Hubble Space Telescope Will a 9-degree moon avoidance angle gaurantee a zodi-limited background level?
Scattered Moon Light:
I see that the HST has a Moon avoidance angle of 9 degrees. We have
to look out for this as we're observing in the ecliptic. Is there some
larger angle at which the scattered light from the Moon starts to raise
the background of the exposures? Or will a 9-degree avoidance gaurantee
a zodi-limited background level?
The background component from the Moon is shown in Figure 6.2 of the
ACS Instrument Handbook. For a near ecliptic target for which minimal
zodiacal background is about V = 22.4 the contribution from the Moon
at 9 degrees can reach V = 21, significantly above the zodi-limited
background. Of course given the rapidity with which the Moon moves
such an elevated background would not last very long as it drops off
quite steeply with angle.
We suggest avoiding the full moon by 12 degrees to be safe on
this. In the unlikely event this turned out to be a limiting factor
for scheduling this should be looked at more carefully.
This assumes that the scattered light contribution shown for the moon
corresponds to full.