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The Webb Update #7 - November 2009

Welcome to the seventh issue of the Webb Update, a newsletter to update the community about the James Webb Space Telescope.  Webb will be the next flagship astrophysics mission for NASA and is planned for launch in 2014.  A text version of this newsletter is emailed to a subscriber list when it is released. If you would like to receive the email newsletter, please visit our main Newsletter page for information on how to subscribe.

In this newsletter:

Optical Telescope Element Critial Design Review Successfully Completed
by Lee Feinberg

The Optical Telescope (OTE) Element Critical Design Review (CDR) was completed successfully on October 5th-9th at Northrop Grumman in Redondo Beach, California. The review board concluded that the CDR was passed and identified 3 liens that will be closed out by the Mission CDR. Leading up to the CDR, many OTE subsystem CDRs were completed over the summer. This included reviews of the deployment systems for the primary mirror and secondary mirror and the large composite backplane that holds the primary mirror segments. The OTE CDR comes as the primary mirror segment engineering design unit nears its very final polishing. Before the end of this calendar year, the mirror will be shipped for a second cryogenic test to assure the polishing of mirror deformations has been successful and then it will go on to coating. The OTE team is now focused on supporting the mission CDR and fabrication and assembly of the OTE.

One-Third Scale Sunshield Testing
by Mark Clampin

The James Webb Space Telescope’s sunshield is designed to shield the telescope from the sun, and allow the telescope to cool to its 50K operating temperature (Figure 1). The sunshield assembly is built from five large Kapton membranes (similar to the Mylar used in party balloons.) Think of five candy wrappers, each the size of a tennis court. To understand how the sunshield will perform in its task of cooling the telescope, Northrop Grumman will perform a thermal test of a one-third scale sunshield assembly this month. One of the five membranes to be used in this test is shown in Figure 2. The test will allow the sophisticated models that predict JWST’s temperature profile to be compared to real test data. This represents one of the last schedule milestones to be completed prior to the Sunshield's Critical Design Review in January 2010.

Sunshield Sunshield

Integrated Science Instrument Module Structure Arrives at GSFC
by Matt Greenhouse

The Integrated Science Instrument Module (ISIM) is the science instrument payload of the JWST. It is a distributed system with subsystems located throughout the observatory. Its cryogenic portion consists of:

  • Four science instruments described at:,
  • A fine guidance sensor that is used to control telescope pointing during science observations,
  • An optical metering structure that supports the instruments,
  • A thermal transition harness system that connects the instruments to control electronics both on the cold side of the observatory and in the warm spacecraft,
  • A command and data handling computer system, and
  • Flight software.

For a technical description of the ISIM see: Greenhouse, M. A., et al., 2006, Proc. SPIE 6265, 33.

The ISIM structure is a high precision M55J-954-6 cynate ester composite material that affords a high stiffness to mass ratio and a low cryogenic expansion coefficient to yield high optical alignment stability with repeated thermal cycling to 40K and through the launch load environment to orbit. The JWST application is the first time that this composite material has been used as a cryogenic optical metering structure.

The flight model structure is now in Goddard’s large cleanroom for integration with secondary structure and kinematic mounts that interface the ISIM to the JWST telescope. Fit checks with Engineering Test Unit science instruments and other testing will occur at Goddard during 2010.

A press release about the arrival of the ISIM's structure at GSFC can be found at:; as well as a movie of the structure being placed into the GSFC clean room: