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Space Telescope Science Institute Wfc3/uvis Bowtie Monitor

We present the results of the WFC3/UVIS bowtie monitoring program based on data acquired during SMOV/Cycle 17 through current observations of Cycle 20, spanning all on-orbit data (June 2009-June 2013). The bowtie program serves as a periodic monitor for intermittent low-level (up to ∼4%) detector hysteresis, i.e. a QE deficit across each UVIS CCD chip. A set of three internal flat fields is used to probe for hysteresis, neutralize any QE deficits via a saturated QE 'pinning' exposure, and assess the effectiveness of the hysteresis mitigation. Image ratios involving the flat fields before and after the saturating exposure indicate that hysteresis features (1) are detectable only in the first bowtie visit after UVIS anneal procedures wherein the detector is warmed then cooled back down to operating temperature and (2) are effectively suppressed at all times, ensuring that science observations are not compromised by QE offsets. For the first time on-orbit, a bowtie-shaped hysteresis pattern, for which the program is named, was observed following the August 2012 UVIS anneal; to date, this incident has been a one-time anomalous event on-orbit. A by-product of the bowtie monitoring data is the discovery of a slow decrease in flux level over time, currently about ∼0.19±0.01%/yr. Assuming the decline is due solely to the aging of the lamp, the lamp output is currently at ∼98% of its original level.