The MultiDrizzle Handbook
1.2 What is New in Multidrizzle?
MultiDrizzle has undergone a number of internal changes since its last major release. Fortran has been converted to C; python code has been improved and better documented. Additionally a fundamental change has been made in the philosophy used to handle image distortions and astrometry. In the past, MultiDrizzle used a set of coefficients separate from the image world coordinate system (WCS) to define nonlinear distortions in the image, and even some small linear corrections in certain cases. The distortion information came from a separate file containing a table called the IDCTAB (or instrument distortion coefficients table), to supply distortion coefficients to the drizzle program. By contrast, the new version of MultiDrizzle incorporates the distortion information directly into the WCS using a standard called the "simple image polynomial" or SIP standard (Shupe et al., 2005). This standard has previously been used to describe the geometry of the Spitzer Space Telescope images, and we expect it to become a FITS standard in the near future. As MultiDrizzle is run by the HST archive during calibration-on-the-fly, whenever a user receives HST images in the future, they will have the distortion information incorporated into the headers of their calibrated data using FITS standard conventions. This means that users will find it easier to write their own programs to combine HST data which take full advantage of our knowledge of distortions in the image plane. Indeed, a number of programs already in use in the astronomical community, such as SAOimage and DS9, are already able to read the SIP coefficients and use them to convert between pixel and sky coordinates.
In the longer term, this change of philosophy will allow even greater improvements to the way we handle image combination and astrometry. For instance, we plan to distribute code that will compare a catalog of object positions derived from an HST image with an astrometric catalog supplied by the user. The code will then calculate a "headerlet" that will contain the changes to the WCS necessary to bring the HST image coordinate system into agreement with the astrometric catalog. The "astrometric catalog" could equally well be a set of positions derived from other HST images. Thus this method will provide a powerful tool both for combining and creating mosaics of HST images as well as aligning them with external catalogs.
Space Telescope Science Institute
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