NAME:
  astromerr
 PURPOSE:   (one line only)
  Compute an estimate of the astrometric error given FWHM and SNR.
 DESCRIPTION:
 CATEGORY:
  Astrometry
 CALLING SEQUENCE:
  astromerr,fwhm,snr,err
 INPUTS:
  fwhm - scalar or vector, full-width at half-max of the source in
           (default unit is [pixels])
  snr  - signal-to-noise ratio.  Size and rank needs to match fwhm
 OPTIONAL INPUT PARAMETERS:
 KEYWORD INPUT PARAMETERS:
  PSCALE - image scale, (default=1), typically arcsec/pixel but you can
             use you own units.  If supplied, the output result will be
             scaled by this scalar value.
  ARCSEC - Flag, if set, the input fwhm will be in arcsec.  In this case
             pscale will be used to scale to pixels for the computation
             and then scaled again for return.
 OUTPUTS:
  err  - positional uncertainty of a centroid measurement for an object
           aperture equal to the FWHM
 KEYWORD OUTPUT PARAMETERS:
 CONFIGURATION:
 COMMON BLOCKS:
 SIDE EFFECTS:
 RESTRICTIONS:
  This tool works from a basis of a pixelated image and the scaling to
     angular units is a later step.
  The valid range for the computation is:
      fwhm  {1.6-10}.  values less then 1.6 are treated like 1.6, values
        greater than 10 are treated as 10.
      snr   {>3}.  You can provide snr <3 and you will get an answer but
        its vailidity is very limited.
  The result will never be less than 0.001 pixel nor will it ever be larger
        than the input fwhm.
 PROCEDURE:
 MODIFICATION HISTORY:
  Written by Marc W. Buie, Southwest Research Institute, 2021/03/24