ACS data requests from the HST Archive are processed using the "On-the-Fly Reprocessing" (OTFR) System, beginning with raw spacecraft telemetry data (POD files) to produce several types of calibrated products.
POD files undergo a process called "Generic Conversion" to create raw image data (RAW files, with suffix raw.fits).
RAW files are processed using the ACS calibration software, CALACS, to perform standard calibrations such as bias and dark subtraction, as well as flat-fielding, to create calibrated products called FLT files. Note that these images have not yet been corrected for geometric distortion.
For ACS/WFC data taken after SM4 data, CALACS corrects artifacts due to post-SM4 electronics, a signal-dependent "bias shift" and bias striping.
All WFC data undergoes pixel-based CTE corrections, resulting in calibrated FLC files where CTE trails are removed and lost charge is restored to sources.
FLT files are processed by AstroDrizzle to create geometric distortion-corrected images, DRZ files. FLC files are similarly processed to create DRC files. If the FLT or FLC data are part of an association, the images are drizzle-combined; otherwise, individual FLT or FLC exposures are drizzled.
A detailed description of CALACS is available at http://www.stsci.edu/hst/acs/performance/calacs_cte/calacs_cte.html
For more information about AstroDrizzle, please consult the DrizzlePac webpage.