In order to reduce the number of damaged pixels ("rogues") and to improve the signal-to-noise ratio nin observations of faint sources, the bias voltage on the Long High array was reduced from 2.0 to 1.6 volts at the start of IRS Campaign 25 (see Table 2.2). This change reduced the detector's quantum efficiency by a wavelength-dependent factor (by an average of ~30%). On the other hand, the change also reduced the number of bad pixels in the array by a factor of two to three, resulting in cleaner spectra.
For the same reason the bias was changed in Long High, in IRS Campaign 45 the bias voltage on the Long Low array was reduced from 1.8 to 1.6 volts and the array temperature was reduced from 4.4 to 4.1 K (see Table 2.2). As for LH, this reduced the dark current and photon sensitivity of the array. In the case of Long Low, the number of detected rogue pixels decreased to 1/3 of its previous value. This combination of effects increased the overall signal-to-noise ratio in the LL array by up to 60%, depending on wavelength.
2.8.2 Array Temperature Control and Annealing
In order to achieve optimum performance, the detector array temperature was closely controlled. Because charged particle radiation damage accumulation on the pixels causes an increase in baseline dark current, periodic annealing was required to re-establish this baseline. Raising the temperature (annealing) of the array to 25 K (±5 K) for a minimum time of 1 minute accomplished this. The IRS detectors were annealed as part of the detector check procedure that ran every three to four days of an IRS campaign, including at the beginning after powering up the IRS and at the end just before shutting down. In addition, all IRS arrays were annealed once every 24 hours, prior to taking dark measurements at the start of each Period of Autonomous Operation (PAO).