List of largest optical telescopes in the British Isles
List of largest optical telescopes in Ireland and the United Kingdom is a list of the largest optical telescopes in the British Isles, including in the United Kingdom and Ireland.
The most famous telescopes include Herschel's reflector, with which he discovered Georgium Sidus, and the Leviathan of Parsonstown which at was for decades the largest aperture telescope in the world. In the 20th century many older telescopes are popular tourist attractions, such as at the Royal Observatory in Greenwich, London. There are also a number of modest instruments at universities used for various astronomical projects or education.
The largest optical telescope in Britain was the Isaac Newton Telescope, which had a mirror; it was located at the Royal Greenwich Observatory, Herstmonceux from 1965 to 1980, but was then relocated to Roque de los Muchachos Observatory on La Palma, Canary Islands.
The list is not representative of the largest telescopes operated by the United Kingdom or Ireland, which by the 20th century were building large telescopes overseas or in the southern hemisphere for better weather or other reasons.
Current list
The following is a non-comprehensive list of optical telescopes currently located in the British Isles with an aperture of or greater:| Name | Effective aperture | Type | Location | Operator | First light | Notes |
| Rosse Six Foot Telescope | Newtonian reflector | Birr, Leinster  convert|38|in|cm|sigfig=3|abbr=onHistorical
ObservationsA noted accomplishment of the biggest telescope at the time, Ross's "six foot" leviathan, was the observation of the spiral structure of M51, which was presented at Cambridge in the summer of 1845. Herschel was also quite prolific discovering a planet and many moons of the Solar system with his reflectors. |