WFC3/IR Reference Pixel Characterization #1: Comparison of Bias Subtraction Methods

In this first of two ISRs about the WFC3/IR reference pixel performance, we compare five different methods for using the reference pixels on the detector to remove bias signal from the measured signal in the light-sensitive science pixels. None of these methods produce a significant improvement over the current approach in the calwf3 calibration pipeline. We also note the existence of an extra signal in addition to the bias signal in the inboard reference pixels. Thought to be a “signal memory”, this signal was observed in ground tests of early, non-flight detectors but was thought not to be present in the current flight detector. The extra signal does not degrade the WFC3/IR calibration. Large (~100DN), long-lasting (~weeks) signal jumps in a subset of the reference pixels as well as nearby science pixels were also observed. A subsequent ISR will report on efforts to characterize these jumps and the long-term behavior of the reference pixels.