Dunsink Observatory|thumb|300px
A plan of the observatory from the Gentleman's Magazine in 1788.|thumb|300px
The Dunsink Observatory is an astronomical observatory established in 1785 in the townland of Dunsink in the outskirts of the city of Dublin, Ireland.
Directors of the observatory include William Rowan Hamilton, who, amongst other things, discovered quaternions, the first non-commutative algebra form, while walking from the observatory to the city with his wife. The annual Hamilton Walk that commemorates this discovery begins at the observatory.
History
The observatory was established by an endowment of £3,000 in the will of Francis Andrews, who was Provost of Trinity College Dublin at his death on 18 June 1774. The site was established on the south slope of a low hill in the townland of Dunsink, 84m above sea level. The South Telescope, a 12-inch Grubb instrument, is a refracting telescope built by Thomas Grubb of Dublin and completed in 1868. The achromatic lens, with an aperture of 11.75 inches, was donated by Sir James South in 1862. He had purchased the lens from Cauchoix of Paris 30 years earlier. He had intended it for a large but troubled equatorial that came to fruition in the 1830s, but was dismantled around 1838. (See also Great refractors)
The entry for the observatory in Thom's Directory (1850) gives the following account of the observatory,