page_title: Optical Telescope Element
section_title: Overview
hierarchical_section_title: Optical Telescope Element / Overview
caption_attribution_description: Ball Aerospace Technician Robin Russell inspects the Webb Telescope Aft Optics Subsystem during mirror integration activities. The Aft Optics bench, made of lightweight beryllium like the mirrors, holds Webb's tertiary and fine steering mirrors. The installed, gold-coated tertiary mirror can be seen in the background. Credit: Ball Aerospace NASA image use policy. NASA Goddard Space Flight Center enables NASA’s mission through four scientific endeavors: Earth Science, Heliophysics, Solar System Exploration, and Astrophysics. Goddard plays a leading role in NASA’s accomplishments by contributing compelling scientific knowledge to advance the Agency’s mission. Follow us on Twitter Like us on Facebook Find us on Instagram
context_page_description: Optical Telescope Element is a sub-section of the James Webb Space Telescope, a large infrared space telescope scheduled to be launched early 2021. The OTE consists of some major parts of the telescopes including the main mirror, the secondary mirrors, the framework and controls to support those mirrors, and various thermal and other systems to support the functioning of the telescope. The other two major sections of the JWST are the Integrated Science Instrument Module and the Spacecraft Element, which includes the Spacecraft Bus and Sunshield. The OTE collects the light and sends it to the science instruments in the ISIM. The OTE has been compared to being the "eye" of the telescope and the backplane of it to being the "spine".
  The primary mirror is a tiled assembly of 18 hexagonal elements, each 1.32 meters from flat to flat. This combination yields an effective aperture of 6.5 meters and a total collecting surface of 27 square meters. Secondary mirrors complete the f/20 anastigmatic imaging optics. The complete system provides an effective f/number of f/16.67 and focal length of 131.4 meters.
context_section_description: The OTE combines a large amount of the optics and structural components of the James Webb Space Telescope, including the Main mirror. It also has the fine steering mirror, which, provides that final precise pointing, and it works in conjunction with the fine guidance sensor and other controls systems and sensors in the Spacecraft Bus.
  The main mirror segments are aligned roughly using a coarse phasing algorithm. Then for finer alignment, special optical devices inside NIRCam are used to conduct a phase retrieval technique, to achieve designed wavefront error of less than 150 nm. To function as focusing mirror correctly the 18 main mirror segments need to be aligned very closely to perform as one. This needs to be done in outer space, so extensive testing on Earth is required to ensure that it will work properly. To align each mirror segment, it is mounted to six actuators that can adjust that segment in 5 nm steps. One reason the mirror was divided into segments is that it cuts down on weight, because a mirror's weight is related to its size, which is also one of the reasons beryllium was chosen as the mirror material because of its low weight. Although in the essentially weightless environment of space the mirror will weigh hardly anything, it needs to be very stiff to maintain its shape. The Wavefront sensing and control sub-system is designed to make the 18 segment primary mirror behave as a monolithic (single-piece) mirror, and it does this in part by actively sensing and correcting for errors. There are nine distance alignment process that the telescope goes through to achieve this. Another important aspect to the adjustments is that the primary mirror backplane assembly is steady. The backplane assembly is made of graphite composite, invar, and titanium.
  The ADIR, Aft Deployable Infrared Radiator is a radiator behind the main mirror, that helps keep the telescope cool. There are two ADIR's and they are made of high-purity aluminum. There is a special black coating on the radiators that helps them emit heat into space.
  Some major parts of the OTE according to NASA:
  Primary mirror (18 segments)
  Secondary mirror (74 cm diameter)
  Tertiary mirror (3rd) (in Aft Optics Subsystem)
  Fine Steering Mirror (in Aft Optics Subsystem)
  Telescope structure
  primary mirror backplane assembly
  main backplane support fixture (BSF)
  secondary mirror support structure
  deployable tower array
  Thermal Management Subsystem
  Aft Deployable ISIM Radiator (ADIR)
  Wavefront sensing and control
  The  Aft Optics Subsystem includes the Tertiary mirror and the Fine Steering Mirror. One of the tasks for the Fine steering mirror is image stabilization.
  The metal Beryllium was chosen for a number of reasons including weight, but also for its low-temperature coefficient of thermal expansion compared to glass. Other infrared telescopes that have used beryllium mirrors include IRAS, COBE, and Spitzer. The  Subscale Beryllium Model Demonstrator (SBMD) was successfully tested at cryogenic temperatures, and one of the concerns was surface roughness at low kelvin numbers. The beryllium mirrors are coated with a very fine layer of Gold to reflect infrared light. There are 18 hexagonal segments that are grouped together to create a single mirror with an overall diameter  of 6.5 meters (650 cm, ~7.1 yards, ~256 inches).
caption: Testing of the Aft Optic Subsytem in 2011, which contains the Tertiary (3rd) mirror and Fine Steering Mirror
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