We have re-investigated the orbit dependent dark signal behaviour for detectors P1, P2, P3, C100 and C200 from all suitable data sets in the ISO archive. For P1, P3 and C100 we find that the dark signals are almost constant during the orbit. For P2 and C200 we find a steady increase of about 40% and 15%, respectively, between orbital positions 0.2 and 0.85. The high level of the dark signal at the beginning and at the end of the science window period as implied by the previous calibration analysis is not confirmed, although we note a larger scatter in the dark signal values at orbital positions before 0.25 and after 0.80. For P3 and C200 time-dependent calibration files are now required to take into account the impact of re-curing introduced in revolution 150. The C200 array behaves homogeneously. For C100, pixel 2 has a slightly increased dark signal, and pixel 6 presents a high dark signal level. No correlation with space weather activity, as monitored by responsivity checks after curing in the activation window, is seen. The results allow us to perform a more accurate determination of the photometric zero point of the ISOPHOT detectors, which is crucial to absolute photometry observations.
The Calibration Legacy of the ISO Mission
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