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? Reply 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.
