A segmented mirror is an array of smaller mirrors designed to act as segments of a single large curved mirror

Curved mirror

A curved mirror is a mirror with a curved reflective surface, which may be either convex or concave . Most curved mirrors have surfaces that are shaped like part of a sphere, but other shapes are sometimes used in optical devices...

. The segments can be either spherical or asymmetric (if they are part of a larger parabolic reflector

Parabolic reflector

A parabolic reflector is a reflective device used to collect or project energy such as light, sound, or radio waves. Its shape is that of a circular paraboloid, that is, the surface generated by a parabola revolving around its axis...

). They are used as objectives

Objective (optics)

In an optical instrument, the objective is the optical element that gathers light from the object being observed and focuses the light rays to produce a real image. Objectives can be single lenses or mirrors, or combinations of several optical elements. They are used in microscopes, telescopes,...

 for large reflecting telescope

Reflecting telescope

A reflecting telescope is an optical telescope which uses a single or combination of curved mirrors that reflect light and form an image. The reflecting telescope was invented in the 17th century as an alternative to the refracting telescope which, at that time, was a design that suffered from...

s. To function, all the mirror segments have to be polished to a precise shape and actively aligned by a computer controlled active optics

Active optics

Active optics is a technology used with reflecting telescopes developed in the 1980s, which actively shapes a telescope's mirrors to prevent deformation due to external influences such as wind, temperature, mechanical stress...

 system using actuators built into the mirror support cell

Mirror support cell

In astronomy the mirror support cell - more commonly mirror cell - is the component of a reflecting telescope which supports the mirror in place to hold optical alignment, allow collimation adjustment, and protect it from falling out...

. The concept and necessary technologies were initially developed under the leadership of Dr. Jerry Nelson

Jerry Nelson (astronomer)

Jerry Earl Nelson is an American astronomer known for his pioneering work designing segmented mirror telescopes, which led to him receiving the 2010 Kavli Prize for Astrophysics....

 at the Lawrence Berkeley National Laboratory

Lawrence Berkeley National Laboratory

The Lawrence Berkeley National Laboratory , is a U.S. Department of Energy national laboratory conducting unclassified scientific research. It is located on the grounds of the University of California, Berkeley, in the Berkeley Hills above the central campus...

 and University of California

University of California

The University of California is a public university system in the U.S. state of California. Under the California Master Plan for Higher Education, the University of California is a part of the state's three-tier public higher education system, which also includes the California State University...

 during the 1980s, and have since spread worldwide to the point that essentially all future large optical telescopes plan to use segmented mirrors.

Application

Because current monolithic mirrors cannot be constructed larger than about eight meters in diameter, the use of segmented mirrors is a key component of current large-aperture telescope

Telescope

A telescope is an instrument that aids in the observation of remote objects by collecting electromagnetic radiation . The first known practical telescopes were invented in the Netherlands at the beginning of the 1600s , using glass lenses...

s. This is because of the technological limit of a primary mirror made of a single rigid piece of glass. Using a monolithic mirror much larger than 5 meters is prohibitively expensive due to the cost of both the mirror, and the massive structure needed to support it. A mirror beyond that size would also sag slightly under its own weight as the telescope was rotated to different positions, changing the precision shape of the surface. Segments are also easier to fabricate, transport, install, and maintain over very large monolithic mirrors.

Segmented mirrors do have the drawback that each segment may require some precise asymmetrical shape, and rely on a complicated computer-controlled mounting system. All of the segments also cause diffraction

Diffraction

Diffraction refers to various phenomena which occur when a wave encounters an obstacle. Italian scientist Francesco Maria Grimaldi coined the word "diffraction" and was the first to record accurate observations of the phenomenon in 1665...

 effects in the final image.

Current segmented-mirror telescopes

These include, but are not limited to:

• The twin W. M. Keck Telescopes
• The Hobby-Eberly Telescope
  Hobby-Eberly Telescope
  The Hobby-Eberly Telescope is a 9.2-meter aperture telescope located at the McDonald Observatory. It combines a number of features that differentiate it from most telescope designs, resulting in greatly lowered construction costs...
  
  
• The Southern African Large Telescope
  Southern African Large Telescope
  The Southern African Large Telescope is a 66m2 area optical telescope with a nominally 9.2 meter aperture but up to about 11.1m x ~9.8 m diameter aperture, and designed mainly for spectroscopy. It is located close to the town of Sutherland in the semi-desert region of the Karoo, South Africa...
  
  
• The Gran Telescopio Canarias
  Gran Telescopio Canarias
  The Gran Telescopio Canarias , also known as GranTeCan or GTC, is a reflecting telescope undertaking commissioning observations at the Roque de los Muchachos Observatory on the island of La Palma, in the Canary Islands of Spain, as of July 2009.Construction of the telescope, sited on a volcanic...
  
   (GTC) 36 segmented mirror

The James Webb Space Telescope

James Webb Space Telescope

The James Webb Space Telescope , previously known as Next Generation Space Telescope , is a planned next-generation space telescope, optimized for observations in the infrared. The main technical features are a large and very cold 6.5 meter diameter mirror, an observing position far from Earth,...

's 18 mirror segments have mostly been fabricated as of spring 2011 but the spacecraft is not expected to be launched until 2017 or 2018.

Proposed segmented-mirror telescopes

These telescopes represent the next generation of segmented mirror telescope

Telescope

A telescope is an instrument that aids in the observation of remote objects by collecting electromagnetic radiation . The first known practical telescopes were invented in the Netherlands at the beginning of the 1600s , using glass lenses...

s; they are currently at least a decade from completion.

• TMT, the North American Thirty meter telescope
  Thirty meter telescope
  The Thirty Metre Telescope is a proposed ground-based large segmented mirror reflecting telescope to be built on Mauna Kea in Hawaii. The telescope is designed for observations from the near-ultraviolet to the mid-infrared . An adaptive optics system would correct for image blur caused by the...
  
  
• OWL, the European Overwhelmingly Large Telescope
  Overwhelmingly Large Telescope
  The Overwhelmingly Large Telescope is a conceptual design by the European Southern Observatory organization for an extremely large telescope, which was intended to have a single aperture of 100 meters in diameter...
  
  
• E-ELT, European Extremely Large Telescope
• The Giant Magellan Telescope
  Giant Magellan Telescope
  The Giant Magellan Telescope is a ground-based extremely large telescope planned for completion in 2018. It will consist of seven diameter primary segments, with the resolving power of a primary mirror and collecting area equivalent to one...
  
  , uses very large segments and is either grouped with segmented mirrors telescopes or its own category.