Mathematics – Logic
Scientific paper
Jul 2001
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2001hst..prop.8983s&link_type=abstract
HST Proposal ID #8983
Mathematics
Logic
Hst Proposal Id #8983
Scientific paper
NICMOS coronagraphy is extremely sensitive to small centering errors which can result from imperfections in the target acquisition {TA} process. During HST Cycle 7 the on-board {TA} process was augmented which resulted in post-acquisition dispersions of target placements w.r.t. the fiducial position in the occulting system with 1-sigma RSX dispersions of 0.08 mas {or 1/10 pixel}. This level of acquisition precision is required to enable high-contrast imaging near occulted targets of which the system is capable. With the elevated detector temperatures {relative to Cycle 7} which we anticipate in Cycle 11, the change in detector characteristics, if incorrectly compensated in on-board mode-2 image processing, would result in degraded TA accuracy, and ultimately in coronagraphic gain. In addition, we posit the susceptibility of the system to metrological changes with further dewar stress/relaxation which could alter TA performance. This test will evaluate the TA performance, following a number of apriori changes to "tunable constants" in the FSW data, which we anticipate would result in performance comparable to that seen in Cycle 7. This test will verify the operability of the TA FSW, the precision of the "hole finding" algorithm, the dynamic range {exposure time} requirements for acquiring coronagraphic targets to high precision, the astrometric calibration of the TA process {image scales and rotation}, the stability of sub-orbital and multi-orbital acquisitions. We will specifically re-evaluate the efficacy of the Target Location, Image Centration, and Coordinate Transformation algorithms in the on-board S/W working with contemporaneously acquired TA mode imaging, and "hole finding" reference flats. Flight S/W data table updates derived from the pre and post target acquisition images and FGS data may follow the execution of this test as noted for Cycle 7 in SMO-2021/2.13.9. We will also explore the operating characteristics of the target acquisition process in the regime of a low S/N {underexposed} targets and saturated targets.
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