1.2 What is DrizzlePac

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DrizzlePac is a suite of tasks for aligning, distortion-correcting, cosmic-ray cleaning, and combining HST images. It also includes several other tasks, such as a task for applying distortions to images, and tasks for transforming sky coordinates to image coordinates and vice-versa.

The Drizzle algorithm (Fruchter and Hook, 2002) was developed as a powerful method for combining dithered HST images, i.e., images (exposures) observed with different pointings (offsets). 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 to 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 images. 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 CR (cosmic rays) and bad pixels.  In some situations, such as the case of appropriately chosen dither patterns, users were able regain some of the resolution lost in the original undersampled images. Improved DrizzlePac software replaced the old MultiDrizzle software in the HST pipeline in June 2012. 

The initial concept of the DrizzlePac package which replaced the original MultiDrizzle package 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. DrizzlePac continues to perform the same functions as MultiDrizzle. It significantly improves the processing of the data, and particularly the astrometric information from the image header. These features will be covered in greater detail in the subsequent chapters.

The DrizzlePac software package includes the following tasks:

  • AstroDrizzle to sky-subtract, CR clean, and co-register multiple distorted images onto a single distortion-corrected frame;
  • TweakReg to perform image alignment by finding the offsets and rotation between HST images, and computing a corrected WCS (World Coordinate System) from these offsets and rotations;
  • SkyMatch to subtract (or equalize) sky background in HST images;
  • Runastrodriz to control the operation of AstroDrizzle.
  • Tweakback to resample and apply distortions to undistorted images (it is a reverse of AstroDrizzle);
  • Pixtopix to transform image coordinates of a source in one image to image coordinates in another image using full WCS information;
  • Skytopix transforms sky coordinates to X,Y pixel positions. A reverse transformation can be done using the task pixtosky;

  • MapReg to read and convert DS9 region files based on the WCS information in the image header;

  • PhotEq to equalize the data values of chips within an image so they can all be analyzed in the same way, which is particularly useful for WFPC2 and WFC3;

  • PixReplace to replace certain pixel values with another value;

  • ResetBits to change specific values in the data quality arrays to zero;

  • UpdateNpol to update ACS image headers with new NPOL and D2IM reference filenames;

  • ImageFindPars to set the parameters used by the internal DrizzlePac source finding algorithm, which is based on the DAOFIND algorithms (Stetson 1987);

  • RefImageFindPars to set the parameters in a similar manner to imagefindpars, but only applied to source detection in the reference images.

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