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Overview

The Hubble Source Catalog (HSC) combines more than 64,000 visit-based source lists derived from images from the Hubble Advanced Products (HAP) Single-Visit Mosaics and the Hubble Legacy Archive (HLA) into a single master catalog. The astrometric accuracy of the input source lists is highly variable.  The HAP products are often calibrated using Gaia DR3, which leads to good accuracy in the positions.  The older HLA source lists used for WFPC2 preceded the Gaia mission and so have lower accuracy positions.  The positions of the instrument apertures in the focal plane change with time, and the accuracy with which those positions are calibrated also changes.  The cameras have significant geometric distortions that also change over time.  The Guide Star positions have uncertainties that affect the pointing accuracy; those errors have been as large as an arcsecond in the past, but the current HST guide star catalog is based on Gaia and has much smaller errors.  Even with perfect guide star positions, occasionally the HST fine guidance system locks onto the wrong guide star, which can lead to very large errors in the coordinates (up to 90 arcsec, which is a large fraction of the field of view for the cameras).

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