Telescopes

The list of telescopes has been organised according to mounting type or function. The date of manufacture is given in parenthesis where known. Where a date is inscribed on the instrument, this is what is recorded.

Within each section, instruments are listed in the order they arrived at the Observatory. Some were acquired from new, but others were existing instruments that were either purchased second hand, donated or on loan.

When a dimension is mentioned in the name of a telescope it is the diameter of the objective or primary mirror in the case of the equatorial telescopes and photoheliographs and the length of the tube for the others. The practice of describing telescopes by their length ceased during the first half of the nineteenth century.

 

Mural Instruments

These instruments were rigidly mounted on a meridian wall. Like the later transit instruments, they could only move in the in the plane of the meridian. Their purpose was to measure the position of the heavenly bodies in terms of their declination and right ascension. In practice, the later instruments were used to determine only the declination; right ascension being found with an adjacent transit telescope.

Hooke's 10-foot Mural Quadrant (1676)
Flamsteed's 'Slight' Mural Arc (1683)
Flamsteed's Mural Arc (1689)
Halley's 8-foot Iron Mural Quadrant (1725)
Bradley's 8-foot Brass Mural Quadrant (1750)
Troughton 6-foot Mural Circle (1810)
Jones 6-foot Mural Circles (1821 & 1822)

Transit Instruments

These instruments were mounted between two piers in such a way that they could only rotate in the plane of the meridian. They were used to determine the right ascensions of stars. The transit circles were also used to determine declination. The Halley, Bradley, Troughton and Airy instruments were also used for time determination. From 1927, the small reversible transits replaced the Airy Transit Circle for this purpose. The Bamberg Broken Transit was also used for time determination. The small transits and Bamberg Broken Transit were superseded for time determination by the Photographic Zenith Tube.

Halley's 5-foot Transit Instrument (1721)
Bradley's 8-foot Transit Instrument (1750)
Troughton 10-foot Transit Instrument (1816)
Airy's Transit Circle (1850)
Portable Transit by Brauer (1866)
Small Reversible Transits – five in total (1870)
Small Transit (originally ordered for Singapore)
Bamberg Broken Transit (early 20th C)
Cooke Reversible Transit Circle (1933)
Melbourne Reversible Transit Circle (1883)
Horizontal Transit Instrument (developed in the 1940s & 50s but never built)
Mirror Transit Circle (developed in the 1940s & 50s but never built)

 

Prismatic Astrolabes

Invented in the early twentieth century, prismatic astrolabes were used to determine the latitude of a location and the clock correction from observations of the passages of stars at various azimuths across a given almucantar. They could also be used to determine the equatorial coordinates of stars and planets.

4-inch Danjon Prismatic Astrolabe OPL. No. 9 (1956)

 

Large Altazimuth Instruments

The main purpose of these instruments in the Greenwich context, was to measure the position of the Moon, particularly on the days either side of new moon when it couldn’t be observed with the transit instruments due to glare from the Sun.  They were also useful when a transit observation of the Moon was missed due to cloudy weather. Unlike modern altazimuth instruments, these instruments were designed to be locked in azimuth during use.

Airy's Altazimuth (1847)
Christie's Altazimuth (1896)

 

Zenith Instruments

This class of telescope was restricted by its mounting to observations of the zenith – the point directly overhead. A greater degree of positional accuracy could be obtained than with either the mural or transit instruments because the light from stars directly overhead is not refracted by the Earth’s atmosphere on its journey to the telescope and because errors arising from tube flexure were reduced or eliminated. Some of the instruments were highly successful. Others were failures. The earlier instruments were built to detect and measure stellar parallax – something they failed to do, although Bradley did discover the phenomena of aberration and nutation in the process. The later instruments were used to check the line of collimation of the meridian telescopes, to measure the constants of aberration and nutation, and measure the variation of latitude due to polar wander. The Photographic Zenith Tube (1955) was designed specifically for time determination.

Flamsteed's Well Telescope (1676)
Bradley's 12½-foot Zenith Sector (1727)
Pond's 9½-foot Zenith Tube (1812)
Pond's 8-foot Achromatic Zenith Telescope (c.1820?)
Pond's 25-foot Great Zenith Tube (1833)
Airy's Reflex Zenith Tube (1851)
Airy's Water Telescope (1870)
Cookson's Floating Zenith Telescope (1900)
Photographic Zenith Tube (1955)

 

Equatorial Sectors

These instruments, as their name implies, were mounted equatorially. They were used for measuring the angle between pairs of heavenly bodies. Before the invention of pendulum clocks they were used to measure differences in right ascension. Flamsteed used his for just such a purpose to compile his star catalogue, abandoning it in 1689 in favour of a pendulum clock and his newly acquired mural arc. The seventeenth century instruments were used to measure the paths of comets by comparison with nearby stars whose right ascension and declination were known.

Flamsteed's 7-foot Equatorial Sextant (1676)
Graham's 2½-foot Equatorial Sector (c.1735)
Sisson's 5-foot Equatorial Sectors (1773)

 

Large telescopes carried on equatorial mounts

Equatorially mounted telescopes, by virtue of their design, are able to lock onto celestial objects and follow them across the sky as the Earth rotates on its axis. There are three types of equatorial mount: the English, the German and the Fork type. The Observatory had examples of each. The English mounting has two piers and the advantage (if properly constructed) of being the most stable, but the disadvantage that the Polar Regions are inaccessible.

Until the development of the thrust bearing, the size of equatorial instruments was severely constrained by their weight. The size of refractors was further constrained initially by the ability of glassmakers to cast large optical blanks of sufficient optical quality, and then by problems of flexure as lenses got larger. When originally built, the Shuckburgh was the world’s largest equatorial telescope on a fixed mounting. The Greenwich 28-inch and 26-inch refractors are amongst the largest refractors in the world.

At Greenwich the three largest equatorial instruments either had their original telescope replaced at some point, and / or simultaneously carried more than one telescope. These were: the Great Equatorial (also known as the South-East Equatorial), the Lassell Equatorial and the Thompson Equatorial.

Shuckburgh 4.1-inch Refractor (1791) – also known as the Shuckburgh Equatorial
Western Equatorial (c.1824)
Sheepshanks 6.7-inch Refractor (1838) – also known as the Sheepshanks Equatorial
Merz 12.8-inch Visual Refractor (1859)
Lassell 2-foot Reflector (1845)
13-inch Astrographic Refractor (1890)
Thomson 9-inch Photographic Refractor (c.1888)
28-inch Refractor (1893)
Thompson 26-inch Photographic Refractor (1896)
Thompson 30-inch Photographic Reflector (1896)
Yapp 36-inch Reflector (1932)
Isaac Roberts 20-inch reflector (1885)
30-inch Steavenson Reflector (1939)
6-inch Franklin Adams wide-angle Star Camera (1898)
38-inch Hargreaves Reflector (1960)

 


 

Telescopes mounted on the three largest equatorials at Greenwich
       
Telescope Great Equtatorial Lassell Equatorial Thompson Equatorial
Merz 12.8 inch Visual Refractor 1860–1891 1892–1895 1897 onwards
Corbett Equatorial (see below) 1893 onwards 1887–1891
Hodgson Equatorial (see below)
1897 onwards
Dallmeyer Photoheliograph (see below)
1901
Lassell 2-foot Reflector   1884–1892  
Thompson 9-inch Photographic refractor / Photoheliograph   1891–1894 (or 5?) 1897 onwards (temp removed for eclipse expeditions)
28-inch Refractor 1893 onwards    
Thompson 26-inch Photographic Refractor     1897 onwards
Thompson 30-inch Photographic Reflector     1897 onwards
6-inch Franklin Adams camera 1911/2–1914 (+?)

 


 

6-inch Equatorials (used on Transit of Venus Expeditions)

Of the six 6-inch Equatorials originally assembled for the British 1874 Transit of Venus expeditions, all were subsequently transferred to the Royal Observatory in 1876. The two instruments marked (*) in the list below were later used as sighting telescopes on the large equatorials at Greenwich and Herstmonceux. The first four instuments in the list were acquired second hand and are named after their previous owners. The two instruments by Simms were new at the time of purchase.

Various new instruments including two 6-inch Equatorials by Cooke (Cooke No. 1 and Cooke No.2) were ordered in 1881 for the 1882 Transit of Venus expeditions (RGO6/283). Following the loss at sea of the Naylor on its return from the 1882 Transit, Cooke No.2 was transferred to the Royal Observatory (RGO6/283). The subsequent history of Cooke No. 1 is unknown.

Acquired after 1874 Transit

Lee Equatorial (1829)
Corbett Equatorial*
Hodgson Equatorial*
Naylor Equatorial (1863)
Simms No. 1
Simms No. 2

Acquired after 1882 Transit

Cooke No. 2

 

Other small Equatorials

Newbegin 6¼-inch refractor (c.1888)

 

Photoheliographs

This class of telescope designed specifically for taking photographs of the Sun. The Thompson 9-inch Photoheliograph was created from the Thomson 9-inch Photographic Refractor (above) in 1892.

3½-inch Kew Photoheliograph (1854)
4-inch Dallmeyer Photoheliographs – five in total (1873)
Thompson 9-inch Photoheliograph (c.1888/1891)

 

Large Herschelian Reflectors

The Herschelian reflector is named after William Herschel, who used this design to build very large telescopes including a 49.5 inch (126 cm) diameter telescope in 1789. The primary mirror was tilted so the observer's head did not block the incoming light. Although this introduced geometrical aberrations, Herschel employed this design to avoid the use of a Newtonian secondary mirror since the speculum metal mirrors of that time tarnished quickly and could only achieve 60% reflectivity.

Ramage's 25-foot telescope (c.1820)

 

Fixed telescopes for special investigations

These telescopes which were rigidly fixed to a wall pointed at a specific part of the sky. They were constructed specifically for their intended purpose. The Sirius Telescope was used to investigate the rotation of the Earth. The α Aquilae and α Cygni Telescopes were used to investigate the parallaxes of the two stars.

Sirius Telescope (1677)
α Aquilae and α Cygni Telescopes (1816)

 

Flamsteed’s Mast Telescopes

Until the invention of the achromatic doublet in the mid 1700s, images produced by telescope lenses would always suffer from the distorting effects of chromatic and spherical aberration. These could be reduced by using lenses which were less curved. Such lenses however had longer focal lengths and necessitating the use of long telescope tubes, which were usually supported by hanging them from a mast.

16-foot Refractor (pre 1675)
60-foot Mast Telescope (c.1677)
27-foot Refractor (c.1682)

 

Telescopes operated on behalf of the wider astronomical community

Until its closure in 1998, The RGO had responsibility for a number of telescopes which were funded for use by the whole of the British astronomical community.

98-inch/100-inch Isaac Newton Telescope (1967/1984)
34-inch Hewitt Camera (1962)
Carlsberg Automatic Meridian Circle (1952/1983)
1.0 metre Jacobus Kapteyn Telescope (1984)
4.2 metre William Herschel Telescope (1987)

Apart from the Hewitt Camera which is at Herstmonceux, the telescopes are currently located at Roque de los Muchachos Observatory on La Palma in the Canary Islands, the Isaac Newton Telescope having been moved there from Hertmonceux in the early 1980s. The Isaac Newton, acobus Kapteyn and William Herschel Telescopes are known as the Isaac Newton Group (ING).

 

Other telescopes

Over the years, there were a number of minor telescopes housed at the Observatory. Most were only used ocassionally for observing special events such as eclipses. Some were listed by Howse in Greenwich Observatory Vol. 3 (London: Taylor and Francis, 1975). To find out more, the various inventories should be consulted.