WFC L - Flats Post Cooldown

Following recovery of ACS with Side 2 electronics the temperature setpoint was lowered from a nominal value of-77 C to-81 C for the WFC in order to minimize impacts of the continuously growing hot pixel contamination. The CCDs are expected to have changes of 1-3% in overall sensitivity, with some spatial dependence (Sirianni et al. 2006) – the latter is calibrated via changes to the L-flats. We find that required L-flat changes range from about 0.6% peak-to-peak at F435W to 0.15% at F814W. Separate ISRs will review evidence for possible pixel-to-pixel flat field and overall sensitivity changes post cooldown. L-flat changes for F435W, F625W and F814W are derived by comparison of internal tungsten exposures taken before and after cooldown. Flat field updates for other filters within this wavelength range follow from linear interpolation. New LP-flats with a separate useafter date have been delivered.