review, COSMOS astrometry

I worked on my Marseille review.

Burles assigned me the task of re-solving the astrometric WCS for all of the HST/COSMOS images, because (as he rightly pointed out) this is a perfect overlap of the PRIMUS and astrometry.net projects (see sidebar). I worked out the necessary changes to our system to make this happen. The big deal is that if you want high precision, you can't just fit to USNO-B1.0, you have to propagate faint sources in the HST imaging onto the celestial sphere, and simultaneously fit for (1) the pointing and rotation of each ACS exposure, (2) the pixel scale and camera distortions, and (3) the sky coordinates (RA, Dec) of all the faint sources that appear in any two exposures. Though some people have implemented such systems (it's not rocket science), no-one, to my knowledge, has a general, plug-and-play system. That's one of the long-term goals of astrometry.net, of course.