ft (12.2 m) tube was made of iron
. The telescope was mounted on a fully rotatable alt-azimuth mount. It was paid for by King George III, who granted £4,000 for it to be made. During construction, whilst the telescope tube lay on the ground, the King as well as the Archbishop of Canterbury
visited the telescope. Just prior to them entering the open mouth of the tube, the King commented "Come, my Lord Bishop, I will show you the way to Heaven!"
Two 48 inches (121.9 cm) concave metal mirrors
were made for the telescope, each with a focal ratio of f/10. The first was cast in a London foundry on 31 October 1785, and was made of speculum
(an alloy of mostly copper
and tin
) with arsenic
to improve the finish. It weighed 1023 lb after being cast, but it was found to be 0.9 inches thinner at the centre than at the edge (where it was around 2 inches thick). Over a year was spent grinding and polishing the mirror, however Herschel found it to be "much too thin to keep its figure when put into the telescope" (despite weighing half a ton). A second mirror with twice the thickness of the original was cast a few years later, and this was used rather than the original. However, this required more frequent polishing due to the fast tarnishing nature of the metal, and the original mirror was used when the second was being polished. The mirrors remained the largest in the world until 1845.
Herschel eliminated the small diagonal mirror of a standard newtonian reflector from his design and instead tilted his primary mirror
so he could view the formed image when he stood in an observing cage directly in front of the telescope. This saved on the severe light loss the image would suffer if he had used a speculum metal diagonal mirror. This design has come to be called a Herschelian telescope.
, Herschel's house in Slough, between 1789 and 1840. The first observation with the telescope was on 19 February 1787, when Herschel pointed the then-incomplete telescope towards the Orion nebula
, which he observed by crawling into the telescope and using a hand-held eyepiece
:
The one achievement of the telescope was to discover Enceladus
and Mimas
, the 6th and 7th moons of Saturn, although this is not certain, as Herschel used other telescopes at the same time. Herschel described the view of Sirius
through the telescope:
As part of the funding deal with the telescope, Caroline Herschel
was granted a pension of £50 per year to be William's assistant. As a result, she was the first woman in England to be paid to carry out astronomy.
The telescope was a local tourist attraction, visited by rich and famous people on their way to the nearby Windsor Castle
to visit the King, and was featured on Ordnance Survey
maps. It was the largest telescope in the world for 50 years. It was called the "40-foot telescope" as at the time telescopes were referred to by the length of their tube rather than the diameter of the mirror.
Due to problems with the mirrors and because the telescope was unwieldy, the telescope did not prove to be a substantial improvement over smaller telescopes. The weather was rarely suitable for the telescope, and most objects observed by Herschel were also visible in his smaller telescopes. The final observation made by the telescope was in 1815.
The telescope was featured in Herschel's coat of arms:
The telescope's frame was dismantled at the end of 1839 by William Herschel's son, John Herschel
, on his return from carrying out observations in South Africa
. It was dismantled as it was feared that the frame might collapse due to rot
, and John feared for the safety of his young children. A small ceremony was conducted to commemorate its dismantling.
The tube was left lying horizontally in the garden, supported by stone blocks at either end, where it was crushed in 1867 by a falling tree. The remaining piece is a 10 feet (3 m) length of the mirror end, which is 3,048 by 1,465 mm (120" x 57.7"). This was still located in the garden of Observatory House in 1955, but was subsequently moved and is now located in the Herschel Collection of the National Maritime Museum
, in the Royal Observatory, Greenwich
, London
.
The first mirror was last polished in 1797, and was subsequently stored away and lost. When John Herschel moved from Observatory House to Hawkhurst
in 1840, a number of items (including the 40-foot telescope) were left behind. In an inventory written at the time, he recorded "In the Observatory, beneath stair, one 40-foot mirror, with case and cover." A workman later reported that only a light metal cover of a 4-foot mirror was present, rather than the mirror itself. The mirror was rediscovered on 2 February 1927:
On 4 March 1927 the mirror was moved to the Cottage library, and was once more polished some 130 years after the mirror was last properly polished. The original mirror now resides in the Science Museum
, London
. The second mirror was left in place in the telescope when it was dismantled, but was removed when the tube was crushed. In 1871 it was moved into the hall of Observatory House.
The 40 feet (12.2 m) telescope was surpassed in 1845 as the largest ever built by Lord Rosse
's great 72 inches (1.8 m) reflecting telescope
. The image of the 40-foot telescope remains as one of the great icon
s of astronomy.
William Herschel
Sir Frederick William Herschel, KH, FRS, German: Friedrich Wilhelm Herschel was a German-born British astronomer, technical expert, and composer. Born in Hanover, Wilhelm first followed his father into the Military Band of Hanover, but emigrated to Britain at age 19...
's 40-foot telescope, also known as the Great Forty-Foot telescope, was a reflecting telescope
Reflecting telescope
A reflecting telescope is an optical telescope which uses a single or combination of curved mirrors that reflect light and form an image. The reflecting telescope was invented in the 17th century as an alternative to the refracting telescope which, at that time, was a design that suffered from...
constructed between 1785 and 1789 at Observatory House
Observatory House
Observatory House was an observatory in Slough, England. It was built, run and used by the astronomer William Herschel, and his sister Caroline. The famous '40-foot telescope' - at that time the largest in the world - was housed there in the late 18th century and early 19th century.The main house...
in Slough
Slough
Slough is a borough and unitary authority within the ceremonial county of Royal Berkshire, England. The town straddles the A4 Bath Road and the Great Western Main Line, west of central London...
, England
England
England is a country that is part of the United Kingdom. It shares land borders with Scotland to the north and Wales to the west; the Irish Sea is to the north west, the Celtic Sea to the south west, with the North Sea to the east and the English Channel to the south separating it from continental...
. It used a 120 cm (48-inch) diameter 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...
with a 12 m (40 foot) long 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...
(hence its name "Forty-Foot"). It was the largest telescope in the world for 50 years. It may have been used to discover Enceladus
Enceladus (moon)
Enceladus is the sixth-largest of the moons of Saturn. It was discovered in 1789 by William Herschel. Until the two Voyager spacecraft passed near it in the early 1980s very little was known about this small moon besides the identification of water ice on its surface...
and Mimas
Mimas (moon)
Mimas is a moon of Saturn which was discovered in 1789 by William Herschel. It is named after Mimas, a son of Gaia in Greek mythology, and is also designated Saturn I....
, the 6th and 7th moons of Saturn. It was dismantled in 1840; today the original mirror and a 10 feet (3 m) section of the tube remain.
Construction
The telescope was constructed by Sir William HerschelWilliam Herschel
Sir Frederick William Herschel, KH, FRS, German: Friedrich Wilhelm Herschel was a German-born British astronomer, technical expert, and composer. Born in Hanover, Wilhelm first followed his father into the Military Band of Hanover, but emigrated to Britain at age 19...
, with the assistance of Caroline Herschel
Caroline Herschel
Caroline Lucretia Herschel was a German-British astronomer, the sister of astronomer Sir Friedrich Wilhelm Herschel with whom she worked throughout both of their careers. Her most significant contribution to astronomy was the discovery of several comets and in particular the periodic comet...
, between 1785 and 1789 in Slough
Slough
Slough is a borough and unitary authority within the ceremonial county of Royal Berkshire, England. The town straddles the A4 Bath Road and the Great Western Main Line, west of central London...
, with components made in Clay Hall near Windsor
Windsor, Berkshire
Windsor is an affluent suburban town and unparished area in the Royal Borough of Windsor and Maidenhead in Berkshire, England. It is widely known as the site of Windsor Castle, one of the official residences of the British Royal Family....
. The 40 ft (12.2 m) tube was made of iron
Iron
Iron is a chemical element with the symbol Fe and atomic number 26. It is a metal in the first transition series. It is the most common element forming the planet Earth as a whole, forming much of Earth's outer and inner core. It is the fourth most common element in the Earth's crust...
. The telescope was mounted on a fully rotatable alt-azimuth mount. It was paid for by King George III, who granted £4,000 for it to be made. During construction, whilst the telescope tube lay on the ground, the King as well as the Archbishop of Canterbury
Archbishop of Canterbury
The Archbishop of Canterbury is the senior bishop and principal leader of the Church of England, the symbolic head of the worldwide Anglican Communion, and the diocesan bishop of the Diocese of Canterbury. In his role as head of the Anglican Communion, the archbishop leads the third largest group...
visited the telescope. Just prior to them entering the open mouth of the tube, the King commented "Come, my Lord Bishop, I will show you the way to Heaven!"
Two 48 inches (121.9 cm) concave metal mirrors
Curved mirror
A curved mirror is a mirror with a curved reflective surface, which may be either convex or concave . Most curved mirrors have surfaces that are shaped like part of a sphere, but other shapes are sometimes used in optical devices...
were made for the telescope, each with a focal ratio of f/10. The first was cast in a London foundry on 31 October 1785, and was made of speculum
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...
(an alloy of mostly copper
Copper
Copper is a chemical element with the symbol Cu and atomic number 29. It is a ductile metal with very high thermal and electrical conductivity. Pure copper is soft and malleable; an exposed surface has a reddish-orange tarnish...
and tin
Tin
Tin is a chemical element with the symbol Sn and atomic number 50. It is a main group metal in group 14 of the periodic table. Tin shows chemical similarity to both neighboring group 14 elements, germanium and lead and has two possible oxidation states, +2 and the slightly more stable +4...
) with arsenic
Arsenic
Arsenic is a chemical element with the symbol As, atomic number 33 and relative atomic mass 74.92. Arsenic occurs in many minerals, usually in conjunction with sulfur and metals, and also as a pure elemental crystal. It was first documented by Albertus Magnus in 1250.Arsenic is a metalloid...
to improve the finish. It weighed 1023 lb after being cast, but it was found to be 0.9 inches thinner at the centre than at the edge (where it was around 2 inches thick). Over a year was spent grinding and polishing the mirror, however Herschel found it to be "much too thin to keep its figure when put into the telescope" (despite weighing half a ton). A second mirror with twice the thickness of the original was cast a few years later, and this was used rather than the original. However, this required more frequent polishing due to the fast tarnishing nature of the metal, and the original mirror was used when the second was being polished. The mirrors remained the largest in the world until 1845.
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...
so he could view the formed image when he stood in an observing cage directly in front of the telescope. This saved on the severe light loss the image would suffer if he had used a speculum metal diagonal mirror. This design has come to be called a Herschelian telescope.
Use
The telescope was located on the grounds of Observatory HouseObservatory House
Observatory House was an observatory in Slough, England. It was built, run and used by the astronomer William Herschel, and his sister Caroline. The famous '40-foot telescope' - at that time the largest in the world - was housed there in the late 18th century and early 19th century.The main house...
, Herschel's house in Slough, between 1789 and 1840. The first observation with the telescope was on 19 February 1787, when Herschel pointed the then-incomplete telescope towards the Orion nebula
Orion Nebula
The Orion Nebula is a diffuse nebula situated south of Orion's Belt. It is one of the brightest nebulae, and is visible to the naked eye in the night sky. M42 is located at a distance of and is the closest region of massive star formation to Earth. The M42 nebula is estimated to be 24 light...
, which he observed by crawling into the telescope and using a hand-held eyepiece
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 so named because it is usually the lens that is closest to the eye when someone looks through the device. The objective lens or mirror collects light and brings...
:
The one achievement of the telescope was to discover Enceladus
Enceladus (moon)
Enceladus is the sixth-largest of the moons of Saturn. It was discovered in 1789 by William Herschel. Until the two Voyager spacecraft passed near it in the early 1980s very little was known about this small moon besides the identification of water ice on its surface...
and Mimas
Mimas (moon)
Mimas is a moon of Saturn which was discovered in 1789 by William Herschel. It is named after Mimas, a son of Gaia in Greek mythology, and is also designated Saturn I....
, the 6th and 7th moons of Saturn, although this is not certain, as Herschel used other telescopes at the same time. Herschel described the view of Sirius
Sirius
Sirius is the brightest star in the night sky. With a visual apparent magnitude of −1.46, it is almost twice as bright as Canopus, the next brightest star. The name "Sirius" is derived from the Ancient Greek: Seirios . The star has the Bayer designation Alpha Canis Majoris...
through the telescope:
As part of the funding deal with the telescope, Caroline Herschel
Caroline Herschel
Caroline Lucretia Herschel was a German-British astronomer, the sister of astronomer Sir Friedrich Wilhelm Herschel with whom she worked throughout both of their careers. Her most significant contribution to astronomy was the discovery of several comets and in particular the periodic comet...
was granted a pension of £50 per year to be William's assistant. As a result, she was the first woman in England to be paid to carry out astronomy.
The telescope was a local tourist attraction, visited by rich and famous people on their way to the nearby Windsor Castle
Windsor Castle
Windsor Castle is a medieval castle and royal residence in Windsor in the English county of Berkshire, notable for its long association with the British royal family and its architecture. The original castle was built after the Norman invasion by William the Conqueror. Since the time of Henry I it...
to visit the King, and was featured on Ordnance Survey
Ordnance Survey
Ordnance Survey , an executive agency and non-ministerial government department of the Government of the United Kingdom, is the national mapping agency for Great Britain, producing maps of Great Britain , and one of the world's largest producers of maps.The name reflects its creation together with...
maps. It was the largest telescope in the world for 50 years. It was called the "40-foot telescope" as at the time telescopes were referred to by the length of their tube rather than the diameter of the mirror.
Due to problems with the mirrors and because the telescope was unwieldy, the telescope did not prove to be a substantial improvement over smaller telescopes. The weather was rarely suitable for the telescope, and most objects observed by Herschel were also visible in his smaller telescopes. The final observation made by the telescope was in 1815.
The telescope was featured in Herschel's coat of arms:
Decommissioning
John Herschel
Sir John Frederick William Herschel, 1st Baronet KH, FRS ,was an English mathematician, astronomer, chemist, and experimental photographer/inventor, who in some years also did valuable botanical work...
, on his return from carrying out observations in South Africa
South Africa
The Republic of South Africa is a country in southern Africa. Located at the southern tip of Africa, it is divided into nine provinces, with of coastline on the Atlantic and Indian oceans...
. It was dismantled as it was feared that the frame might collapse due to rot
ROT
The aviation term ROT stands for rate one turn, also known as a standard rate turn. All aircraft must be able to perform a standard rate turn....
, and John feared for the safety of his young children. A small ceremony was conducted to commemorate its dismantling.
The tube was left lying horizontally in the garden, supported by stone blocks at either end, where it was crushed in 1867 by a falling tree. The remaining piece is a 10 feet (3 m) length of the mirror end, which is 3,048 by 1,465 mm (120" x 57.7"). This was still located in the garden of Observatory House in 1955, but was subsequently moved and is now located in the Herschel Collection of the National Maritime Museum
National Maritime Museum
The National Maritime Museum in Greenwich, England is the leading maritime museum of the United Kingdom and may be the largest museum of its kind in the world. The historic buildings forming part of the Maritime Greenwich World Heritage Site, it also incorporates the Royal Observatory, Greenwich,...
, in the Royal Observatory, Greenwich
Royal Observatory, Greenwich
The Royal Observatory, Greenwich , in London, England played a major role in the history of astronomy and navigation, and is best known as the location of the prime meridian...
, London
London
London is the capital city of :England and the :United Kingdom, the largest metropolitan area in the United Kingdom, and the largest urban zone in the European Union by most measures. Located on the River Thames, London has been a major settlement for two millennia, its history going back to its...
.
The first mirror was last polished in 1797, and was subsequently stored away and lost. When John Herschel moved from Observatory House to Hawkhurst
Hawkhurst
Hawkhurst is a village and civil parish in the borough of Tunbridge Wells, Kent, England. The parish lies to the south-east of Tunbridge Wells. Hawkhurst itself is virtually two villages...
in 1840, a number of items (including the 40-foot telescope) were left behind. In an inventory written at the time, he recorded "In the Observatory, beneath stair, one 40-foot mirror, with case and cover." A workman later reported that only a light metal cover of a 4-foot mirror was present, rather than the mirror itself. The mirror was rediscovered on 2 February 1927:
On 4 March 1927 the mirror was moved to the Cottage library, and was once more polished some 130 years after the mirror was last properly polished. The original mirror now resides in the Science Museum
Science museum
A science museum or a science centre is a museum devoted primarily to science. Older science museums tended to concentrate on static displays of objects related to natural history, paleontology, geology, industry and industrial machinery, etc. Modern trends in museology have broadened the range of...
, London
London
London is the capital city of :England and the :United Kingdom, the largest metropolitan area in the United Kingdom, and the largest urban zone in the European Union by most measures. Located on the River Thames, London has been a major settlement for two millennia, its history going back to its...
. The second mirror was left in place in the telescope when it was dismantled, but was removed when the tube was crushed. In 1871 it was moved into the hall of Observatory House.
The 40 feet (12.2 m) telescope was surpassed in 1845 as the largest ever built by Lord Rosse
William Parsons, 3rd Earl of Rosse
William Parsons, 3rd Earl of Rosse, Knight of the Order of St Patrick was an Irish astronomer who had several telescopes built. His 72-inch telescope "Leviathan", built 1845, was the world's largest telescope until the early 20th century.-Life:He was born in Yorkshire, England, in the city of...
's great 72 inches (1.8 m) reflecting telescope
Leviathan of Parsonstown
Leviathan of Parsonstown is the unofficial name of the Rosse six foot telescope. This is a historic reflecting telescope of 72 in aperture, which was the largest telescope in the world from 1845 until the construction of the 100 in Hooker Telescope in 1917...
. The image of the 40-foot telescope remains as one of the great icon
Icon
An icon is a religious work of art, most commonly a painting, from Eastern Christianity and in certain Eastern Catholic churches...
s of astronomy.