The XMM-Newton Optical Monitor data available directly through MAST consists of 
two types of data: images in several different filters, and source lists.  
The XMM images are simple primary array FITS files and should be readable with 
most FITS readers. The header contains the WCS keywords.

All the XMM-OM data has been reprocessed using the standard omichain/omfchain 
pipelines in SAS 6.5.0. See a brief description of the OM which is an excerpt of 
a paper by Kuntz et.al. (in prep). A section describing the processing and 
catalogue creation was also excerpted from this paper at
http://archive.stsci.edu/xmm-om/omproc.html 

The following is an excerpt of the paper by Kuntz et.al. in prep describing the 
filenames, and some details about the formats and contents.

The following table summarizes the file naming convention. Note that mosaic 
image files taken in different modes have different names: this is to ensure 
that all of the mosaicked images are retained if a given observation has a 
mixture of imaging modes; one could possibly have multiple images in the 
same filter but different modes.


Overview
nnnnnnnnnn_filt.fits	      mosaic from default imaging mode
nnnnnnnnnn_filt_HR.fits	      coadded hi-res image center from default imaging mode
nnnnnnnnnn_filt_E.fits	      mosaic from "ENG4" imaging mode
nnnnnnnnnn_filt_EM.fits	      mosaic from "ENG2" imaging mode
nnnnnnnnnn_filt_FA.fits	      coadded hi-res image from fast imaging mode
nnnnnnnnnn_om_source_sas.fits original source list produced by SAS
nnnnnnnnnn_om_source_v00.fits original source list reformatted
nnnnnnnnnn_om_source_v01.fits source list with corrected coordinates


Low-Resolution and High-Resolution Mosaics: A mosaic image is contained in a 
FITS file with two extensions. The first extension is the actual image. 
The second extension is the exposure map.

To be consistent with the standard omichain output format, all images are 
normalized to a 1000s exposure; the image units are counts per 1000s. The part 
of the image that is outside the OM FOV contains "NULL" values. The header 
contains the standard WCS keywords. As should be clear from the description of 
the processing, there are multiple generations of coordinates to be considered, 
and all of those generations have been retained in the header. The original 
coordinates of the mosaic are retained in the CRVAL1R and CRVAL2R keywords. 
These coordinates are from the first science window used in the construction of 
the mosaic, and these coordinates do not reflect any of the values in the source 
lists. We first corrected the mosaic coordinates to the values in the RA and DEC 
columns of the source lists; these coordinates are listed in the CRVAL1S and 
CRVAL2S keywords. The OFFSET1R=CRVAL1S-CRVAL1R and OFFSET2R=CRVAL2S-CRVAL2R 
keywords contain the offset between the two coordinate systems. We then 
corrected the mosaic coordinates to the values in the RA_CORR and DEC_CORR 
columns of the final source list; these coordinates are listed in the standard 
CRVAL1 and CRVAL2 keywords. 
The OFFSET1S=CRVAL1-CRVAL1S and OFFSET1S=CRVAL2-CRVAL2S keywords contain the 
offset between the two coordinate systems. The original 
pre-coordinate-correction CDELT1 and CDELT2 keywords have been moved to CDELT1R 
and CDELT2R and replaced with CDn_n keywords. Finally, the MEAN_EXP keyword is 
the mean exposure time for the image as calculated from the exposure map. 
Since the mosaicked image is calculated from several science windows that may 
overlap, the MEAN_EXP keyword reflects the amount of overlap.

Source Lists: Three generations of source lists have been retained. The 
om_source_sas.fits file contains the source list output by SAS 6.5.0 in binary 
FITS table format. The om_source_v00.fits file is a reformatted version of the 
om_source_sas.fits file which preserved most (but not all) of the columns in the 
SAS produced master source list. The only columns in the om_source_sas.fits 
which are not contained in the om_source_v00.fits files are those containing the 
colors of the sources in the standard photometric bands. This information can be 
reconstructed from the instrumental magnitudes using the formulae in 
XMM-SOC-CAL-TN-0019 *gzipped ps file. For the purpose of creating a single OMCat, 
(Kuntz et.al. in prep) the source lists for individual obsids have several small 
format differences from those output by SAS; columns containing vectors of 
values, one element for each filter, have been split into a single column for 
each filter. Note that there are two sets of coordinates listed for each source. 
The coordinates in RA and DEC are the coordinates before SAS pipeline correction 
by the USNO-B1 catalogue; the coordinates in RA_CORR and DEC_CORR are post 
correction.

The om_source_v01.fits file is identical to the om_source_v00.fits file with the 
exception of the RA_CORR, DEC_CORR, and RMSRESID columns which contain the 
results of the post-pipeline coordinate correction. If the RMSRESID value is 
zero, then no coordinate correction ws made.

See a list of the columns at 
http://archive.stsci.edu/xmm-om/file_format.html