Computer Science – Performance
Scientific paper
Jan 2009
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2009aas...21342614b&link_type=abstract
American Astronomical Society, AAS Meeting #213, #426.14; Bulletin of the American Astronomical Society, Vol. 41, p.256
Computer Science
Performance
Scientific paper
The telescope of the James Webb Space Telescope (JWST) is an f/20, three mirror anastigmat design. To fit within the launch vehicle envelope (Arianne V), the 6.6 meter primary mirror and the secondary mirror support structure are folded for launch and deployed in space. The primary mirror is composed of 18 individual, 1.3 meter hexagonal segments, each having seven degrees of freedom (six rigid body and radius of curvature) provided by a set of precision actuators. The actuated secondary mirror can be similarly positioned in six degrees of rigid body motion. To achieve diffraction limited performance at two microns, each primary mirror segment and the secondary mirror must be precisely positioned and aligned following deployment. A process of wavefront sensing and control (WFSC) has been developed to adjust the telescope components, progressively reducing the observatory wavefront error. The latter steps of this process use a set of defocused images from the NIRCam camera for phase retrieval. Data obtained at a single field point can result in ambiguity in the primary figure and secondary alignment configuration, producing a highly corrected system over a limited field of view. This ambiguity can be removed, producing optimal correction over the entire field of view by obtaining wavefront information at larger field angles. A simple, linear sensing algorithm has been developed to determine the dominant ambiguity errors (focal plane tilt and field dependent astigmatism) and has been applied both in simulations and laboratory tests. In order to sense wavefront errors over the full field of view of the four JWST science instruments, various novel techniques have been developed, which permit wavefront sensing in these instruments even though they were not originally designed for this purpose (typically undersampled and broadband). We here review the WFSC process and status, and outline the multi-instrument, multi-field alignment approach.
Acton Daniel S.
Bowers Charles W.
Contos Adam R.
Dean Brian
Hayden William
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