# 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 (the planet Uranus), and the Leviathan of Parsonstown which at 1.83 meters (72 inches) was for decades the largest aperture telescope in the World. In the 20th century many older telescopes are popular tourist attractions, such as at 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 98 inches (2.5 m) 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 really 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 24 inches or greater:
Reflecting telescopes :
| Name | Effective aperture | Type | Location | Operator | First light | Comments |
| Rosse Six Foot Telescope (reconstructed) | 72 in (183 cm) | Newtonian reflector | Birr, Leinster, Ireland | Birr Castle | 1999 | Largest optical telescope in Ireland |
| 38-inch Congo Schmidt | 38 in (96.5 cm) | Reflector | Herstmonceux, East Sussex, England | The Observatory Science Centre | 1960 | Largest optical telescope in UK, but never used due to flawed optics |
| James Gregory Telescope | 37 in (94.0 cm) | Cassegrain reflector | St Andrews, Fife, Scotland | University of St Andrews | 1962 | Largest operational optical telescope in the UK |
| Cambridge 36-Inch telescope | 36 in (91.4 cm) | Reflector | Cambridge, Cambridgeshire, England | University of Cambridge | 1955 | Largest optical telescope still in use in England |
| 36-inch Yapp telescope | 36 in (91.4 cm) | Reflector | Herstmonceux, East Sussex, England | The Observatory Science Centre | 1932 | |
| Edinburgh 36-inch telescope | 36 in (91.4 cm) | Reflector | Edinburgh, Scotland | Royal Observatory Edinburgh | 1930 | No longer operational |
| 34-inch Hewitt Camera | 34 in (86.4 cm) | Reflector | Herstmonceux, East Sussex, England | The Observatory Science Centre | 1950s | |
| Perren Telescope | 31.5 in (80.0 cm) | Ritchey–Chrétien reflector | Mill Hill, London, England | UCL Observatory | 2019 | |
| Thomson/Regan/Owen Reflector | 30 in (76.2 cm) | Reflector | Great Sutton, Cheshire | David Thomson | 2023 | |
| John Wall refractor | 30 in (76.2 cm) | Refractor | Hanwell, Oxfordshire, England | Hanwell Community Observatory | 1999 | Largest refractor in the British Isles |
| 30" Dobsonian | 30 in (76.2 cm) | Reflector | Todmorden, West Yorkshire, England | The Astronomy Centre | 1986 | |
| Thompson 30-inch Reflector | 30 in (76.2 cm) | Reflector | Herstmonceux, East Sussex, England | The Observatory Science Centre | 1896 | |
| Greenwich 28 inch refractor | 28 in (71.1 cm) | Refractor | Greenwich, London, England | Royal Observatory, Greenwich | 1893 | |
| Moses Holden Telescope | 27.6 in (70.1 cm) | Reflector | Preston, Lancashire, England | University of Central Lancashire | 2015 |
| Thompson 26-inch Refractor | 26 in (66.0 cm) | Refractor | Herstmonceux, East Sussex, England | The Observatory Science Centre | 1897 | |
| 24 / 17" Schmidt Camera | 24 in (61.0 cm) | Reflector | Knighton, Powys, Wales | The Spaceguard Centre | 1950 | Largest optical telescope in Wales |
| Thornton Telescope | 24 in (61.0 cm) | Reflector | Keele, Staffordshire, England | Keele University | 1975 | |
| 24" Telescope | 24 in (61.0 cm) | Reflector | Sherwood Observatory | Sherwood Observatory | 1984 | |
| 24" Telescope | 24 in (61.0 cm) | Reflector | Bayfordbury, Hertfordshire, England | University of Hertfordshire | 2021 | |

## Historical

* Isaac Newton Telescope at Herstmonceux, 98 inches (1965-1979)
* Leviathan of Parsonstown, 1842- ~1890
* 3-foot telescope at Parsons
* RGO telescopes at different points in its history
** 38-inch Hargreaves Reflector (1960)
** Yapp 36-inch Reflector (1932)
** 30-inch Steavenson Reflector (1939)
** 28-inch Refractor (1893)
** Thompson Telescope with a 26-inch refractor and 30-inch reflector on one mounting (1896)
** Lassell 2-foot Reflector (1845)
** Isaac Roberts 20-inch reflector (1885)
** Western Equatorial (c.1824)
** 13-inch Astrographic Refractor (1890)
** Merz 12.8-inch Visual Refractor (1859-1893) (this was replaced by the 28 inch Grubb in the onion dome)
** Thomson 9-inch Photographic Refractor (c.1888)
** Sheepshanks refractor 6.7-inch (1838) (aka Sheepshanks Equatorial)
** 6-inch Franklin Adams Camera (1898)
** Shuckburgh telescope a 4.1-inch aperture Refractor (1791)
* At the Observatory Science Center (at Herstmonceux)
** Hargreaves 38 inch Congo Schmidt
** Yapp 36 Inch reflector
** Thompson 30 inch reflector
** Thompson 26 inch reflector
* Markree Observatory 13.3" Cauchoix (the largest refractor of the early 1830s)
* A.A. Commons reflectors (later reworked into Crossley and Harvard telescopes)
* Lassel's reflector, this 24 inch metal mirror telescope was used to discover the moons Triton and Hyperion.
* Newton's reflector
* 40-foot telescope (England)
* Armagh Observatory 15-inch Grubb reflecting telescope. Specula metal mirror mounted on an equatorial, with clockwork-drive.
* Bedford Observatory Tully 5.9 inch refractor (8.5 feet focal length); Dollond mount with Sheepshanks clockwork drive.
* Cambridge Observatory 36 inch (3 feet = 91.44 cm) aperture reflector

## Observations

A 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 quite prolific discovering a planet and many moons of the Solar system also with his reflectors.