#mwmw, day 1

Today was the first day of the Milky Way Mapper Workshop, at the Max Planck for Solar System Research in Goettingen. The meeting is about points of target selection, operations, commissioning, and planning. I am very excited about Milky Way Mapper, which is part of the SDSS-V family of projects; it will take infrared and optical spectra of millions of stars. From my perspective a few important things happened at the meeting (note the subjectivity and unfairness of this; it is not a summary):

MWM will operate in a robotic mode, with robotic fiber positioners. This permits us to observe enormous numbers of stars, but it means that our default calibration strategy of arcs and flats between exposures that we have used in SDSS through SDSS-IV will not be tenable. That's good! Because it causes us to do some commissioning work at the start where we quantitatively analyze the calibration strategy.

We discussed principles underlying target-selection in our various target categories. Hans-Walter Rix (MPIA) and I intend to write a general paper for the astrophysics community about this question, because there are some hard-won lessons from previous projects and things we and others have done wrong. I will say more about this in future blog posts as I try to write some of it up, but the extremely important underlying principle is the likelihood principle: If information comes through the likelihood function, then you have to select your targets such that, at the end of the day, you can write down a computationally tractable likelihood function for the parameters of interest. That's perhaps a Duh! point, but I'd like to point out that many of the complex, multi-stage projects (like RV surveys, or time-domain follow-up spectroscopic projects) fail to meet this requirement! More on this over the next weeks.

I learned a few crazy simple things today. One is that SDSS-IV APOGEE is taking multiple hot-star standards per plate! That means that the survey has, through its calibration work, created a huge time-domain survey of hot stars over a huge part of the sky. That's pretty important for science. And at this point, they have not been fully exploited as a scientific project. It's many thousands of stars!

Another crazy thing is that the SDSS projects have obtained enormous numbers of white-dwarf spectra, sometimes deliberately and sometimes by accident. These cover large parts of the white-dwarf sequence in ESA Gaia data, and this sequence contains lots of informative and intriguing structure. That suggests an interesting Gaia Sprint project.