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NASA's James Webb Space Telescope:

Optics

The James Webb Space Telescope (JWST ) is a 6.5-meter-aper ture obser vator y optimized for infrared astronomy (0.6 - 28 m).

Science Themes:
Discover the First Stars and Follow the Universe's Ionization Histor y Track the Assembly and Evolution of Gala xies Across Cosmic Time Study the Bir th of Stars and Protoplanetar y Systems Measure Planetar y Atmospheres and Study the Origins of Life

JWST is P a s si v e w here a E ar th an

the first infrared space telescope to employ passive cooling to achieve its operating cryogenic temperature of 40 K. cooling is achieved by loc ating the telescope beyond the orbit of the Moon at the S econd L agrange Point, sunshield consisting of a deployable array of K apton membranes c an block the infrared radiation of the Sun, d M o o n.

Optical Systems
JWST's telescope features a 3-mirror anastigmat optimized for wide fieldof-view imaging. The telescope's optical train feeds a fine steering mirror that compensates for spacecraf t jit ter. The ter tiar y and fine steering mirrors are mounted on the af t-optical system bench, which also ser ves as a stray light baf fle. The primar y and secondar y mirrors are an active optical system. Each primar y mirror segment is mounted on an actuator array that provides six degrees of freedom motion, and an additional mirror radius of cur vature adjustment. The secondar y mirror is also mounted with actuators to provide six degrees of freedom. JWST has a segmented mirror architecture that facilitates folding and stowing of the primar y mirror for launch.

Fine Steering Mirror

Ter tiar y Mirror

Secondar y Mirror

Primar y Mirror Segments


Building the Mirrors

Ambient SFE

Cryogenic SFE at 40 K
JWST mirrors are made of ber yllium, selected for its stif fness, light weight and cr yogenic stabilit y. The mirror billets are generated from ber yllium powder using a hot, isostatic press process. Each billet is sawed in half to yield t wo mirror blanks. Each blank is machined to light weight the mirror in a process that removes ~92% of its mass. The mirrors are then polished by SSG-Tinsley in Richmond, CA. JWST's mirrors must operate at cr yogenic temperature (40 K ), but they are polished at room temperature. When the polishing reaches a sur face figure error (SFE) of ~100 nm, the mirrors are shipped to the X-Ray Calibration Facilit y (XRCF), where their cr yogenic SFE is measured. This SFE is then used to generate requirements for the final polishing phase, so the mirrors will meet their 17 nm SFE at cr yogenic temperature. Each mirror is then gold-coated to provide ma ximum infrared reflectivit y. Finally, the mirrors undergo threea xis sine-vibration testing followed by a final cr yo-optical measurement of the mirror's SFE.

View of mirror rear sur faces showing hexagonal wall structure af ter lightweighting. Mirrors are shown to same relative scale.

The JWST project has completed polishing its flight mirrors. All of the JWST flight mirrors, including the 18 primar y segments, the secondar y, ter tiar y, and fine steering mirror, have completed polishing and gold coating. The JWST flight segments have all completed final cr yo-optical acceptance testing at the end of 2011. Deliver y of the first JWST science instruments to the Goddard Space Flight Center will occur in the summer of 2012.

Follow the Webb Space Telescope at WebbTelescope.org and jwst.nasa.gov

Operated for NASA by AURA

AC SP E TELESCOPE SCIENCE INSTITUTE

I m a g e s c o ur t e s y of N A S A