T_APER

       Driver procedure (for APER) to compute concentric aperture photometry.

       Data is read from and written to disk FITS ASCII tables.
       Part of the IDL-DAOPHOT photometry sequence

       T_APER, image, fitsfile, [ apr, skyrad, badpix, PRINT=, NEWTABLE=,
                       /EXACT, /SILENT, SETSKYVAL = ]

       IMAGE   - input data array
       FITSFILE  - disk FITS ASCII table name (from T_FIND).  Must contain
               the keywords 'X' and 'Y' giving the centroid of the source
               positions in FORTRAN (first pixel is 1) convention.   An
               extension of .fit is assumed if not supplied.

       User will be prompted for the following parameters if not supplied.
       APR    -  Vector of up to 12 REAL photometry aperture radii.
       SKYRAD  - Two element vector giving the inner and outer radii
               to be used for the sky annulus
       BADPIX  - Two element vector giving the minimum and maximum
               value of a good pixel (Default [-32765,32767])

       /EXACT - If this keyword is set, then intersection of the circular
               aperture is computed exactly (and slowly) rather than using
               an approximation.   See APER for more info.
       /PRINT - if set and non-zero then NSTAR will also write its results to
               a file aper.prt.   One can specify a different output file
               name by setting PRINT = 'filename'.
       /SILENT - If this keyword is set and non-zero, then APER will not
               display photometry results at the screen, and the results
               will be automatically incorporated in the FITS table without
               prompting the user
       NEWTABLE  - Name of output disk FITS ASCII table, scalar string.
               If not supplied, then the input FITSFILE will be updated with
               the aperture photometry results.
       SETSKYVAL - Use this keyword to force the sky to a specified value
               rather than have APER compute a sky value.    SETSKYVAL
               can either be a scalar specifying the sky value to use for
               all sources, or a 3 element vector specifying the sky value,
               the sigma of the sky value, and the number of elements used
               to compute a sky value.   The 3 element form of SETSKYVAL
               is needed for accurate error budgeting.

       T_APER requires the number of photons per analog digital unit
       (PHPADU), so that it can compute Poisson noise statistics to assign
       photometry errors.    It first tries to find the PHPADU keyword in the
       original image header, and if not found will look for the GAIN,
       CCDGAIN and finally ATODGAIN keywords.   If still not found, T_APER
       will prompt the user for this value.

       APER, FTADDCOL, FTGET(), FTINFO, FTPUT, READFITS(), SXADDPAR,
       SXPAR(), WRITEFITS

       Written   W. Landsman   ST Systems Co.            May 1988
       Store results as flux or magnitude                August 1988
       Added SILENT keyword  W. Landsman                 Sep. 1991
       Changed ERR SKY to ERR_SKY W. Landsman   March 1996
       Replace TEXTOUT keyword with PRINT keyword  W. Landsman  May 1996
       Check CCDGAIN or ATODGAIN keywords to find phpadu W. Landsman May 1997
       Converted to IDL V5.0   W. Landsman   September 1997
       Updated for new FTINFO calling sequence   W. Landsman  May 2000
       Added /EXACT keyword                      W. Landsman  June 2000

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