Today Lily Zhao (Yale) visualized for me some of the calibration data they have for the EXPRES spectrograph at Yale. What she showed is that the calibration does vary at very high signal-to-noise, and that the variations are systematic or smooth. That is, the instrument varies only a tiny tiny bit, but it does so very smoothly and the smooth variations are measured incredibly precisely. This suggests that it should be possible to pool data from many calibration exposures to build a better calibration model for every exposure than we could get if we treated the data all independently.
Late in the day, we drew a graphical model for the calibration, and worked through a possible structure. As my loyal reader knows, I want to go to full two-dimensional modeling of spectrographs! But we are going to start with measurements made on one-dimensional extractions. That's easier for the community to accept right now, anyways!