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Segmented mirror - Segmented mirror

Size comparison of the primary mirrors. Segmented mirrors are typically hexagonal and arranged in a honeycomb pattern.

A segmented mirror is an array of smaller mirrors intended to serve as segments of a single large curved mirror. The segments can be either spherical or asymmetrical (if they are part of a larger parabolic reflector). They are used as lenses for large reflector telescopes. In order to function, all mirror segments must be polished to a precise shape and actively aligned by a computer-controlled active optics system using actuators built into the mirror support cell. The concept and required technologies were originally developed in the 1980s under the direction of Dr. Jerry Nelson at Lawrence Berkeley National Laboratory and the University of California and have since become so widespread around the world that essentially all future large optical telescopes plan to use segmented mirrors.


Segmented mirror from SALT

There is a technological limit for primary mirrors made from a single rigid piece of glass. Such not segmented or monolithic mirror cannot be constructed larger than about eight meters in diameter. The largest monolithic mirror used is currently the two main mirrors of the Large Binocular Telescope, each with a diameter of 8.4 meters. The use of segmented mirrors is therefore a key component for large aperture telescopes. Using a monolithic mirror much larger than 5 meters is prohibitively expensive because of the cost of both the mirror and the massive structure required to support the mirror. A mirror beyond this size would also sag slightly under its own weight if the telescope were rotated to different positions, thereby changing the precision shape of the surface. Segments are also easier to manufacture, transport, install, and maintain via very large monolithic mirrors.

Segmented mirrors have the disadvantage that each segment can require a precisely asymmetrical shape and is based on a complicated computer-controlled assembly system. All of the segments also cause diffraction effects in the final image.

Telescopes with segmented mirrors

Primary mirror assembly of the James Webb Space Telescope

Some of the largest optical telescopes in the world use segmented primary mirrors. These include the following telescopes:

The twin telescopes are the most famous of the Mauna Kea observatories at an altitude of 4,145 meters near the summit of Mauna Kea in Hawaii, USA. Both telescopes have 10 m (33 ft) primary mirrors.
The HET is a 9.2 meter telescope located at the McDonald Observatory in West Texas at an altitude of 2,026 meters. Its primary mirror consists of 91 hexagonal segments. The telescope's primary mirror is fixed at a 55 degree angle and can rotate around its base. A goal is pursued by moving the instruments in the focus of the telescope. This allows access to approximately 70-81% of the sky in its location and a single target can be tracked for up to two hours.
The SALT is a 10-meter telescope that devotes most of its observation time to spectroscopy. It is similar to the Hobby Eberly telescope and consists of 91 hexagonal mirror segments, each 1 m in diameter, resulting in a total hexagonal mirror measuring 11.1 mx 9.8 m. It is located near the town of Sutherland in the semi-desert region of the Karoo in South Africa. It is a facility of the South African Astronomical Observatory, the national optical observatory of South Africa.
The Canaries Great Telescope, also known as GranTeCan, uses a total of 36 segmented mirrors. With a primary mirror of 10.4 m, it is currently the world's largest optical telescope, which is located at the Roque de los Muchachos observatory on the island of La Palma in the Canary Islands in Spain.
The Multi-Object Fiber Spectroscopic Telescope with a large sky area is a surveying telescope in the Chinese province of Hebei. It consists of two rectangular mirrors made up of 24 or 37 segments. Each hexagonal segment is 1.1 meters tall.
Most of the 18 mirror segments of the JSWT were manufactured in 2011. While the total estimated cost has been increasing for several years, it has been called the the telescope criticized who ate astronomy as the portion of NASA's astrophysics budget it had consumed grew. As of 2013, the estimated cost was $ 8.8 billion. The space telescope is scheduled to be launched by an Ariane 5 from the Guiana Space Center in March 2021.

Next generation telescopes

Three extremely large telescopes will be the next generation of segmented reflector telescopes and should be put into operation between 2021 and 2024. The Giant Magellanic Telescope uses very large segments and is either grouped with segmented reflecting telescopes or a category of its own. Completion is planned for 2021. The 30-meter telescope will be built in the Mauna Kea observatories in Hawaii (estimated first light by 2022). The European extremely large telescope will be the largest of the three, using a total of 798 segments as the primary mirror. The first light is expected for 2024.

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