#tellurics, day 4

Today was the last day and wrap-up from the Telluric Line Hack Week at Flatiron. What an impressive meeting it was; I learned a huge amount. Here are a few highlights from the wrap-up, but I warn you that these highlights are very subjective and non-representative of the whole meeting! If you want to see more, the wrap-up slides are here.

The most surprising thing to me—though maybe I shouldn't be surprised—was the optimism expressed at the wrap-up. The theoretical modelers of atmospheric absorption were optimistic that data-driven techniques could fill in the issues in their models, and the data-driven modelers were optimistic that the theory is good enough to do most of the heavy lifting. That is, there was nearly a consensus that telluric absorption can be understood to the level necessary to achieve 10-cm/s-level radial-velocity measurements.

Okay maybe just as surprising to me was the demos that various people showed of the Planetary Spectrum Generator that can take your location, a time, and an airmass, and make a physical prediction for the tellurics you will see, even broken down by molecular species. It is outright incredible, and remarkably accurate. It is obvious to me that our data-driven techniques would be much better applied to residuals away from this PSG model. That's an example of the kind of hybrid methods many participants at the meeting were interested in exploring.

One of the main things I learned at the meeting (and I am embarrassed to say this, since in retrospect it is so damned obvious) came from Sharon X Wang (DTM): Even if you have a perfect tellurics model, dividing it out even from your extremely high signal-to-noise spectrum is not exactly correct! The reason is duh: The spectrum is generated by a star times tellurics, convolved with the LSF. That's not the same as the LSF-convolved star times the LSF-convolved tellurics. That is a bit subtle, but seriously, Duh! Foreman-Mackey, Bedell, and I spoke a tiny bit about the point that this subtlety could be incorporated into wobble without too much trouble, and we might need to do that for infrared regions of the spectrum, where the tellurics are very strong. We have gotten away with the wobble approximation because HARPS is high resolution, and in the visible.

And finally (but importantly for me), many of the participants tried out the wobble model or understood it or applied it to their data. We have new users and the good ideas in that code (the very simple, but good, ideas) will propagate into the community. That's very good for us; it justifies our work; and it makes me even more excited to be part of the EPRV community.