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DrizzlePac 2012 Handbook
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The DrizzlePac Handbook > Chapter 1: Introduction to AstroDrizzle and DrizzlePac > 1.1 What are AstroDrizzle and DrizzlePac?

1.1
AstroDrizzle replaced MultiDrizzle in the HST data pipeline in June 2012 (please check the DrizzlePac website for the exact date). It is also available to users who wish to reprocess data using non-default parameter settings that could improve the quality of their images. AstroDrizzle, an abbreviation for Astrometric Drizzle, was designed from the ground-up to improve the handling of astrometry and geometric distortion.
DrizzlePac is a software package that replaces the Dither package in STSDAS. It is a suite of supporting tasks for astrodrizzle which include:
tweakreg and tweakback for aligning images in different visits
pixtopix transforms an X,Y pixel position to its pixel position after distortion corrections
skytopix transforms sky coordinates to X,Y pixel positions. A reverse transformation can be done using the task pixtosky
The Drizzle algorithm (Fruchter and Hook, 2002) was developed as a powerful method for combining dithered HST images. MultiDrizzle was later created to serve as a wrapper script that sequentially performs the steps needed to combine dithered images using Drizzle, as well as remove cosmic rays in the final combined image.
For many years, MultiDrizzle (Koekemoer et al., 2002) served as the pipeline software workhorse for combining HST observations. It was particularly suited for dithered HST images in the same visit, where the telescope pointing is offset between exposures. MultiDrizzle used pointing information in the headers to align dithered data, then combined them while correcting for geometric distortion and removing most artifacts such as cosmic rays and bad pixels. Users were also able to use MultiDrizzle to reprocess their data to include images from other visits, and in some situations, regain some of the resolution lost in the original undersampled images.
A description of the DrizzlePac package which replaced the Dither package, including information about AstroDrizzle which replaced MultiDrizzle, was first published in the paper by Fruchter, A., et al, “BetaDrizzle: A Redesign of the MultiDrizzle Package” in the STSCI Calibration Workshop Proceedings, Baltimore, MD, 2010. AstroDrizzle continues to perform the same functions as MultiDrizzle. But it also holds significant improvements in how the data is processed, including better handling of astrometric information. Bugs in MultiDrizzle have also been fixed in this new software. In order to implement the new astrometric features, calibrated flt.fits1 files from the data now have a slightly modified structure2. These issues will be covered in greater detail in subsequent chapters.

1
flt.fits files are flat-field calibrated images that have not been corrected for geometric distortion. New calacs data products are now available with the suffix flc.fits; these are identical to flt.fits except for CTE corrections to the images.

2
When old- and new-format flt.fits files are run in multidrizzle, almost identical images are created. While multidrizzle will continue to be available in STSDAS, it is no longer supported.


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