Lily Zhao (Yale), Megan Bedell (Flatiron), and I spoke about Zhao's attempt to understand the variations in the data in the EXPRES precision spectrograph. We have the genius (to us, anyway) idea of using the trace positions in the cross-dispersion direction to calibrate the dispersion direction. Sound wrong-headed? It isn't, because the small number of instrument degrees of freedom that affect the wavelength solution will also affect the trace positions. So they will co-vary. We can even show that they do! The big issue we are having—and we are having great trouble diagnosing this—is that the calibration data (laser-frequency comb, in this case) are offset from the science data in strange ways. Which makes no sense, given the incredible stability of the spectrograph. In our call today, we came up with ways to further investigate and visualize what's going on.
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