40-foot telescope

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40-foot telescope
Herschel 40 foot.jpg
Alternative namesGreat Forty-Foot telescope Blue pencil.svg
Location(s) Slough, United Kingdom Blue pencil.svg
Coordinates 51°30′30″N0°35′43″W / 51.5082°N 0.5954°W / 51.5082; -0.5954 Coordinates: 51°30′30″N0°35′43″W / 51.5082°N 0.5954°W / 51.5082; -0.5954 Blue pencil.svg
Built1785  Blue pencil.svg –1789  Blue pencil.svg  (1785  Blue pencil.svg –1789  Blue pencil.svg ) Blue pencil.svg
First light 19 February 1787  Blue pencil.svg
Decommissioned1840  Blue pencil.svg
Telescope style Altazimuth mount
Reflecting telescope   Blue pencil.svg
Diameter48 in (1.2 m) Blue pencil.svg
Focal length 40 ft (12 m) Blue pencil.svg
United Kingdom relief location map.jpg
Red pog.svg
Location of 40-foot telescope

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 48-inch (120 cm) diameter primary mirror with a 40-foot-long (12 m) 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-foot (3.0 m) section of the tube remain.

William Herschel 18th and 19th-century German-born British astronomer and composer

Frederick William Herschel, was a German-born British astronomer, composer and brother of fellow astronomer Caroline Herschel, with whom he worked. Born in the Electorate of Hanover, Herschel followed his father into the Military Band of Hanover, before migrating to Great Britain in 1757 at the age of nineteen.

Reflecting telescope telescopes that reflect light with a combination of mirrors

A reflecting telescope 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 alternative to the refracting telescope which, at that time, was a design that suffered from severe chromatic aberration. Although reflecting telescopes produce other types of optical aberrations, it is a design that allows for very large diameter objectives. Almost all of the major telescopes used in astronomy research are reflectors. Reflecting telescopes come in many design variations and may employ extra optical elements to improve image quality or place the image in a mechanically advantageous position. Since reflecting telescopes use mirrors, the design is sometimes referred to as a "catoptric" telescope.

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 telescope was constructed by Sir William Herschel, with the assistance of his sister Caroline Herschel, between 1785 and 1789 in Slough, with components made in Clay Hall near Windsor. The 40 ft (12 m) tube was made of iron. [1] 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. [1] 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!" [2]

Caroline Herschel 18th and 19th-century German-British astronomer

Caroline Lucretia Herschel was a German astronomer, whose most significant contributions to astronomy were the discoveries of several comets, including the periodic comet 35P/Herschel–Rigollet, which bears her name. She was the younger sister of astronomer William Herschel, with whom she worked throughout her career.

Slough Place in England

Slough is a large town in Berkshire, England, on the western fringes of the Greater London Urban Area, 20 miles (32 km) west of Charing Cross, central London, 2 miles (3 km) north of Windsor, 5 miles (8 km) east of Maidenhead, 11 miles (18 km) south-east of High Wycombe and 17 miles (27 km) north-east of the county town of Reading. It is between the Thames Valley and London and at the intersection of the M4, M40 and M25 motorways.

Clay Hall building in Oklahoma

Clay Hall is a mid-twentieth century women's dormitory located on the campus of Northern Oklahoma College in Enid, Oklahoma that has been listed on the National Register of Historic Places since 2012. Architect Roy W. Shaw designed it for Phillips University in 1941. The building was named after Robert Henry Clay, the husband of Sadie Clay, who had given a $25,000 donation to the project. A cornerstone ceremony was held on October 9, 1941. Mefford construction had completed the exterior by 1942, and the interior was completed in 1946, having been delayed by the onset of World War II. The dormitory cost $175,000 to build, and the University held a dedication ceremony on October 11, 1946. In 1951 and 1959, a north and a south wing were added to the building in order to accommodate an expanding student population. These additional wings increased Clay Hall's residential space from 150 women to 258, and its building size to 59,000 square feet. Phillips University's enrollment peaked in the 1970s, and the dormitory closed briefly in 1985, was reopened in 1986, and then permanently shut down in 1987. Clay Hall is the oldest dormitory on the campus. Its predecessor, Athenian Hall, was demolished in 1952, and a men's dormitory, Earl Butts Hall, was completed in 1955.

Two 48-inch (120 cm) concave metal mirrors were made for the telescope, each with a focal ratio of f/10. [2] The first was cast in a London foundry on 31 October 1785, [3] and was made of speculum (an alloy of mostly copper and tin) with arsenic to improve the finish. [4] [5] 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" [3] (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. [4] [5]

Curved mirror type of mirror

A curved mirror is a mirror with a curved reflecting surface. The surface 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. The most common non-spherical type are parabolic reflectors, found in optical devices such as reflecting telescopes that need to image distant objects, since spherical mirror systems, like spherical lenses, suffer from spherical aberration. Distorting mirrors are used for entertainment. They have convex and concave regions that produce deliberately distorted images.

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 was used historically to make different kinds of mirrors from personal grooming aids to reflecting telescope optical mirrors until it was replaced by more modern materials.

Copper Chemical element with atomic number 29

Copper is a chemical element with symbol Cu and atomic number 29. It is a soft, malleable, and ductile metal with very high thermal and electrical conductivity. A freshly exposed surface of pure copper has a pinkish-orange color. Copper is used as a conductor of heat and electricity, as a building material, and as a constituent of various metal alloys, such as sterling silver used in jewelry, cupronickel used to make marine hardware and coins, and constantan used in strain gauges and thermocouples for temperature measurement.

Herschelian telescope. Herschel-Lomonosov reflecting telescope.svg
Herschelian telescope.

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. [6]

Primary mirror

A primary mirror is the principal light-gathering surface of a reflecting telescope.


Photograph of the 40-foot telescope's frame taken in 1839 by William Herschel's son, John Herschel. Herschel first picture on glass 1839 3 pozitiv.jpg
Photograph of the 40-foot telescope's frame taken in 1839 by William Herschel's son, John Herschel.

The telescope was located on the grounds of Observatory House, Herschel's house in Slough, between 1789 and 1840. [1] 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: [3] [5] "The apparatus for the 40-foot telescope was by this time so far completed that I could put the mirror into the tube and direct it to a celestial object; but having no eye-glass fixed, not being acquainted with the focal length which was to be tried, I went into the tube, and laying down near the mouth of it I held the eye-glass in my hand, and soon found the place of the focus. The object I viewed was the nebula in the belt of Orion, and I found the figure of the mirror, though far from perfect, better than I had expected. It showed four small stars in the nebula and many more. The nebula was extremely bright."

First light (astronomy) term in astronomy for the first time a telescope is used to look at the Universe

In astronomy, first light is the first use of a telescope to take an astronomical image after it has been constructed. This is often not the first viewing using the telescope; optical tests will probably have been performed in daylight to adjust the components. The first light image is normally of little scientific interest and is of poor quality, since the various telescope elements are yet to be adjusted for optimum efficiency. Despite this, a first light is always a moment of great excitement, both for the people who design and build the telescope and for the astronomical community, who may have anticipated the moment for many years while the telescope was under construction. A well-known and spectacular astronomical object is usually chosen as a subject.

Eyepiece type of lens attached to a variety of optical devices such as telescopes and microscopes; usually the lens that is closest to the eye when someone looks through the device, placed near the focal point of the objective to magnify the image

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 it to focus creating an image. The eyepiece is placed near the focal point of the objective to magnify this image. The amount of magnification depends on the focal length of the 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. [1] Herschel described the view of Sirius through the telescope: [2] "... the appearance of Sirius announced itself, ... and came on by degrees, increasing in brightness, till this brilliant star at last entered the field of view of the telescope, with all the splendour of the rising sun, and forced me to take the eye from that beautiful sight."

Mimas (moon) moon of Saturn

Mimas, also designated Saturn I, 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.

Moons of Saturn The natural satellites of the planet Saturn

The moons of Saturn are numerous and diverse, ranging from tiny moonlets less than 1 kilometer across to the enormous Titan, which is larger than the planet Mercury. Saturn has 62 moons with confirmed orbits, 53 of which have names and only 13 of which have diameters larger than 50 kilometers, as well as dense rings with complex orbital motions of their own. Seven Saturnian moons are large enough to be ellipsoidal in shape, yet only two of those, Titan and Rhea, are currently in hydrostatic equilibrium. Particularly notable among Saturn's moons are Titan, the second-largest moon in the Solar System, with a nitrogen-rich Earth-like atmosphere and a landscape featuring dry river networks and hydrocarbon lakes found nowhere else in the solar system; and Enceladus since its chemical composition is similar to that of comets. In particular, Enceladus emits jets of gas and dust, which could indicate the presence of liquid water under its south pole region, and may have a global ocean below its surface.

Sirius Brightest star in the night sky

Sirius is a binary star and 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 system has the Bayer designation α (Alpha) Canis Majoris. The binary system consists of a main-sequence star of spectral type A0 or A1, termed Sirius A, and a faint white dwarf companion of spectral type DA2, designated Sirius B. The distance between the two varies between 8.2 and 31.5 astronomical units as they orbit every 50 years.

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. [1]

The telescope was a local tourist attraction, [7] visited by rich and famous people on their way to the nearby Windsor Castle to visit the King, [4] and was featured on Ordnance Survey maps. [7] It was the largest telescope in the world for 50 years. [1] 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. [2]

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. [5] The weather was rarely suitable for the telescope, and most objects observed by Herschel were also visible in his smaller telescopes. [2] The final observation made by the telescope was in 1815. [1]

The telescope was featured in Herschel's coat of arms: "Argent on a mount vert a representation of the forty-feet reflecting telescope with its apparatus proper; a chief azure thereon the astronomical symbol of Uranus or Georgium Sidus irradiated Or." [8]

Decommissioning and preservation

The telescope's first mirror on display in the Science Museum, London. 40-foot telescope mirror.jpg
The telescope's first mirror on display in the Science Museum, London.

The telescope's frame was dismantled at the end of 1839 by William Herschel's son, John Herschel, [1] [3] 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. [9]

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. [3] The remaining piece is a 10-foot (3.0 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, [10] but was subsequently moved and is now located in the Herschel Collection of the National Maritime Museum, in the Royal Observatory, Greenwich, London. [1]

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: "All that could be seen on a casual inspection was a somewhat rusty iron ring, about 4 feet in diameter and 5 inches thick ... covered in front with a close-fitting lid of thin metal. The iron ring, which was not unlike the tyre of a cart-wheel, was obviously the cell of a large mirror and was quite separate from the tin cover. On removing the latter, which was provided with six handles, the mirror itself was at once seen, occupying the front portion of the cell, close under the cover." [3]

The remaining 10 ft section of the telescope, now at the Royal Observatory, Greenwich. Remaining section of the 40ft telescope.jpg
The remaining 10 ft section of the telescope, now at the Royal Observatory, Greenwich.

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. [3] The original mirror now resides in the Science Museum, London. [11] 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. [3]

A scale model of the telescope at the Herschel Museum of Astronomy in Bath. William Herschel Museum - 40-foot telescope model 1.jpg
A scale model of the telescope at the Herschel Museum of Astronomy in Bath.

A scale model of the telescope, as well as an early photo of it that is framed in wood from the telescope, is on display at the Herschel Museum of Astronomy in Bath. [12]

The 40-foot (12 m) telescope was surpassed in 1845 as the largest ever built by Lord Rosse's great 72-inch (1.8 m) reflecting telescope. [10] The image of the 40-foot telescope remains as one of the great icons of astronomy. [2]

See also

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