Computer Science – Performance
Scientific paper
Jul 2008
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008hst..prop11920d&link_type=abstract
HST Proposal ID #11920. Cycle 17
Computer Science
Performance
Scientific paper
The IR imaging performance over the detector will be assessed periodically {every 4 months} in two passbands to check for image stability. The field around star 58 in the open cluster NGC188 is the chosen target because it is sufficiently dense to provide good sampling over the FOV while providing enough isolated stars to permit accurate PSF {point spread function} measurement. It is available year-round and used previously for ACS image quality assessment. The field is astrometric, and astrometric guide stars will be used, so that the plate scale and image orientation may also be determined if necessary {as in SMOV proposals 11437 and 11443}. Full frame images will be obtained at each of 4 POSTARG offset positions designed to improve sampling over the detector in F098M, F105W, and F160W. The PSFs will be sampled at 4 positions with subpixel shifts in filters F164N and F127M. This proposal is a periodic repeat {once every 4 months} of the visits in SMOV proposal 11437 {activity ID WFC3-24}. The data will be analyzed using the code and techniques described in ISR WFC3 2008-41 {Hartig}. Profiles of encircled energy will be monitored and presented in an ISR. If an update to the SIAF is needed, {V2,V3} locations of stars will be obtained from the Flight Ops Sensors and Calibrations group at GSFC, the {V2,V3} of the reference pixel and the orientation of the detector will be determined by the WFC3 group, and the Telescopes group will update and deliver the SIAF to the PRDB branch. The specific PSF metrics to be examined are encircled energy for aperture diameter 0.25, 0.37, and 0.60 arcsec, FWHM, and sharpness. {See ISR WFC3 2008-41 tables 2 and 3 and preceding text.} 20 stars distributed over the detector will be measured in each exposure for each filter. The mean, rms, and rms of the mean will be determined for each metric. The values determined from each of the 4 exposures per filter within a visit will be compared to each other to see to what extent they are affected by "breathing". Values will be compared from visit to visit, starting with the values obtained during SMOV after the fine alignment has been performed, to see if the measures of the compactness of the PSF indicate degradation over time. The analysis will be repeated for stars on the inner part of the detector and stars on the outer part of the detector to check for differential degradation of the PSF. As an example of the analysis, one can examine the sharpness of the F160W PSF exposures made during thermal vacuum testing {ISR WFC3 2008-41}. To compare two samples, one can define the PSFs on each half of the detector {lower and upper} as a sample {with 7 and 8 PSFs, respectively}. The mean, rms, and rms of the mean sharpness are 0.0826, 0.0067, and 0.0027 for one half, and 0.0773, 0.0049, and 0.0019 for the other. The difference of the means is 0.0053 and the statistical error in that difference is 0.0033, so the difference is not significant.;
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