WFC3 Data Handbook V. 4.0

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7.9	Time Variable Background Contamination

7.9.1 Scattered Earthlight

7.9.2 Metastable Helium 1.083 μm Emission Line

7.9.3 Mitigation Strategies

7.9.1 Scattered Earthlight

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.

Figure 7.12: IR Image affected by scattered earth-shine.

7.9.2 Metastable Helium 1.083 μm Emission Line

An additional source of background contamination is caused by the He line at 1.083 μm. 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.

7.9.3 Mitigation Strategies

For archival data, one solution is to keep only those reads unaffected by earth shine and/or the He line, with a loss of signal of noise. Users will need to manually reprocess the images, as in Example 4 in Section 3.7.2, masking out the affected reads. To find out which reads are affected, one can plot statistics for each read, e.g.

from wfc3tools import pstat

pstat.pstat(‘rootname_ima.fits’)

 

With respect to the He emission line, sensitive observations should be scheduled mid-orbit, where the effects are less. For the future, the WFC3 team is exploring possible observing strategies without decreasing the efficiency of HST observations.