For certain HST orientations, the WFC3/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 7.12
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 degrees.
An additional source of background contamination is caused by the He line at 1.083 μ
m. Affected filters are F105W, F110W and the G102 grism. This emission line originates from metastable helium in the Earth’s upper atmosphere that can increase the IR background by up to factors of 6 above the nominal zodiacal background. This spatially diffuse source affects portions of HST orbits where both the telescope and the atmosphere are illuminated by sunlight. WFC3 ISR 2014-03
describes this effect more fully.