WFC/ACS Post-Flash Capability Now Available for Cycle 21
The WFC/ACS post-flash has been tested and a reference file has been created. The flash level is highly varied across both WFC CCDs, with a factor of two difference in signal level between the brightest part of the flash, and the darkest. The diagram below shows the level of flash across both CCDs. This image is in units of percentage, where 100% is the level of post-flash specified in APT and the ETC: 14 electrons/second. The image range is from 50-110%, and the green portion of the image shows the area where the flash reaches 95-105%. The pattern of the lamp is circular and slightly off center of the chip gap between the two CCDs. The direction of the variation is such that the post-flash is brightest far from the readout amplifiers, where the CTE trailing is worse. Unfortunately, this variation in the post-flash leads to different levels of Poisson noise in different areas of the CCDs. The added noise from the post-flash, and the success of the CTE correction already in place in CALACS results in a limited set of cases where post-flash may be helpful.
Post-flash is not recommended for most science observations. At present, post-flash is unnecessary in cases where the object is large or bright, as these objects are self-shielded from the worst effects of CTE trailing. Post-flash may be useful in cases of extremely faint unresolved/compact (less than ~10 pixels in size) targets imaged with short exposure times and/or narrow-band filters that would result in a background < 20 electrons. The ETC has now been modified to include a post-flash exposure, and will print a warning in the cases where post-flash is recommended. Unfortunately, the ETC does not account for CTE, so the user must manually adjust the brightness of their source in order to obtain the effective count rate. For point sources, this adjustment can be estimated using the equations from Chiaberge ISR 2012-05) which have been incorporated into the ACS/WFC CTE Correction Calculator. More information on CTE trailing in ACS and observation recommendations are available in the CTE White Paper and Ogaz, et. al, 2013 (in prep). In future cycles, when CTE degradation will be even more dramatic, post-flash might become useful in other cases. The ACS team will continue to monitor the CTE losses and the efficiency of the post-flash in mitigating CTE effects with a series of dedicated calibration programs. We encourage the users to periodically check the ACS website for updates.