Dating from 1885, the telescope is named after its first owner Isaac Roberts (1829-1904). The instrument was on loan to the Observatory from the Science Museum from 1956–1961.
Isaac Roberts was 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. The telescope consisted of a 20-inch Newtonian photographic reflector by Sir Howard Grubb of Dublin (who also made the mount), counterbalanced by a 7-inch refractor by Thomas Cooke & Son of York, that acted as the guidescope. Together such combinations are more usually referred to as Twin Equatorials. Roberts’ instrument was inspired by a similar instrument built for Huggins at Tulse Hill.
Roberts originally set his telescope up at Maghull, near Liverpool, but in 1890, he moved to a purpose-built house-cum-observatory at Crowborough in Sussex, in the hope of getting clearer skies. After his death, the instrument was acquired by J G Bower, an industrialist, amateur astronomer and instrument collector based in Norwich. When his death, the telescope was bought by the Science Museum in 1936 (Inventory number: 1936-231).
In 1956/7, with no immediate prospect of getting any of the large equatorials installed and working at Herstmonceux, Woolley approached the Science Museum to see if it might allow the 20-inch Isaac Roberts reflector to be used as a stop gap measure. In recommending the loan, HR Calvert wrote: ‘It will have the advantage that the telescope will be overhauled and put into working order so that it will be ready for use in our own observatory if we can get nothing better.’ The telescope was collected on 24 September 1956 and installed in Dome C by August 1957 where it was used for photometric work. Observing conditions at Herstmonceux proved unsuitable for this type of work and the telescope was returned to the Science Museum on 4 July 1961.