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17 May 2007
Lightweight cryogenic mirrors and a wavefront sensing system are just two of the ten new technologies that NASA will deploy on the James Webb Space Telescope.
More than a year ahead of schedule, a team of independent experts has approved all 10 new technologies developed for NASA's James Webb Space Telescope (JWST). Many of the technologies have never before been used on any satellite or space telescope such as lightweight cryogenic mirrors and a wavefront sensing and control (WFS&C) system.
Scheduled to launch in 2013, the JWST will be a 6.6 m infrared-optimized space telescope with wide wavelength coverage capable of performing both imaging and spectroscopy. The mirror will be over 2.5 times larger than the diameter of the Hubble Space Telescope's and will allow scientists to peer back to when the first galaxies formed just after the Big Bang.
The large telescope consists of 18 beryllium mirror segments; each segment is 1.3 m in diameter which when combined form a total area of 25 m2. The segments will be brought into optical alignment on-orbit through a process of periodic WFS&C. This aligns the segments so that their wavefronts match, creating a diffraction-limited 6.6 m telescope, rather than overlapping images from 18 individual 1.3 m telescopes. The primary mirror then performs like a single monolithic mirror with an effective f/number of 20 and focal length of 131.4 m over a wavelength ranging 0.6–29 µm. The telescope has a Strehl ratio of approximately 0.8 at a wavelength of 2 µm.
"This is the first time a large segmented telescope is being launched into space where it is desirable to align the entire telescope from images obtained by the main science camera without requiring a dedicated wavefront sensor," said Lee Feinberg, JWST Telescope Manager from NASA, told optics.org. "There are ground-based segmented telescopes like Keck that have dedicated wavefront sensor hardware but this hardware would be too large and heavy to be practical for a lightweight space telescope."
The WFS&C process uses a set of algorithms and software programs to determine the optimum position of each of the telescope mirrors. The positions of individual mirrors will be adjusted as necessary, causing them to function as one very sensitive telescope.
"The key advantage of this technology is that we were able to avoid a dedicated wavefront sensor that would have consumed a lot of mass and volume. This saving allowed for four very powerful science instruments and a 25 square meter primary mirror," said Feinberg.
According to Feinberg, the biggest challenge was to evaluate the technology on a testbed that was a 1⁄6th scale model of the full telescope and correctly simulate space-like conditions.
"We hope to speed up the turnaround time and minimize the amount of iterations it takes to get a fully aligned telescope. Future space telescopes may be able to incorporate similar capabilities as JWST but entirely on-orbit without the ground being in the loop," concluded Feinberg.
The next milestone for NASA scientists is the Mission Preliminary Design Review and the associated Non-Advocate review, scheduled for March 2008. "At that time we will seek to convince a panel of independent experts that we have an executable and affordable plan for the implementation phase of the program," John Decker, NASA's Deputy Associate Director of the JWST Project, told optics.org.
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