16871

This program has been put togehter quickly to be inserted into the November 2021 schedule while ACS is the only instrument in operation. Originally comma we had planned to use Omega Cen for this comma but it is not visibile in this special window of opportunity comma so I searched for appropriate fields that could be used and found this globular cluster. The goal of this program is to calibrate table-based CTE corrections for astrometry. Photometry comes along for the ride comma but we already have corrections for that from Marco Chiaberge.s long-term effort comma so the focus here is astrometry. The photometric component of this will just be a useful cross-check. The reason that the standard CTE program can.t measure astrometry is because we have to measure the shift of stars across the gap. In the standard program comma full-chip offsets are used that map bottom-chip to top-chip. We can measure relative astrometric shifts this way comma but not absolute shifts. We need to have one reference exposure that d_commastraddles the gapd_comma to do this. This program remedies that. We will explore 8 different background levelsdoublePoint 10e (natural for this field-filter) comma 20e comma 30e comma 40e comma 60e comma 80e comma 100e comma and 100e. The 40 background exposure will be the d_commareferenced_comma exposures. The result of analyzing these data will be a 2-dimensional table of corrections for photometry that will allow users and the hst1pass routine to interpolate the astrometric shift as a function of instrumental magnitude and background. We will put together an ISR describing the analysis that went into the construction of the table.