16473

Recently ACS-WFC geometric distortion have been assessed w.r.t standard astrometric catalog significantly improved with the access to globally accurate positions from the Gaia DR2 as it discussed by Kozhurina-Platais (ACS-ISR-2018-02). A newly derived and improved IDCTAB have been also converted into HST V2V3 system with improved SIAF of ACS-WFC w.r.t Gaia DR2 (J. Sahlmann TEL-ISR-2019). This IDCTAB has been tested using the 47Tuc observations over 20 years with the access to globally accurate positions from the Gaia DR2. The results of test have shown the significant improvement in the global astrometric accuracy of ACS-WFC compare to ICRS based on Gaia now (S. Hoffman comma ACS_ISR-2020). The results of this test have also shown that around 2017 Xscale and Yskew (linear terms of distortion) in both WFC CCD chips suddenly changed introducing errors at the far edges of ACS-WFC images by <0.2 pixels. The galactic globular cluster 47Tuc (NGC104) 47Tuc has been used to examine the geometric distortion of ACS-WFC in multicycle calibration programs over last 20 years of ACS on HST board. Traditional ACS-WFC multi-cycle calibration program observing 47Tuc d_commaACS CCD Stability Monitord_comma consists of 4 HST orbits comma repeated observations in F606W and wide pass-band ACS-WFC filter 2 x times a year. This is too sparse observations to keep track on the time-depended scale and skew variations comma as indicated by their pattern since 2017. In order to secure an accurate calibration of these two indirect terms (Xscale and Yskew) comma we request additional 3 HST orbits well-spread over a year of Cyc 28 to observe 47 Tuc through F606W.