At the beginning of each MULTIACCUM
observation, when the detector is reset, a net DC bias with a value of order 11,000 DN is introduced. For details, see
Section 7.7.2 of the WFC3 Instrument Handbook
. The overall bias level is somewhat different within each readout quadrant. In addition, there is a fixed pattern signal representing the inherent pixel-to-pixel reset levels. In standard MULTIACCUM
processing a two-step process is used to remove these signals.
First, reference pixels are used to remove any changes in the overall bias level that may occur during the exposure. The 5 columns of reference pixels on the left and right sides of the IR detector are insensitive to illumination and are used to measure the bias level of the detector at the time of each readout. During calwf3
processing, the mean measured signal of the reference pixels in each read is calculated and subtracted from all of the science pixels on the detector for that read. This method has the advantage of removing any bias drifts with time.
WFC3 ISR 2002-06
details the method of reference pixel bias subtraction, while
WFC3 ISR 2012-05
shows reference pixel behavior, and compares various methods of subtracting the reference pixel signal. A long-term drift in the raw pixel values of the reference pixels and the zeroth read signals has been observed over the course of on-orbit observations (see Figures 5 and 6 of
WFC3 ISR 2017-04
) but has had no adverse effect on the final calibration.The reference pixel level subtraction is performed in the BLEVCORR step of calwf3
Next, pixel-to-pixel and quadrant-to-quadrant differences in reset levels are removed by subtracting the zeroth readout of the exposure from all subsequent readouts. This is performed in the ZOFFCORR
step of calwf3
.The IR bias signal is therefore not a component of any of the calibration reference files, because it is removed using data contained in the science exposure itself.