Dan Foreman-Mackey (UW) appeared for a few days in New York City. I had various conversations with him, including one in which I sanity-checked my data-driven model for radial velocities. He was suspicious that I can take the first-order (linear) approximation on the velocities. I said that they are a thousandth of a pixel! He still was suspicious. I also discussed with him the point of—and the mathematical basis underlying—the project we have with Adrian Price-Whelan (Princeton) on inferring companion orbits from stellar radial-velocity data. He agrees with me that we have a point in doing this project despite its unbelievably limited scope! Remotely, I worked a bit more on the wide-separation binaries in Gaia DR1 with Price-Whelan.
Not sure from this brief snippet if it's relevant to your discussion (or if it's Dan's entire point), but the velocity shifts of stars, of course, are dozens of pixels because of the barycentric motion of the Earth. The shifts can be much wider than the lines themselves, and cause lines to cross weak, uncataloged (but important!) telluric absorption features along the way. It's the *precision* of that measuring that shift despite the telluric mess and pixel imperfections that needs to be good to 1/1000 of a pixel.
ReplyDeleteAlso, to be pedantic, you're describing the problem of measuring a redshift, not a radial velocity. The transformation between the two is not linear at these precisions (the nonlinear term is ~3 m/s) and filled with nuisance terms.