The Photometric Performance of WFC 3 / IR : Temporal Stability Through Year 1

We analyze multiple observations of four bright HST spectrophotometric standard stars obtained with WFC3/IR in order to establish the temporal stability of the instrument over its first 1.5 years of science operations. Observations of one or more of the four stars, P330E, GD 153, GD 71, and G191B2B were obtained at multiple dither positions near the center of the array, and in all 15 imaging filters. A number of different read out modes and subarray sizes were used to ensure that a very high signal-to-noise measurement of the star was obtained in each observation. The first data sets followed shortly after installation of WFC3 in HST in May 2009 and over two dozen epochs have been observed up to late 2010. Analysis of the over 1200 individual observations shows that point source photometry with WFC3/IR is temporally stable to more than 99%. The infrared (IR) channel of WFC3 uses a high quantum efficiency 1k x 1k HgCdTe array, with phenomenal sensitivity between 900 and 1700 nm (see e.g., Kalirai et al. 2009). The IR array offers several advantages over conventional CCD detectors, and similar systems will be used in top-priority missions by NASA and ESA over the next decade. Unlike CCD detectors, the accumulated signal over an exposure can be read out non-destructively multiple times, which reduces the effective read-out noise and provides a very high dynamic range to measure both very bright and very faint objects in the same exposure. For example, a 0.91 second exposure on WFC3/IR can be read out 15 times in the smallest subarray, with each read out being just 0.061. At the opposite end, the SPARS200 readout sequence provides non-destructive reads ranging from 2.9 seconds in the first sample all the way to 2802.9 seconds in the 15th sample. The non-destructive read out capabilities of IR arrays also provide very efficient cosmic-ray rejection by identifying the sample in which a given cosmic ray impacted the array and removing it