OPREP: Open a Batch Surface Photometry Preparation File

OPREP opens a "batch photometry preparation file" used for batch-type surface photometry reductions. The files contain the following information (one record per image):

SKY value
RMAX variable (max. radius for photometry)
estimated HWHM for the PSF
contents of the AXES block
image header.

This information is written by WPREP command, and read by RPREP. This routine initialized the PREPCON common block, which records the status of the prep-file (whether open to read or write). The filename is supplied as a keyword. Default directory is V_DATADIR, with the default extension ``.pre''. If no name is given on the command line, the default name PREPFILE is used. If there is a prep-file already open, this routine closes it and opens a new one.

NOTE: You must compute the image parameters above ahead of time, either outside of VISTA or using the AXES command without the CORR option.