Primary mirror

The largest non-segmented mirror in an optical telescope in 2009, one of the Large Binocular Telescope's two mirrors
Six of the primary mirrors of the James Webb Space Telescope being prepared for acceptance testing

A primary mirror (or primary) is the principal light-gathering surface (the objective) of a reflecting telescope.


The primary mirror of a reflecting telescope is a spherical or parabolic shaped disks of polished reflective metal (speculum metal up to the mid 19th century), or in later telescopes, glass or other material coated with a reflective layer. One of the first known reflecting telescopes, Newton's reflector of 1668, used a 3.3 cm polished metal primary mirror. The next major change was to use silver on glass rather than metal, in the 19th century such was with the Crossley reflector. This was changed to vacuum deposited aluminum on glass, used on the 200-inch Hale telescope.

Solid primary mirrors have to sustain their own weight and not deform under gravity, which limits the maximum size for a single piece primary mirror.

Segmented mirror configurations are used to get around the size limitation on single primary mirrors. For example, the Giant Magellan Telescope will have seven 8.4 meter primary mirrors, with the resolving power equivalent to a 24.5 m (80.4 ft) optical aperture.[1]

Superlative primary mirrors

The largest optical telescope in the world as of 2009 to use a non-segmented single-mirror as its primary mirror is the 8.2 m (26.9 ft) Subaru telescope of the National Astronomical Observatory of Japan, located in Mauna Kea Observatory on Hawaii since 1997;[2] however, this is not the largest diameter single mirror in a telescope, the U.S./German/Italian Large Binocular Telescope has two 8.4 m (27.6 ft) mirrors (which can be used together for interferometric mode).[3] Both of these are smaller than the 10 m segmented primary mirrors on the two Keck telescope. The Hubble Space Telescope has a 2.4 m (7 ft 10 in) primary mirror.

The correctly ground backup primary mirror built by Eastman Kodak for the Hubble space telescope (the mirror was never coated with a reflective surface, hence its honeycomb support structure can be seen). It now resides in the National Air and Space Museum in Washington, DC.[4]

Radio and submillimeter telescopes use much larger dishes or antennae, which do not have to be made as precisely as the mirrors used in optical telescopes. The Arecibo Observatory uses a 305 m dish, which is the world largest single-dish radio telescope fixed to the ground.[5] The Green Bank Telescope has the world's largest steerable single radio dish with 100 m in diameter.[6] There are largest radio arrays, composed of multiple dishes which have better image resolution but less sensitivity.[7]

See also


  1. Maggie McKee (2007-10-04). "Giant telescope in race to become world's largest". New Scientist. Retrieved 2009-03-27.
  2. - "The Subaru Telescope"
  3. "Giant telescope opens both eyes". BBC News. London. 2008-03-06. Retrieved 2008-03-06.
  4. "Mirror, Primary Backup, Hubble Space Telescope". National Air and Space Museum. Archived from the original on October 13, 2007. Retrieved 2008-04-26.
  5. "The 305 meter radio telescope". National Astronomy and Ionosphere Center - Arecibo Observatory. Retrieved 22 January 2015. External link in |website= (help)
  6. "Green Bank". National Radio Astronomy Observatory. Retrieved 22 January 2015. External link in |website= (help)
  7. "Radio Telescope". University of Oregon. Retrieved 22 January 2015. External link in |website= (help)
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