WFC3-IR observations have revealed that for certain HST orientations, the IR detector can be subject to elevated and/or irregular background levels. Observations made when
HST is pointing near the bright Earth limb can result in the left most ~200 columns of the detector being subjected to background levels up to twice as bright as that on the rest of the chip. This is due to scattered Earth light, most likely reaching the detector through an unintended path through the optics.
Figure 6.17 shows an example of this behavior. The overall shape of this high background region is very similar from one affected image to another, but the brightness of the scattered light varies as the
HST pointing approaches or recedes from the bright Earth limb. Details on the nature of this effect in IR darks can be found in
WFC3 ISR 2009-21. This effect can often be present for observations made when the limb angle, which is the angle between
HST's pointing direction and the nearest limb of the bright Earth, is less than ~30
o.
