Optical Telescope Element

Optical Telescope Element (OTE) is a sub-section of the James Webb Space Telescope, a large infrared space telescope scheduled to be launched in October 2018. 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 (holds instruments) and the Spacecraft Element (SE), 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 main mirror system has an area of 25 square mirrors; it is a 3-mirror f/20 Anastigmatic design. This yields an effective f/number of f/16.67 and focal length of 131.4 meters. The main three-mirror telescope is a Korsch-type design, and it feeds into the Aft Optics Subsystem (part of OTE), which in turn feeds into the Integrated Science Instrument Module which holds the science instruments and fine guidance sensor.

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 course 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 to ensure it will work is required. 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 mirrors 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 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.

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