predicting wavelength calibration from housekeeping data

I did some of my favorite thing today, which is fitting flexible models. The context was my attempt to predict the wavelength solution for the SDSS-IV BOSS spectrographs using only the housekeeping data, like the state of the telescope, temperatures, and so on. It doesn't work accurately enough, or at least not with the housekeeping data I've tried so far. It looks like there might be a hysteresis or a clank or something like that. If this is right, it bodes poorly for reducing the number of arcs we need to take in SDSS-V, which is supposed to move fast and not break things.

But all that said, I still have one card left to play, which is to see if we can look at sky lines in science frames and learn enough from sky lines—plus the historical behavior of all arcs ever taken—such that we can lock everything else down without taking an arc for every visit.