||HST/WFC3: Understanding and Mitigating Radiation Damage Effects in the CCD Detectors
||495, Astronomical Data Analysis Software and Systems XXIV (ADASS XXIV)
||Baggett, S.; Anderson, J.; Sosey, M.; MacKenty, J.; Gosmeyer, C.; Noeske, K.; Gunning, H.; Bourque, M.
||At the heart of the Hubble Space Telescope Wide Field Camera 3
(HST/WFC3) UVIS channel resides a 4096x4096 pixel e2v CCD array. While
these detectors are performing extremely well after more than 5 years
in low-earth orbit, the cumulative effects of radiation damage cause a
continual growth in the hot pixel population and a progressive loss in
charge transfer efficiency (CTE) over time. The decline in CTE has two
effects: (1) it reduces the detected source flux as the defects trap
charge during readout and (2) it systematically shifts source centroids
as the trapped charge is later released. The flux losses can be
significant, particularly for faint sources in low background images.
Several mitigation options exist, including target placement within the
field of view, empirical stellar photometric corrections, post-flash
mode and an empirical pixel-based CTE correction. The application of a
post-flash has been remarkably effective in WFC3 at reducing CTE losses
in low background images for a relatively small noise penalty.
Currently all WFC3 observers are encouraged to post-flash images with
low backgrounds. Another powerful option in mitigating CTE losses is
the pixel-based CTE correction. Analagous to the CTE correction
software currently in use in the HST Advanced Camera for Surveys (ACS)
pipeline, the algorithm employs an empirical
observationally-constrained model of how much charge is captured and
released in order to reconstruct the image. Applied to images (with or
without post-flash) after they are acquired, the software is currently
available as a standalone routine. The correction will be incorporated
into the standard WFC3 calibration pipeline.