NAME:
  snrpred
 PURPOSE:   (one line only)
  Compute an estimated signal-to-noise ratio for a point source
 DESCRIPTION:
 CATEGORY:
  Photometry
 CALLING SEQUENCE:
  snrpred,system,exptime,smag,omag,snr
 INPUTS:
  system - anonymous structure with the following tags:
             jy0mag:   Janskys for zero Vega magnitude at lambda
             lambda:   center wavelength [nanometers]
             topt:     optical surface temperature of telescope [K]
             emis:     net emissivity to sky
             rdnoise:  read noise [e-]
             aper:     telescope aperture [cm]
             fnum:     f/# at focal plane
             bandpass: bandpass of instrument [nanometers]
             pixel:    size of pixel [microns]
             sky:      sky brightness [mag/sq arcsec]
             dark:     dark signal [e-/sec/pixel]
             tput:     throughput, central obs, reflectsion and QE
             seeing:   FWHM arcseconds
  exptime - scalar or vector if exposure time values [seconds]
  smag - Magnitude of the occultation star source (at lambda)
  omag - Magnitude of the occultating body (at lambda)
 OPTIONAL INPUT PARAMETERS:
 KEYWORD INPUT PARAMETERS:
  VERBOSE - if set will generate lots of printed output.  This is rarely
              useful if "smag" is a vector.
 OUTPUTS:
  snr - Estimated signal-to-noise estimation per point
 KEYWORD OUTPUT PARAMETERS:
 CONFIGURATION:
 COMMON BLOCKS:
 SIDE EFFECTS:
 RESTRICTIONS:
  The input values exptime, smag, and omag can be either scalars or vectors.
    If any are vectors, any that are must be the same length.  This is
    NOT enforced.  Note that the value of /VERBOSE may be limited with
    vector inputs as the output printed will be poorly formatted (and long).
 PROCEDURE:
 MODIFICATION HISTORY:
  Written by Marc W. Buie, Southwest Research Institute, 2022/02/04 and
     based on a spreadsheet provided by Mike Skrutskie, UVa.
  2022/02/22, MWB, added object magnitude and scintillation estimates
  2024/11/14, MWB, added exposure time as an argument instead of including
                 it in the input structure.