6.2 Temperature Change of the ISO Focal Plane

The temperature of the focal plane steadily increased after the helium boil-off. Figure 6.1 shows how after some days it exceeded the dynamic range of the temperature sensors on the optical support structure. The temperature trend of the focal plane further in the Post-Helium Phase can be inferred from a similar trend seen in the temperatures of the upper baffle of the ISO telescope. It is estimated that the temperature of the SWS detectors increased from the nominal operating temperature (1.8 $\pm$ 0.1 K) to 40 K $\pm$ 5 K over the 30 days of post-helium operations.

Figure 6.1: The temperatures of the optical support structure (lower short curve, dashes) and of the upper baffle of the ISO telescope (upper curve, solid line) increased smoothly during the Post-Helium Phase.

Before the helium boil-off, it was anticipated that the temperature of the focal plane would increase sufficiently to allow operating the JFET cryogenic readout without heating. The slow temperature increase however made heating required throughout the entire Post-Helium Phase for the JFETs to function properly. This was verified during every revolution with SWS operations. The instrument controllers manually uplinked a command sequence to the spacecraft to switch off the JFET-heaters for one minute. It was verified offline that this indeed corrupted the detector readout.