AstroDrizzle: Image Combination, Astrometry Included

Andy Fruchter, fruchter@stsci.edu

If you have retrieved imaging data from the Hubble archive recently, you have probably seen a notice saying that a new program, AstroDrizzle, has replaced MultiDrizzle in the Hubble pipeline. AstroDrizzle is the key program in a new package of programs designed to reduce dithered data. This package is called DrizzlePac and replaces the older Dither package, which contained MultiDrizzle. For a number of years now, MultiDrizzle has been the primary program used by the archive—as well as most Hubble users—to combine Hubble images. MultiDrizzle has produced a great deal of important science and some wonderful images. However, its coding made it difficult to maintain, and its design left out an important consideration: the incorporation of the full astrometric solution into the input images themselves.

When you flat-field an image, for instance, the flat field is directly incorporated into the image. You do not need to maintain the flat-field image itself to interpret the calibrated image. This has not been true, however, when it comes to astrometry and Hubble images. Additional, large reference files have been required to accurately interpret the astrometry of the flat-fielded images (the .flt files). This problem has been solved. AstroDrizzle uses polynomial coefficients in the image header and small fits extensions with interpolatable look-up tables, to fully represent the distortions of the Advanced Camera for Surveys (ACS) and Wide Field Camera 3 (WFC3). As a result, a calibrated image needs no other files to describe its astrometry. The “Astro” in AstroDrizzle, therefore, refers to astrometry.

This change in design is far more powerful than it might seem at first. The presence of the full astrometric solution in the image means that users can fit the astrometry of one image to another image, or to an external catalog, without drizzling the image, as precise coordinate information can be extracted directly from the .flt image itself. And when an image is aligned to an astrometric catalog, its astrometry is fully updated to reflect that change. However, the user can store the original astrometric solution, as well as astrometric solutions corresponding to alignments to different catalogs, in the image and can swap these solutions into the primary header position at will. Finally, users can extract an astrometric solution from an aligned image and put it in a small fits file we call a “headerlet.” Users can send these headerlets to collaborators or, in the near future, provide them back to the archive itself, so that others may take advantage of an astrometric solution without having to re-copy the much larger image file as well.

Users will be happy to hear that in spite of these changes to the astrometric header, the pixels of the images created by AstroDrizzle generally agree with the ones produced by MultiDrizzle to many significant digits. There may, however, be small differences in the position on the sky assigned to a pixel.