Webb Update

Massimo Stiavelli, mstiavel@stsci.edu

Developing the James Webb Space Telescope is now in full swing. Components representing 14% of the mass of the observatory have been fabricated, and other parts for 47% of the mass are under construction. The remaining 39% of the mass will be completed over the next two years. Important components of the observatory—and, indeed, the mission itself—recently passed major reviews. A test of figuring primary-mirror segments was a success. With these milestones accomplished, the phase of designing and analyzing Webb winds down, and the era of integrating and testing ramps up.

In February 2010, the demonstration model of the Near Infrared Spectrograph was delivered to the Goddard Space Flight Center to begin testing. The large clean room used for work on Hubble servicing missions is now being used for Webb. NASA has installed a “Webb cam” in the clean room to provide continuous coverage of the assembly of the Webb instruments and telescope (view it here).

Passed Reviews

In October 2009, Webb’s Optical Telescope Element (OTE) passed its critical design review (CDR). The OTE comprises the primary, secondary, tertiary, and fine steering mirrors.

In January 2010, the Webb sunshield passed its CDR. The sunshield is a critical component of the observatory, as it enables the telescope optics and instruments to radiatively cool down to 40 K. At launch, the sunshield will be folded, but will deploy to its full tennis-court size en route to L2. NASA had delayed manufacturing the sunshield so the design team could work through several prototypes before selecting the design that offered the greatest deployment reliability and protection during integration and launch.

In April 2010, Webb passed its mission CDR, which was held at Northrop Grumman’s Redondo Beach facility near Los Angeles. Over the course of several days, 1,854 viewgraphs were presented by the leads of all subsystems. The final conclusion was that Webb‘s development is on track, despite its complexity.

Verification of Figuring Method

Ensuring that the primary mirror segments have the correct figure when cooled to the 40 K operating temperature is a major challenge. The segments are polished at room temperature to a shape that is not the desired flight shape, but the mirrors will deform to the correct shape when cooled to flight temperatures. This strategy requires extensive numerical simulations and iterative polishing and testing. In the final stages, the mirror segments circulate around the country: First, they are figured, polished, and measured at Tinsley Laboratories in California. Next, the segments travel to Colorado for the support structures to be installed at Ball Aerospace Technology Corporation. Next, the units travel to Alabama for optical measurements at cryogenic temperatures at the X-Ray and Cryogenic Facility (XRCF) at the Marshall Space Flight Center. Then the cycle is repeated, with further polishing at Tinsley, remounting at Ball, and retesting at Marshall, until the required optical figure is achieved.

As of January 2010, this complex process of figuring Webb’s primary mirror segments was tested and verified, when a flight spare successfully completed this polishing and testing cycle. Many lessons were learned on this pathfinder program, which should substantially reduce the polishing andtesting times for the remaining segments. Many of the segments are ready for testing at the XRCF.