page_title: Greenwich 28 inch refractor
section_title: Domes
hierarchical_section_title: Greenwich 28 inch refractor / Domes
caption_attribution_description: English: Royal Observatory Former Great Equatorial BuildingThis is a photo of listed building number 1078998.
context_page_description: The Greenwich 28-inch refractor is a telescope at the Royal Observatory, Greenwich, where it was first installed in 1893. It is a 28-inch aperture objective lens telescope, otherwise known as a refractor, and was made by the telescope maker Sir Howard Grubb.
  The achromatic lens was made Grubb from Chance Brothers glass.
  The mounting is older however and dates to the 1850s, having been designed by Royal Observatory director George Airy and the firm Ransomes and Simms. The telescope is noted for its spherical dome which extends beyond the tower, nicknamed the "onion" dome.
  Another name for this telescope is "The Great Equatorial" which it shares with the building, which housed an older but smaller telescope previously.
  The telescope was re-commissioned by Queen Elizabeth II in May 1975 after it was brought back from Herstmonceux in Sussex. It was placed in the renovated original dome at Greenwich for the tricentenntial celebration of the observatory.
context_section_description: The dome for the older, smaller telescope was taken down in 1892, and the new, larger dome for the 28-inch was finished by 1893.The older dome for the 12.8 inch refractor has been called a 'drum dome' whereas the 28-inch is called the 'onion dome'. 
  The original 1893 dome was made by Messrs T. Cooke and Sons.
  The new dome was needed because the 28-inch Grubb was longer than the 12.8 inch aperture Merz. The Grubb has a focal length of 27 feet and 10 inches, while the old Merz had a focal length of 17 feet and 10 inches. The tower diameter is smaller than the length of the new Grubb telescope, so the dome had to be bigger to accommodate its length.
  With refractors of this type, the focal length meant a real physical length between the objective and the focus point. Reflecting telescopes can 'fold' their focal length by using mirrors, so they can have a physical length shorter than their optical focal length. Two important figures for telescopes are the aperture and focal length, which affect the equations that describe their properties of magnification.
  The dome for the 28-inch is noted for being called the "onion dome" and the original manufacture from 1893 was made of an iron grid and papier-mâché.  This dome was damaged in a V-1 flying bomb strike during the Second World War, and was taken down in 1953.
  When the telescope was moved back to Greenwich in 1971, a new dome made of fibreglass in the style of the older dome was installed. 
  The telescope is installed in the 'Great Equatorial Building' at Greenwich. When it was installed at Herstmonceux, the 28-inch Grubb was installed in Dome F. The whole observatory was moved in the 1950s to a new installation at Herstmonceux in Sussex, however, astronomy was changing rapidly at the time with a major focus on observatory location; astronomers could review data from instruments elsewhere.
  In 1987 a second staircase into the dome was added, to ease the flow of visitors in and out, the quantity of which was enough to make foot traffic on a single winding staircase difficult. The entrance door to the telescope is also noted for being made of iron, in the Victorian style.
caption: The Great Refractor dome, 2013
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