Observatories began to be built in the Muslim world towards the end of the first millennium. These housed ever larger geometric measuring instruments used to compile astronomical tables. Although useful in part for determining the hour of prayer and observing the new moon that heralded each month they rarely won favour with civil or religious authorities, were dependent upon the support of powerful patrons and were usually destroyed within a few years.
- 1125 - The vizier of Cairo was executed for crimes including communication with Saturn.
|
| Johannes Kepler |
Tycho Brahe's observatory at Uraniborg castle on the island of Hven in the Oresund, built 1576-80, has been described as 'the first scientific research institution of the modern era', but it still did not include any optical instruments.
Tycho's assistant in his later career at Prague, the German astronomer Johannes Kepler (1571-1630/1), is considered to be the father of 'celestial physics' as the old geometric studies faded from fashion. He was also author of two important optical works: Ad Vitellionem Panalipomena (1604) and Dioptrics (1610). In these works Kepler was the first to remark upon the principle of the telescope i.e. two convex lenses and was also the first to give a correct explanation of stereoscopic vision.
The contemporaneous invention of the telescope by Dutch opticians in the first decade of the seventeenth century provided a new possibility of looking at things that had not previously been observable. In Italy Galileo was the first noted user of a telescope. This permitted him to discover:
- new stars
- four satellites (moons) of Jupiter
- rings of Saturn (though these were not identified as such until later in the century)
- phases of Venus
- mountains on the moon
- sun spots
|
Sir Isaac Newton's early scholarship was governed by his enthusiasm for the Cartesian view of the universe.
Huygen's aerial telescope (right) features on a qualification certificate for an ophthalmic optician sitting the
|
|
William Herschel's 'discovery' of Uranus captured the imagination of powerful supporters. King George III granted him a pension and allowed him to move to Windsor Castle. In 1783 he commissioned his 'large' reflecting telescope. Its focal length was 20 feet whilst its mirrors measured 18 inches in diameter. With such a large scale instrument it was vital for the supporting structure to be as firm and stable as possible. In the year of this image his son John had dismantled the telescope due to the decay of the wooden frame. A replacement was immediately built, which later saw service in South Africa from 1825.
|
John Herschel (1792-1871) is the only astronomer to have observed the entire celestial sphere (i.e. both northern and southern hemispheres) with the same major telescope. He also served alongside his friend George Dollond (I) and Michael Faraday on a Royal Society committee of 1824 to investigate ways of improving optical glass.
In England silver-coated glass mirrors were rapidly adopted by telescope makers such as George With and George Calver. James Nasmyth introduced several innovations in casting telescope mirrors. William Lassell pioneered the precision heavy engineering necessary to produce large reflectors. In France Leon Foucault, although not the first, was one of the first to apply a silvering process to glass telescope mirrors (which had been made for him by the well-known instrument maker Secretan).
Fraunhofer and Heliometers.
Fraunhofer (1787-1826) made refracting telescopes of the highest quality for stargazing. These travelled all over the world. One with an objective lens of 240mm was used by Wilhelm Struve in Estonia in the 1830s to observe the star Vega. The heliometer was developed by Fraunhofer for F.W. Bessel (1784-1846) in Koenigsberg. This instrument could measure the apparent change in the sun's diameter (or indeed the angle separating any pair of stars). A lens of sufficient quality was split in half. The two resulting half circles could be slid against each other until aspects of the images in each coincided, allowing the necessary displacement to be read off a scale.
|
|
Reflecting Telescopes, illustrated in the Encyclopedia Londoniensis (1820).
An A3 poster featuring these two images, amongst others, may be purchased from the museum shop.
In 1838 William Parsons (1800-1867) the future Earl of Rosse, began to assemble his great telescope at Birr Castle in Ireland - the Leviathan of Parsonstown' - with mirrors of 6 feet diameter, weighing four tons and requiring the support of masonry walls. Its mirror was a 'speculum' mirror. That is to say it was made from an alloy of copper and tin. In the damp Irish climate this soon tarnished. Although the great nebula in the constellation of Orion had been visible to the naked eye for centuries, this telescope was the first that was powerful enough to observe the individual stars embedded within it. The Great Rosse Telescope survived from 1845-1914. It remained the largest telescope in the world until overtaken by the 100-inch Mount Wilson Telescope in the USA.
In 1845 J.C. Adams (Cambridge) predicted the presence of the planet Neptune. The following year astronomers in Berlin observed its movement.
Max Wolf (1863-1932) in Heidelburg in 1891 used a telescope to both track and photograph a large star field over a period of time, thus simplifying the search for asteroids.
|
Our picture on the right shows a 1920s equatorial telescope.
- 1932 - Karl Jansky (USA) picked up the first radio waves from beyond the solar system.
- 1957 - construction of the world's first giant radio telescope by the University of Manchester at Jodrell Bank, Cheshire. Astronomic observation is now often conducted by tracking radio waves and no longer depends upon optics. The wavelengths they intercept would be imperceptible to the human eye.
Optical telescopes for use in astronomy still exist but are usually to be found at high altitude where the atmosphere is thinner. The very large Hale reflector telescope (5 metres in size) was sited on Palomar Mountain in California in 1947. The Keck I multi-mirror telescope on Mauna Kea in Hawaii was set up in 1992 with an aperture twice that size. With today's sophisticated telescopes, computer controlled 'active optics' can compensate for atmospheric changes to ensure a greater accuracy of reading. The day of the humble telescope may not be over yet, however. In August 2004 it was reported by the Harvard-Smithsonian Centre for Astrophysics that that a network of four-inch telescopes had discovered a planet in another solar system for the first time. The gas giant in the constellation of Lyra is five hundred light years away but it was detected by the 'transit method' in which the brightness of a star was observed to diminish ever so slightly as the new planet crossed in front of it.
A Very Large Telescope
The largest optical telescope in the world, imaginatively named the 'Very Large Telescope' (VLT) is in the desert of northern Chile. It is actually a complex of four telescopes, each with an 8 metre mirror just 18cm thin. To mark the fact that native peoples had been star-gazing for centuries the four observatories are named in the local language after 'Sun', 'Moon', 'Southern Cross' and 'Brightest Star in the Sky' (which to those peoples might have been either Venus or Sirius). The shape of the thin mirrors is preserved by an 'adaptive optic system' of 162 computerised actuators, installed in 2001, that can correct the mirror's position up to 500 times per second thus compensating for atmospheric turbulence. Down-to earth astronomers have described the corrective effect of this system as 'like fitting the telescope with spectacles!'
|
This detail is from an engraving by Robert Meadows of George Romney's painting of 'Newton with the Prism' which was an illustration in William Hayley's biography of the painter, 1809. It shows a model for demonstrating planetary motion. Newton had predicted the existence of what we now call exoplanets, but the first one would not be confirmed until 1995. In 2007 it was discovered that these are not all gaseous but include rocky planets, potentially capable of sustaining life. |
Watch out, however, for something even bigger in the future. The European Extremely Large Telescope (E-ELT) will be 21-storeys high and feature 1000 mirrors when it begins its search for exoplanets (planets orbiting stars outside of our own solar system) around 2018. An additional 2mm thick glass mirror placed over 5000 actuators will reshape itself up to 1000 times a second to sharpen the image and help us to learn whether there really is anyone else out there. The E-ELT is being planned by a consortium of eleven European nations but it won't be located in Europe. Possible sites for what will take over the title of the world's largest optical telescope include the Canary Islands, Chile, Argentina, Morocco and the Australian Antarctic Territory.