An Instrument Anomaly in the Mars Exploration Rover Pancam 1,009-nm Filter (R7): Characterization, Simulation, Correction, and Preliminary Verification

During pre-flight calibration of the panoramic camera (Pancam) instrument on board the Mars Exploration Rovers MER A (Spirit) and MER B (Opportunity), a discrepancy was noted between 11-band spectra extracted from Pancam images of the camera's radiometric calibration target and reflectance spectra obtained with a spectrometer. This discrepancy was observed in the longest-wavelength filter of the camera (the longpass R7 filter with system λ _eff  = 1,009 nm) and consisted of a reduction in contrast between bright and dark regions. Here we describe and characterize this effect. We propose that the effect arises because long-wavelength photons close to the silicon band-gap at 1,100 nm are allowed through the R7 filter, pass through the bulk charge-coupled device, scatter from the backside, pass through the charge-coupled device again, and are registered in a pixel other than the pixel through which they originally entered. Based on this hypothesis we develop a model capable of accurately simulating the effect, and correct for it. We present preliminary results from testing this correction on preflight, as well as in-flight, images. The effect is small, but in some specific cases in small regions of high contrast, the effect is significant. In in-flight images of Martian terrain we observed the signal in dark shadows to be artificially inflated by up to ∼ 33% and analysis of early-mission calibration target images indicated that the reduced contrast due to the artifact is equivalent to >100 DN (full well = 4095 DN) for a hypothetical perfectly dark pixel.