self-calibration for stellar abundances

The self-calibration idea is extremely powerful. There are many ways to describe it, but one is that you can exploit your beliefs about causal structure to work out which trends in your data are real, and which are spurious from, say, calibration issues. For example, if you know that there is a set of stars that don't vary much over time, the differences you see in their magnitudes on repeat observations probably have more to do with throughput variations in your system than real changes to the stars. And your confidence is even greater if you can see the variation correlate with airmass! This was the basis of the photometric calibration (that I helped design and build) of the Sloan Digital Sky Survey imaging, and similar arguments have underpinned self-calibrations of cosmic microwave background data, radio-telescope atmospheric phase shifts, and Kepler light curves, among many other things.

The idea I worked on today relates to stellar abundance measurements. When we measure stars, we want to determine absolute abundances (or abundances relative to the Sun, say). We want these abundances to be consistent across stars, even when those stars have atmospheres at very different temperatures and surface gravities. Up to now, most calibration has been at the level of checking that clusters (particularly open clusters) show consistent abundances across the color–magnitude diagram. But we know that the abundance distribution in the Galaxy ought to depend strongly on actions, weakly on angles, and essentially not at all (with some interesting exceptions) on stellar temperature, nor surface gravity, nor which instrument or fiber took the spectrum. So we are all set to do a self-calibration! I wrote a few words about that today, in preparation for an attempt.