See the ACS dither web page for information on designing and using ACS pointing patterns. The following list of documents provide information about reducing dithered or mosaiced data:

• How such data are handled by the CALACS calibration pipeline, including the drizzling of associated datasets by PyDrizzle
• The use of MultiDrizzle (Koekemoer et al. 2002, HST Calibration Workshop, p. 337) to associate data, refine shift measurements, and reject cosmic rays.
• Reproduceable examples which illustrate the use of all of the above

Drizzling mechanics

The ACS Data Handbook
Mack et al., 2003, Chapter 3 describes pipeline processing, including PyDrizzle; Chapter 4 provides detailed drizzling guidance, including the rejection of cosmic rays and image registration with MultiDrizzle

The PyDrizzle webpage
Hack & Jedrzejewski, 2002, PyDrizzle is included in IRAF/STSDAS, and is a part of the ACS calibration pipeline; requires PyRAF

The MultiDrizzle webpage
Hack & Jedrzejewski, 2002, MultiDrizzle is currently a stand alone IRAF/STSDAS task which must be downloaded and installed; requires PyRAF; does not yet include automated shift tweaking

Reduction of the Helix Nebula mosaic
Mutchler, 2003, a detailed example of drizzling a large ACS mosaic (the F656N WFC mosaic is displayed above), which can be reproduced with FITS data also provided

Drizzling reference

Drizzle: A Method for the Linear Reconstruction of Undersampled Images
Fruchter & Hook, PASP, 114: 144-152, 2002 February

Anton Koekemoer's MultiDrizzle webpage
Koekemoer, 2002 (describes use of the full-featured version with shift tweaking)

MultiDrizzle: An Integrated Pyraf Script for Registering, Cleaning and Combining Images (PDF)
Koekemoer, Fruchter, Hook, Hack, 2002, HST Calibration Workshop Proceedings (details about Multidrizzle steps and parameter settings)

HRC mosaic simulation (PDF)
Mutchler, 2001, previous version of this webpage; pre-flight testing of drizzle software with simulated data