As my loyal reader knows, Lily Zhao (Yale) has been working with Megan Bedell (Flatiron) and me on how we can parameterize and measure the physical state of a spectrograph, for wavelength-calibration purposes. What she's shown is that the spectrograph is indeed a low-dimensional object, in the sense that the wavelength solutions you find fall in a tiny subspace of all possible wavelength solutions (not surprising). Her calibration method Excalibur (which we are writing up now) is built on these results.
As I mentioned a few weeks ago, if the spectrograph state is low-dimensional, in general any change to the state will appear not just in the wavelength solution but also in anything else we can precisely measure about the device. And we realized that we can precisely measure the locations of the spectral traces in the spatial (as opposed to spectral) direcction. Today Zhao showed very good evidence that such measurements are extremely precise measures (in the EXPRES spectrograph) of the calibration state of the device. Our next step is to explicitly upgrade these to an effective “simultaneous reference” that effectively keeps track of wavelength-solution variations throughout the observing night, making every exposure more precisely calibrated, and reducing the demands for calibration data. If this works, we have potentially provided a simultaneous reference for many spectrographs that don't have one, and made many spectrographs both more precise and more efficient.
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