James Webb Space Telescope optical technology milestones achieved
Three optical technology development milestones were recently reached for NASA's James Webb Space Telescope (JWST).1,2 Successor to the celebrated Hubble Space Telescope, JWST is a 6.6m infrared telescope with a planned launch in 2013. JWST is designed to produce images in the wavelength range 1.7–28µm. To ensure that thermal infrared radiation from the mirrors will be no more than a minor source of photon noise, the telescope will be passively cooled to 50K (-223°C). The 25m2 primary mirror consists of 18 hexagonal beryllium segments supported on a composite backplane structure. Wavefront sensing and control (WFS&C) algorithms will be used to align the mirror segments on-orbit to a fraction of a wavelength (less than 0.1µm). In flight the mirror alignments will be adjusted every 14 days and held stable over that time period by the backplane. All three of these technologies—the mirror segments, WFS&C, and primary mirror backplane—have been tested and shown to satisfy NASA's criteria for Technology Readiness Level 6, a prerequisite for a mission's passage into the development phase.
Mirror technology development for JWST began in 1996 with the joint NASA-US Department of Defense Advanced Mirror Systems Demonstrator project. Five lightweight mirror architectures were studied under the project, and two mirrors were selected for fabrication and testing, one made of glass and another made of beryllium. Beryllium was chosen for JWST by a panel of NASA and contractor team experts because it is thermally conductive and has a coefficient of thermal expansion at 50K superior to that of glass (glass is better behaved at room temperature). An Engineering Development Unit (EDU) was used to demonstrate the mirror's ability to survive under flight-like conditions. The EDU is built to the flight design and consists of a 1.32m lightweight beryllium mirror in a six-degree-of-freedom hexapod mount with a radius of curvature actuation system (see Figure 1). The EDU was subjected to vibro-acoustic levels enveloping the three-axis loads we can expect it to encounter in flight. Precise interferometric measurements indicated that the mirror survived the vibro-acoustic tests with no detectable distortion.
Previous experience indicated that mirror fabrication could delay development of the entire observatory. With 18 beryllium mirrors to prepare for flight, the JWST mirror fabrication and polishing process was started in 2004. A major milestone was achieved in the past month with the completed lightweight machining of the 18th mirror blank at Axsys Technologies in Cullman, AL. The lightweighted mirror blanks have been shipped to L3 Communications SSG-Tinsley in Richmond, CA, where they are being ground and polished. Grinding of the EDU mirror is nearly complete, and the test mirror will soon enter the polishing phase. Ten flight mirrors are in various stages of grinding.
Performance verification of the JWST WFS&C subsystem represents the second major optical technology accomplishment. WFS&C algorithms are implemented in software to align the 18 primary mirror segments and the six-degree-of-freedom secondary mirror. The algorithms combine the following elements: phase retrieval algorithms originally developed to determine the optical prescription of the spherically aberrated Hubble Space Telescope primary mirror to correct defocused science images; and coarse phasing algorithms based on dispersed star images used in the alignment of the 36-segment Keck telescope. A three-segment testbed at NASA's Goddard Space Flight Center in Greenbelt, MD, was used in an early demonstration of WFS&C algorithm performance, but the testbed provided fewer degrees of freedom than the flight mirrors. A coarse phasing demonstration was performed on the Keck telescope's inner 18 segments in 2005.
In 2006, Ball Aerospace completed a 1/6th scale WFS&C testbed (see Figure 2) and demonstrated the performance and functionality of the JWST WFS&C flight algorithms in a system traceable to flight. The final performance tests, which stepped through the entire sequence from initial on-orbit errors to final alignment, were finished in December 2006. Three critical measurements were made. First, the final WFS&C ‘phasing’ was compared with that of a calibrated interferometer, the industry standard in measuring optical systems. Second, the alignment algorithms were evaluated for repeatability. Third, the WFS&C algorithm's capability to yield sharp images over the entire large field of view of the telescope was demonstrated. Excellent image quality is critical to the four JWST instruments that will share the field of view. The WFS&C test results were so good that they were limited only by the testbed and not by the flight algorithms.
Finally, the JWST project has completed tests of the Backplane Stability Test Article. The JWST primary mirror backplane was designed to preserve mirror segment alignment during telescope slews and between biweekly alignment adjustments. The Backplane Stability Test Article is a 1/6th section test article representative of the flight backplane (see Figure 3). It contains three full-scale primary mirror bays and measures 2.5×2.8m. The Backplane Stability Test Article was tested at cryogenic temperatures in the X-ray Calibration Facility chamber at NASA's Marshall Space Flight Center in Huntsville, AL, from August to October 2006. The test simulated the most extreme hot-to-cold operational environment changes the structure will experience in space by cycling multiple times through the backplane operational temperature range from 30 to 60K. Structure deformations were measured as a function of temperature. The test data, taken with an Electronic Speckle Pattern Interferometer developed by 4D Technologies of Tucson, AZ, specifically for this application, was compared with detailed finite element models of the thermal deformations. The model was found to predict the true performance to the desired accuracy. This test confirmed that the backplane design and manufacturing process will yield a large cryogenic structure that will satisfy the challenging JWST thermal stability requirements.
In addition to the three optical technologies described above, six additional JWST technologies also matured to Technology Readiness Level 6. Cryocooler technology development is still under way for the mid-infrared detector arrays on JWST. The cryocooler was a late addition to the technology program, but its development is on track to reach Technology Readiness Level 6 in the coming months. At that point the JWST project will transition to its development phase, during which the new technologies will be flight qualified.
