T_SUBSTAR

       Driver procedure (for SUBSTAR) to subtract scaled PSF values

       Computes residuals of the PSF fitting program

       T_SUBSTAR, image, fitsfile, id,[ psfname, /VERBOSE, /NOPSF ]

       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

       FITSFILE  - scalar string giving the name of the disk FITS ASCII
               produced as an output from T_NSTAR.

       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.
               (IDL convention - zero-based subscripts)
       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 non-zero, then the value of each
               star number will be displayed as it is processed.
       /NOPSF - if this keyword is set and non-zero, then all stars will be
               be subtracted *except* those used to determine the PSF.
               An improved PSF can then be derived from the subtracted image.
               If NOPSF is supplied, then the ID parameter is ignored

       T_SUBSTAR does not modify the input FITS table.

       FTGET(), FTINFO, READFITS(), REMOVE, SUBSTAR

       Written, R. Hill, ST Sys. Corp., 22 August 1991
       Added NOPSF keyword   W. Landsman        March, 1996
       Use FITS format for PSF resduals         July, 1997
       Converted to IDL V5.0   W. Landsman   September 1997
       Call FTINFO first to improve efficiency   W. Landsman  May 2000

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