MAST is serving a OM Mosaic product that uses a pipeline described by Kuntz et. al. OMCat: Catalog of Serendipitous Sources Detected with the XMM-Newton Optical Monitor PASP, 120:740-758
Please see discussion on Processing of OM Mosaics, excerpted from a draft of the paper.
The XMM images are simple primary array FITS files and should be readable with most FITS readers. The header contains the WCS keywords.
The XMM-Newton Optical Monitor data available directly through MAST consists of two types of data: images in several different filters, and source lists. Both images and source lists are produced using the standard XMM-Newton pipeline (SAS v.6.0).
Each image is taken as five sub-exposures. The standard pipeline combines each of the sub-exposures into a single mosaic. Tracking corrections, bad pixel corrections, flat-field corrections, and fixed pattern noise corrections are made to the image. Source detection, astrometry and photometry are all done automatically.
After using the standard pipeline to process the OM data, the coordinates are then corrected by cross-correlating the pipeline-produced source lists and the Guide Star Catalogue. An interative shift-and-twist solution is used to minimize the offsets between the XMM and GSC coordinates; the resulting coordinate corrections are applied to the coordinates in the source lists and the headers of the image mosaics. In all cases the original pipeline-produced coordinates are conserved, as CRVALnX, CDELTnX in the image headers (where the original image mosaic has no rotation) and as the RA_OM and DEC_OM columns in the source lists. The source lists have be restructured to some extent and have been augmented with "postage stamp" images of each source.
Caution should be exercised:
1) The individual sub-exposures are not always well-aligned with one another; this mis-alignment can sometimes be seen by comparing images of the same region in different filters or by comparing different observations of the same field. As a result, the quality of the astrometry may vary strongly within an image.
2) Since the source detection is automated, ghost images and very bright extended sources (such as the bulge of M31) produce spurious sources.
3) Incomplete exposures are not handled by the pipeline in a
consistent manner; sometimes incomplete exposures are saved as
partial mosaics by the pipeline and sometimes are not. Source
detection is done at an earlier stage of the pipeline, so the
source catalogues may contain information from observations/filters
that do not exist as mosaics.