Twin equatorial telescope

From Object Wiki

Jump to: navigation, search
Twin equatorial telescope
Manufacturer Howard Grubb, Thomas Cooke & Son
Production years 1885
Production location Dublin, York

This telescope was used by Isaac Roberts to photograph the night sky, particularly in studying nebulae.


How it works

As its name suggests, the Twin Equatorial is a pair of telescopes, sharing the same mount. The main telescope is a reflector with a 20-inch diameter main mirror. It is a photographic telescope, specially designed for photographing the night sky, and was never used for visual observations. Near the top of the instrument, instead of the usual secondary mirror, is a carrier for a 4-inch photographic plate.

On the other side of the mount is a 7-inch refracting telescope, which not only serves as a counterpoise to the big reflector, replacing the usual counterweight, but also acted as a guidescope, ensuring that the main reflector remained accurately pointed at its target during a long photographic exposure.


The ‘Twin Equatorial’ design was not an original one, but was inspired by a similar refractor-cum-reflector built for William Huggins, a nineteenth-century British astronomer and pioneer of astronomical spectroscopy.

The 20-inch reflector and mount were built in 1885 by Sir Howard Grubb of Dublin. His firm (after the First World War known as Grubb Parsons and located in Newcastle-Upon-Tyne) built many of Britain’s greatest telescopes in the nineteenth and twentieth centuries, including the Isaac Newton and William Herschel telescopes on La Palma in the Canary Islands. The 7-inch refractor was built by Thomas Cooke & Son of York, a well-known nineteenth-century instrument-maker.

Both telescopes and the mount were made for Isaac Roberts (1829-1904), a wealthy British amateur astronomer who made a considerable fortune in the building trade and devoted himself full-time to astronomy after his retirement in 1888. From 1885 until his death in 1904, Roberts used the telescope for photographing the night sky (see below). Initially the telescope was set up at Maghull, near Liverpool. In 1890, Roberts moved his home and his telescope to a purpose-built house-cum-observatory at Crowborough in Sussex, in the hope of getting clearer skies. The telescope remained at Crowborough until Roberts’ death. Afterwards, the Twin Equatorial was acquired by J G Bower, an industrialist, amateur astronomer and instrument collector based in Norwich, England. When he died, the telescope was acquired by the Science Museum in 1936.

How this instrument was used

Although photography had been invented c. 1840, only in the 1880s did photographic plates become sensitive enough to record stars and especially nebulae, the fuzzy objects in the night sky, many of which had been discovered but whose nature was in many ways unknown to scientists in the 1880s.

Roberts’ greatest discovery with this telescope was two photographs of the Andromeda Nebula, taken in 1888. Up until then, the true nature of this nebula had eluded even the best visual observers, but Roberts’ photographs showed clearly that it was a spiral nebula. At the time, many thought that spiral nebulae were Solar Systems in the process of formation, and Roberts’ images seemed to many to provide proof of this theory, because they showed what looked like debris condensing to form planets, orbiting a central Sun. We now know that spiral nebulae are, in fact, galaxies, vast systems of stars like our own Milky Way Galaxy, millions of light years from Earth.

Although Roberts’ early photographs were pioneering, other astronomers quickly followed him into the new field of nebular photography, some of them working in much better sky conditions than Roberts ever had. In particular, from 1889 onwards, US astronomer Edward Emerson Barnard began taking spectacular photographs of nebulae and the Milky Way and published his results in the same journal as Roberts, the Monthly Notices of the Royal Astronomical Society. Roberts and Barnard became bitter rivals.

In the Science Museum

Inv. No: 1936-231