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      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

Calling 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.

Optional Inputs

      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])

Optional Keywords Inputs

      /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.



Revison History

      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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