Pixel-to-pixel Flat Field Changes on the WFC

The pixel-to-pixel flat field changes noted by Bohlin and Mack (2005) for the WFC are further quantified. During each period between anneals, a population of pixels with lowered sensitivity develops which is largely reset by the next anneal. The sensitivity deficits are twice as large in the blue as in the red. The pixels with lowered sensitivity appear to be a unique set each anneal cycle, rather than a subset that ‘telegraph’ on and off. The low QE pixels recover ∼90% of their losses on a time scale of a few monthly anneals, but never return fully. Some evidence for spontaneous recovery of the low QE pixels between anneal cycles is developed, but is not conclusive. The number of low pixels would become a large source of error in the absence of performing anneals on a frequent basis. Prior to cooldown in July 2006, the flat field changes that arise continuously within anneal cycles are larger than cumulative persistent changes in the pixel-to-pixel flats. The pre-cooldown reference flat field remained excellent. Post-cooldown, the number of persistent deviant pixels, although still modest in number, may have reached a level justifying delivery of new pixel-to-pixel flats, although providing such will require acquisition of further data.