Orbital Phasing of Sensitivity Monitors
The 6075 program is unique in its manner of obtaining data, and there is not an analogous set of observations to which to compare. However, there are some ways of seeing if similar effects are seen in similar circumstances.
First, we noted that the GHRS Sensitivity Monitor program observes the same star at the same wavelengths four times per year. Examination of the details of that program showed that the same spectrum (at 2000 Å, say) was obtained in very different parts of an orbit during different executions of the monitor. The phenomenology of the Gilliland effect suggests that it starts fresh at the beginning of each orbit, perhaps, or possibly with each Exposure Logsheet line. Thus observations of the same star at the same wavelength may be expected to differ by as much as 10% if obtained in different parts of an orbit.
The differences are, in fact, much smaller (roughly 2%) and are systematic in nature, due to the steadily declining sensitivity of the GHRS. Whatever is causing the Gilliland effect does not appear to be tied to orbital phasing.
Engineering Parameters
J. Skapik has examined the engineering data from the 6075 observations to search for correlations with the declining count rates. None are seen that were not already known to vary over an orbit, and the effects of those thermal changes are reasonably well known and are not likely causes of this effect.
Additionally, spacecraft jitter was investigated once we were able to obtain the jitter files. A bug caused the software to think all the 6075 data were internals and jitter files were not created until Rick Perrine solved the problem. As expected, looking at the jitter over time does not show a significant change in V2 or V3 position except for a few large spikes in V3 which do have corresponding spikes in the data. See Figure 6.
Detector Hysteresis
One aspect of GHRS operation recently came to our attention that appears to be directly correlated with the observed effect. At the beginning of an observation, the GHRS detectors remove hysteresis by running at full and zero deflections. This aspect of operation is only briefly described in SE-01 (the system description for the GHRS). As we have noted, image motion cannot account for the drop in counts because the effect would be lopsided, but conceivably other effects of hysteresis on the detector could lead to an overall sensitivity change. However, no mechanism exists to account for such and effect.