17002

The UVIS detector occasionally exhibits low-level (virgul1%) quantum efficiency offsets (hysteresis) across both chips. This is known as the .bowtie. effect comma due to the unique pattern that results in the image ratios and was revealed during thermal vacuum testing. Ground tests found that this hysteresis could be negated by overexposing the detector to several times the full well amount. In order to detect and mitigate UVIS hysteresis on orbit comma a multi-cycle .bowtie monitoring. program was created. Similar to past bowtie monitoring programs comma each visit of this program acquires a set of three 3x3 binned internal tungsten flatfields. The first image is unsaturated in order to identify hysteresis comma the second image is saturated to neutralize any quantum efficiency offsets comma and the third image is unsaturated to provide an estimate of how well the hysteresis was removed.