SUBSTAR

       Subtract a scaled point spread function at specified star position(s).

       Part of the IDL-DAOPHOT photometry sequence

       SUBSTAR, image, x, y, mag, [ id, psfname, /VERBOSE]

       IMAGE -  On input, IMAGE is the original image array.  A scaled
               PSF will be subtracted from IMAGE at specified star positions.
               Make a copy of IMAGE before calling SUBSTAR, if you want to
               keep a copy of the unsubtracted image array

       X -   REAL Vector of X positions found by NSTAR (or FIND)
       Y -   REAL Vector of Y positions found by NSTAR (or FIND)
       MAG - REAL Vector of stellar magnitudes found by NSTAR (or APER)
               Used to scale the PSF to match intensity at star position.
               Stars with magnitude values of 0.0 are assumed missing and
               ignored in the subtraction.

       ID -  Index vector indicating which stars are to be subtracted.  If
               omitted, (or set equal to -1), then stars will be subtracted
               at all positions specified by the X and Y vectors.
       PSFNAME - Name of the FITS file containing the PSF residuals, as
               generated by GETPSF.  SUBSTAR will prompt for this parameter
               if not supplied.

       VERBOSE - If this keyword is set and nonzero, then SUBSTAR will
               display the star that it is currently processing

       The RINTER common block is used (see RINTER.PRO) to save time in the
       PSF calculations

       DAO_VALUE(), READFITS(), REMOVE, SXOPEN, SXPAR(), SXREAD()

       Written, W. Landsman                      August, 1988
       Added VERBOSE keyword                     January, 1992
       Fix star subtraction near edges, W. Landsman    May, 1996
       Assume the PSF file is in FITS format  W. Landsman   July, 1997
       Converted to IDL V5.0   W. Landsman   September 1997

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