A primary mirror (or primary) is the principal
light-gathering surface (the
objective) of a
reflecting telescope.
Description
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
A segmented mirror is an array of smaller mirrors designed to act as segments of a single large curved mirror. The segments can be either spherical or asymmetric (if they are part of a larger parabolic reflector). They are used as objectives ...
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 optical aperture.
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
The (NAOJ) is an astronomical research organisation comprising several facilities in Japan, as well as an observatory in Hawaii and Chile. It was established in 1988 as an amalgamation of three existing research organizations - the Tokyo Astron ...
, located in
Mauna Kea Observatory on
Hawaii
Hawaii ( ; haw, Hawaii or ) is a state in the Western United States, located in the Pacific Ocean about from the U.S. mainland. It is the only U.S. state outside North America, the only state that is an archipelago, and the only ...
since 1997; 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).
Both of these are smaller than the 10 m segmented primary mirrors on the
dual Keck telescope. The
Hubble Space Telescope
The Hubble Space Telescope (often referred to as HST or Hubble) is a space telescope that was launched into low Earth orbit in 1990 and remains in operation. It was not the first space telescope, but it is one of the largest and most ver ...
has a 2.4 m (7 ft 10 in) primary mirror.
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 Telescope used a 305 m dish, which was the world largest single-dish radio telescope fixed to the ground.
The
Green Bank Telescope has the world's largest steerable single radio dish with 100 m in diameter. There are larger radio arrays, composed of multiple dishes which have better
image resolution
Image resolution is the detail an image holds. The term applies to digital images, film images, and other types of images. "Higher resolution" means more image detail.
Image resolution can be measured in various ways. Resolution quantifies how cl ...
but less
sensitivity
Sensitivity may refer to:
Science and technology Natural sciences
* Sensitivity (physiology), the ability of an organism or organ to respond to external stimuli
** Sensory processing sensitivity in humans
* Sensitivity and specificity, statisti ...
.
See also
*
Active optics
*
Honeycomb mirror
*
Liquid-mirror telescope
*
List of largest optical reflecting telescopes
*
List of telescope parts and construction
*
Mirror mount
*
Mirror support cell
*
Secondary mirror
*
Silvering
Silvering is the chemical process of coating a non-conductive substrate such as glass with a reflective substance, to produce a mirror. While the metal is often silver, the term is used for the application of any reflective metal.
Process ...
References
{{reflist
Optical telescope components
Mirrors