- The ETC shall support a Coronographic imaging mode, which is
distinct from the standard Imaging, Spectroscopic, and
RampFilter modes currently (or soon to be) supported. In
Coronographic mode, only the HRC camera will be used; the
filter set includes all the HRC filters, including Ramp filters.
NOTE: the message "CORONOGAPH IN USE" must appear in the graph and
output data windows when this mode is selected.
- The essence of the coronographic mode is to consider a primary target
that is superimposed upon a background of:
- detector dark current
- sky background, which consists of:
- zodiacal light
- scattered Earth light
- light from the wings of a central source which is
mostly blocked by one of two occulting spots, with diameters
of 1.8 and 3.0 arcsec.
The central source (hereafter, CS) is always considered to have a
point-like spatial distribution, so that the background it generates
can be represented by the wings of a PSF.
- The spatial distribution of light from the CS background can be
approximated by a 1-dimensional function of distance from the CS
(i.e., an azimuthally averaged PSF), normalized such that the total
intensity of the un-occulted CS is unity, in units of
count cm-2 s-1 arcsec-2.
NOTE: the PSF is known to vary with wavelength (or more to the point,
with filter/bandpass).
The ACS group needs to specify the 1-D PSF
for each bandpass that will be considered in the ETC; the format
of the file containing the PSF function(s) is TBD.
- The CS will contribute 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 user will be able to select which occulting
spot to use.
- The user must be able to specify the separation (in arcsec) between the
CS and the target (primary source). The minimum separation is:
the radius of the occulting spot in use, and the maximum separation
is: TBD arcsec.
- The contribution per unit area of the CS to the background will be
computed for the center of the aperture, and is assumed to be constant
over the entire aperture. The CS background, as integrated over the
bandpass, should be summed with the contributions from the zodiacal
and Earth light.
- It must be possible to specify the spectrum of the target
independently from that of the CS. The choices will be the same
that apply for Imaging mode, except that the choices for the
CS spectrum can be a bit more restricted: user-specified emission
lines will not be supported in the CS spectrum.
- It must be possible to specify the spectrum normalization for the
target independently from that of the CS. However, the CS
normalization will be a bit more restricted: the CS will always be
a point-source; and the extinction law and color excess will apply
to both the CS and the target.
The ACS group needs to specify whether the
user-specified input spectra will be supported for both the CS and
the target, or just the target.
- The signal-to-noise (S/N) ratio calculation will proceed exactly as
implemented for Imaging mode, except for the additional,
additive term to the noise discussed above. Note that for extended
sources the S/N should be calculated for a 2x2 pixel box, centered
at the target.
Q to ACS Group: wouldn't a plot of the S/N as a
function of distance from the CS be a very useful output plot to
generate?
- The throughput will be reduced by 48% by a Lyot stop that is used
in Coronographic mode.