page_title: James Webb Space Telescope
section_title: Optics
hierarchical_section_title: James Webb Space Telescope / Features / Optics
caption_attribution_description: A rare view of the James Webb Space Telescope face-on, from the NASA Goddard cleanroom observation window.
context_page_description: The James Webb Space Telescope is a space telescope that is planned to succeed the Hubble Space Telescope as NASA's flagship astrophysics mission. The JWST will provide improved infrared resolution and sensitivity over Hubble, and will enable a broad range of investigations across the fields of astronomy and cosmology, including observing some of the most distant events and objects in the universe, such as the formation of the first galaxies.
  The primary mirror of the JWST, the Optical Telescope Element, is composed of eighteen 1.32-metre hexagonal mirror segments made of gold-plated beryllium which combine to create a 6.5-metre diameter mirror that is considerably larger than the Hubble's 2.4-metre mirror. Unlike the Hubble, which observes in the near ultraviolet, visible, and near infrared spectra, the JWST will observe in a lower frequency range, from long-wavelength visible light through mid-infrared, which will allow it to observe high redshift objects that are too old and too distant for the Hubble to observe.
context_section_description: JWST's primary mirror is a 6.5-meter-diameter gold-coated beryllium reflector with a collecting area of 25.4 m². If it were built as a single large mirror, this would have been too large for existing launch vehicles. The mirror is therefore composed of 18 hexagonal segments which will unfold after the telescope is launched. Image plane wavefront sensing through phase retrieval will be used to position the mirror segments in the correct location using very precise micro-motors. Subsequent to this initial configuration they will only need occasional updates every few days to retain optimal focus. This is unlike terrestrial telescopes, for example the Keck telescopes, which continually adjust their mirror segments using active optics to overcome the effects of gravitational and wind loading. The Webb telescope will use 126 small motors to occasionally adjust the optics as there is a lack of environmental disturbances of a telescope in space.
  JWST's optical design is a three-mirror anastigmat, which makes use of curved secondary and tertiary mirrors to deliver images that are free of optical aberrations over a wide field. In addition, there is a fast steering mirror which can adjust its position many times per second to provide image stabilization.
  Ball Aerospace & Technologies Corp. is the principal optical subcontractor for the JWST project, led by prime contractor Northrop Grumman Aerospace Systems, under a contract from the NASA Goddard Space Flight Center, in Greenbelt, Maryland. Eighteen primary mirror segments, secondary, tertiary and fine steering mirrors, plus flight spares have been fabricated and polished by Ball Aerospace & Technologies based on beryllium segment blanks manufactured by several companies including Axsys, Brush Wellman, and Tinsley Laboratories.
  The final segment of the primary mirror was installed on 3 February 2016, and the secondary mirror was installed on 3 March 2016.
caption: Main mirror assembled at Goddard Space Flight Center, May 2016.
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