introduction, astrometry angular scale

I cranked out an unintelligible, unreferenced, but complete introduction to the post-starburst environments paper. This brings the current version up to what I call a "zeroth" draft—ie, all there but not yet in a form that is ready for the co-authors to read, let alone the public.

Sam and I spent all afternoon playing with SDSS run 745, camcol 3, field 101, looking at its astrometry, the number of USNO-B1.0 stars in the field, and the probability that the field will contain an astrometric index quadruple in our current plan for automated astrometry. It looks like our index ought to make quads with a maximum separation of between 2 and 7 arcmin to have good coverage and good precision when applied to the SDSS data. We realized that if the USNO-B1.0 and all data we ever see is of high precision, the best plan would be simply to index quads at the smallest possible angular scale; the only reason not to do this is that there is positional jitter in both the USNO catalog and any input image, which makes matching more difficult at the smallest scales. Of course at larger scales, camera distortions enter, so it still might be best always to be looking at the smallest available quads.