4.1 Introduction
A major feature of the AstroDrizzle software is in its handling of astrometric information. Calibrated HST images contain header keywords which describe several types of geometric distortion correction. Linear corrections (scale, rotation, and time-dependent skew) are incorporated in the keywords of the CD matrix. Non-linear corrections are populated as high-order polynomial coefficients using the Simple Image Polynomial (SIP) convention (Shupe, D. L., et al., 2005). These are populated in the image headers by the IDCTAB reference file. WFC3/IR data uses only this type of distortion reference file.
A third class of distortion, known as residual or non-polynomial distortion, cannot be expressed as functions. These corrections are applied in the form of look-up tables which are stored as additional FITS extensions in calibrated images. The first type corrects for pixel-grid irregularities due to the detector manufacturing process and is applied to both for WFC3/UVIS and ACS/WFC images using the D2IMFILE reference file. The second type corrects for filter-dependence in the non-polynomial distortion and is applied using the NPOLFILE reference file. Filter-dependent corrections are applied for the WFC3/UVIS detector and for all three ACS channels (WFC, HRC, SBC).
This chapter describes how AstroDrizzle handles geometric distortion information and how the reference files are used in the pipeline to produce calibrated images. Section 4.4 describes the predicted accuracy of HST pointing, its stability, and the precision of any commanded offsets. Section 4.5 summarizes the accuracy of absolute astrometry over the HST mission, which until recently was limited by uncertainties in the coordinates of the Guide Star Catalogs. As the accuracy of these catalogs improved over time, the pointing accuracy of HST has also improved.
Starting in late-2019, HST images were realigned to an absolute reference frame based on Gaia catalogs, making it easier to compare images from different HST instruments or from different telescopes. The key to incorporating this improved astrometry is in the use of headerlets, self-contained FITS extensions containing a World Coordinate System (WCS) transformation which can be attached to an image. Section 4.6 describes how new these FITS extensions can be used to store alternate WCS solutions after aligning to a particular image or astrometric catalog.