At wavelengths below 820nm, the Planetary Camera, with pixel size of
43 mas, undersamples the theoretical
resolution of the 2.4m HST aperture. In spite of this, we chose to observe
Pictor A with the PC because the ability of the CCD detector to accept high
photon count rates would minimize observing time and maximize signal
to noise in the fainter,
defuse regions adjacent to the nucleus. The predicted detectable
flux from the point source nucleus was 100
photons pixel
s
while
the saturation limit on the FOC, which is not undersampled, is
0.5
photons pixel
s
,
requiring a factor of
200 attenuation to avoid saturation
and thus a factor of 200 more observing time to obtain the same S/N in the
faint, diffuse regions, as the PC.
With the aberrated images, it was necessary to increase the observing time by a factor of 7 over the unaberrated exposures to compensate for the increased photon noise from the scattered light in the diffuse regions surrounding the nucleus. This is roughly the same factor by which the nuclear photon count rate is attenuated by the aberration and thus does not render the FOC any more attractive for this project.