In Astronomical Data Group Meeting at Flatiron today, Lily Zhao (Flatiron) asked a great question about how to combine radial-velocity data: Imagine you have a very high-resolution echelle-type spectrograph with many spectral orders. You get a stellar radial-velocity out of every order (or, in some cases, out of many small spectral patches in every order. Now: How to combine those radial-velocity measurements into one, true radial velocity measurement? Obviously you just do an inverse-variance weighted average, no? Well, no! It doesn't work right. There are bad orders, and the only way to know is to see that they make the measurements worse in some end-to-end variance sense. So what to do? How do you empirically determine how to do the combination of data? This problem is simultaneously trivial and impossible. It's a great subject of discussion, and one I've mentioned here previously.
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