The ISOCAM/LW detector dark current behaviour

We describe the calibration, measurements and data reduction, of the dark current of the ISOCAM/LW detector. We point-out the existence of two significant drifts of the LW dark-current, one throughout the ISO mission, on a timescale of days, another within each single revolution, on a timescale of hours. We also show the existence of a dependence of the dark current on the temperature of the ISOCAM detector. By characterizing all these effects through polynomial fittings, we build a model for the LW calibration dark, that depends on the epoch of observation (parametrized with the revolution number and the time elapsed in that given revolution since the activation) and on the temperature of the ISOCAM detector. The model parameters are tuned for each of ISOCAM/LW pixel. We show that the modelling is very effective in taking into account the dark-current variations and allows a much cleaner dark subtraction than using a brute average of several calibration dark images. The residuals of the LW model-dark subtraction are, on average, similar to the pre-launch expectation.