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Calibration of ACS/WFC Absolute Scale and Rotation for Use in Creation of a JWST Astrometric Reference Field

1. ABSTRACT Astrometric calibrations of JWST will use observations of a reference field in the Large Magellanic Cloud. This field will itself be astrometrically calibrated using observations with ACS/WFC on HST. The higher-order geometric distortions of ACS/WFC have previously been calibrated to high accuracy. However, the absolute scale and orientation of ACS/WFC are not known with similar accuracy. We therefore performed observations of the open cluster M35 to calibrate the absolute scale and orientation of ACS/WFC. Astrometric positions for stars in this field are obtained by combining a catalog obtained from many years of HST/FGS observations with the astrometry in the UCAC2 catalog. We also use several years of repeated observations of 47 Tuc to demonstrate the stability with time of the ACS/WFC scale and rotation. The understanding of the ACS/WFC scale and rotation obtained through these analyses is sufficiently accurate to meet the JWST astrometric requirements.