FTHMOD

       Procedure to modify header information for a specified field
       in a FITS table.

       fthmod, h, field, parameter, value

       h - FITS header for the table
       field - field name or number
       parameter - string name of the parameter to modify.  Choices
               include:
                       TTYPE - field name
                       TUNIT - physical units for field (eg. 'ANGSTROMS')
                       TNULL - null value (string) for field, (eg. '***')
                       TFORM - format specification for the field
                       TSCAL - scale factor
                       TZERO - zero offset
               User should be aware that the validity of the change is
               not checked.  Unless you really know what you are doing,
               this routine should only be used to change field names,
               units, or another user specified parameter.
       value - new value for the parameter.  Refer to the FITS table
               standards documentation for valid values.

      Change the units for a field name "FLUX" to "Janskys" in a FITS table
        header,h
      IDL> FTHMOD, h, 'FLUX', 'TUNIT','Janskys'

       The header keyword  is modified
       with the new value.

       version 1, D. Lindler  July 1987
       Converted to IDL V5.0   W. Landsman   September 1997
       Major rewrite to use new FTINFO call   W. Landsman   May 2000

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