Long-term Gain Stability

As indicated in section 4.9, between XRCF Phase I and Ball ISIM Thermal Vacuum tests, the mean detector gains, after correction for effects of CCD and electronics temperature, changed by a mean (for 10 detectors, 40 output nodes) of 0.5%; the RMS fluctuation about this mean was 0.6% per measurement CCD. This is considerably larger than the expected level of about 0.1% per output node (0.05% per CCD). The gain change residuals apper to fall in two groups. In one group (detectors I1,I2,S2 and S3) the mean gain change (XRCF Phase I - Ball ISIM Thermal Vac) is about -1%; in the other group, the mean gain change is less than -0.2%.

The source of these residual gain changes is unknown. It may be that different analog signal chains have different slightly different temperature coefficients of gain. Evidently the effect of DEA temperature, and other factors, on gain must be better understood if long-term gain stability better than 0.5% is to be achieved.