UDF ACS/WFC Raw Data Release - README File
		==========================================
		   Anton Koekemoer, 2 March 2004, STScI


The UDF ACS/WFC data consists of a total of 800 exposures divided
into four bands, namely F435W, F606W, F775W, F850LP. The files in
this directory are the 800 individual ACS/WFC exposures, in the
"raw" format produced by the HST Archive Pipeline, prior to being
calibrated. Each image contains a primary header along with a total
of 6 extensions, corresponding to the science image, error array
and data quality file, for each of the 2 ACS/WFC CCDs.

In addition to the 800 raw exposures, we also provide the calibration
reference files that were used to calibrate them when running the
"calacs" task with IRAF/Pyraf.. In addition to all the standard ACS
reference files, we also provide the following special reference
files that were included in the calibration steps:

 - Hyperdarks, which were formed from all the darks during a 6-month
   period covering the UDF timeframe. These files (provided by M. Mutchler
   of the ACS team) provide a higher S/N than the usual dark reference
   files that are subtracted during calibration, thereby allowing us to
   fully exploit the extreme depth of the UDF. These files are prefixed
   by "wfc_hdrk_".

 - Revised flatfields based on the "L-flat" technique (these files
   provided by J. Mack of the ACS group). These were slightly improved
   over the L-flats present in the ACS pipeline at the time when the
   UDF data were being calibrated, and thus provide a somewhat more
   uniform sky level across the images, to a level of ~1% of the sky
   value. These files are prefixed by "wfc_flat_", together with the
   filter name indicating which filter they are applicable to.

 - Sky-flats, aimed at correcting the ~1% residuals that remain after
   having calibrated the data with the L-flats. These sky-flats were
   generated by the UDF team based on a combination of all the UDF data
   at different orientations, which thereby allow objects to be masked
   out and provide a representation of the true residual sky level
   across the field. These files are prefixed by "wfc_deltaflat_",
   together with the filter name indicating which filter they are
   applicable to, as well as "sci1" or "sci2" indicating which of the
   two CCDs they should be applied to. These residual sky-flats should
   be multiplied into the calibrated files are having run "calacs" with
   the L-flats described above.

 - A revised badpixel file, "wfc_badpix_20040227.fits", which contains
   a number of additional bad columns and other defects not currently
   in the standard data quality arrays. The [dq,1] and [dq,2] extensions
   in this file contain the bad pixels (all set to the value "4") that
   can then be included in the "dq" extensions of all the exposures after
   having run "calacs".

 - A slightly revised geometric distortion IDC-table, "shifted_idc.fits",
   whicih was used running MultiDrizzle to create the combined UDF images.

 - Geometric distortion residual images, provided by J. Anderson, which
   are images representing the residual geometric distortion to be applied
   on top of the 4th-order polynomial model that is represented in the
   IDC-table. Without these images, the distortion model is already good
   to about 0.2-0.3 ACS/WFC pixels across the image, but including these
   distortion residual images improves the accuracy to levels smaller
   than about 0.1 pixels across the entire ACS image. These files are
   prefixed by "dx.*" and "dy.*", followed by "wfc1" and "wfc2" for
   the [sci,1] and [sci,2] groups respectively, as well as indicating
   the filter that each image is applicable to. These files were used
   by a modified version of wdrizzle (R. Hook) when combining all the
   exposures into a final image.


In addition, we provide a file called "readme_keywords.txt" which has
800 lines, one for each individual exposure, showing the values of the
keywords that should be changed before running calacs, in order to
take advantage of the above special calibration reference files.
This file contains the following columns, givin values that can be
applied before running "calacs":
  - raw file name
  - hyperdark file name
  - L-flat filename
  - WCS keywords for sci1: CRVAL, CRPIX, CD matrix
  - WCS keywords for sci2: CRVAL, CRPIX, CD matrix

In addition, the IDCTAB keyword should be set to "shifted_idc.fits"
for all the files, and after calibrating, they should be divided by
the skyflats described above, as well as having the bad pixels inserted
into the [dq] arrays, based on the badpixel file described above.