

Draft requirements for ACS/ETC Coronagraphic Mode

1. The ETC shall suppor a Coronagraphic imaging mode, which is distinct from
   the Imaging, Spectroscopic, and Ramp Filter modes.  In Coronagraphic mode,
   only the HRC camera will be used; the filter set includes all HRC filters,
   including ramp filters.

   NOTE:  The message "CORONAGRAPH IN USE"  must appear in the graph and
   output data windows when this mode is selected.

2. In the Coronagraphic mode there will be a central source (CS) taht is
   placed behind on of two occulting spots (1.8 and 3.0 arcsec diameters),
   and a filed source (FS) that is located within the field of view but
   which is not behind a spot.  The background upon which the FS is superposed
   will consist of:

    1. Detector dark current
    2. Sky background, consisting of;
          + Zodiacal Light
          + Scattered Earth light
    3. Residual diffracted light in the wings of the of the occulted
        central source

   The CS is assumed to be an unresolved point source, such as a star
   or QSO nucleas.

3. The spatial distribution of residual light from the CS can be
   approximated by a 1-dimensional surface brightness profile that is
   a function of distance from the CS, in units of counts cm^-2 s^-1 arcsec^-2,
   normalized such that the total intensity of the unocculted CS in unity.

   This profile will vary with wavelength/filter bandpass.  The profiles
   at five different wavelengths spanning the bandpass range of the HRC will
   be provided, one set for each spot.  The ETC will use the profile that 
   is closest to the central wavelength of the user-specified fileter and 
   for the appropriate spot.

4. The user must be able to specify the speartaion (in arcsec) between the
   CS and the FS.  The minimum separation is the radius of the occulting 
   spot in use, and the maximum separation is 10.0 arcseconds.

5. The CS will contribute to background light over the entire area of a
   square aperture of user-specified size (in detector pixels).  The extent
   of the aperture will be specified exactly as it is for Imaging mode,
   with the additional constraint that it cannot overlap the occulting
   spots.  The suer will be able to select which occulting spot to use.

6. The contribution of the CS to the background will be computed for the
   center of the aperture from the surface brightness profile.  This 
   background level is assumed to be uniform over the entire aperture.

7. It must be possible to specify the spectrum of the FS independently 
   from that of the CS.  The choices will be the same that apply for
   Imaging mode, except that user-specified emmission lines will not be
   supported for the CS spectrum.

8. It must be possible to specify the spectrum nomalization for the 
   FS independently from that of the CS. However, the CS normalization
   will be a bit more restricted as the CS will always be a point-source.
   User-specified input spectra will be supported for both the CS and the
   FS.

9. The signal-to-noise (S/N) ratio calculation will proceed exactly as
   implemented for Imaging mode, except for the additional, additive
   background light term from the CS.  Note that for an extended FS, the S/N
   should be calculated for a 2x2 pixel box, centered on the FS.

10. In Coronagraphic mode, the flux from the FS will be reduced by 48%
    relative to the Imaging mode, due to throughput reduction from the 
    Lyot stop.

11. The peak counts within an aperture will ber reported for the FS if it
    is a point source, and a warning issued if the peak is 90% or greater
    that the saturation level of the detector.  The peak pixel corresponds
    to the PSF ensquared energy in a 1 by 1 pixel aperture.  A table of
    ensquared energies will be provided for selected wavelengths.  
    NOTE:  The ensquared energies of the off-spot field PSF in Coronagraphic
    mode will be different than for normal Imaging mode.

12. The issue is feedback we have been getting when the ACS ETC is compared
    with WFPC2's ETC. The ACS ETCs use the STIS model of calculating S/N in an
    aperture of 80% encircled energy. WFPC2 uses an optimum PSF weighting 
    scheme to calculate S/N. The IDT (who have used the ETC most) have 
    pointed out that quick comparisons between ACS and WFPC2 are confusing as ACS
    does not show much apparent improvement. Currently, we allow users to
    change the aperture size in both ACS ETCs, but I would like to add an 
    option which replicates the WFPC2 algorithm. In the past the IDT have 
    been a good indicator of what to expect from the GOs. Many GOs are likely 
    to start planning proposals from WFPC2 experience so this discrepancy 
    can be confusing. I think we have a duty to ensure that GOs understand 
    the real capabilities of ACS so they can propose good science. I am proposing
    that we simply add another aperture to the pull down menus. This should
    not be much work !
