The unmerged WFC FITS files are merged by Mike Irwin to produce 4 merged files per field (one per CCD) per observing run. On occasions where the same band was observed more than once during the same observing run, both resulting source lists are included. The resultant FITS files contain roughly the same information as the unmerged ones (except that the positions are provided only in terms of X-- and Y--coordinates, not in terms of RA and DEC). These X-- and Y-- coordinates are given relative to a reference point in arcsecs. There is one row per object per band, i.e. if merging files for the i--, z-- and g-- band there would be three rows per object. Object detected only in some of the bands have zeros in the rows corresponding to bands they were not detected in. For more information see the section on merged files in [4]. A typical merged FITS header is shown in Appendix 5.
The original location of the merged files, as well as that of the copies
use for XID work, is the same as for the unmerged files. The merged files are
usually named field_
.mer, or sometimes
field_x_
.mer when there is a slight offset from
the target position.