astrometric distortions

Lang, Roweis, and I spent the full day in Princeton, to talk about and work on extensions and next steps for Astrometry.net. The range of things we talked about was very large, from daily operations to very long-term plans like our conceivable Open-Source Sky Survey.

The problem we talked most about and worked most on was what we call, internally, "tweak". This is the problem of determining the astrometric distortions in an image away from a basic tangent-plane projection of the sky; these distortions are a product of atmosphere and optics and are among the fundamental limitations of cameras. There is a good product out there for determining these distortions called SCAMP (by Bertin and collaborators), and we usually point users to that (we even generate the proper input files that SCAMP wants to see). However, there are issues with all such packages: They need extremely good first guesses to get the correspondences right—the correspondences between sources in the image and sources in whatever catalog is being treated as "truth". Most images that fail for any system fail because of bad correspondences, usually near the edges or in highly distorted parts of the images.

Since linear fitting is trivial, it is not an exaggeration to say that all of the difficulty of fine astrometric calibration is in the finding, managing, and handling of image–catalog correspondences. Even better, we could replace distortion-conveying FITS WCS standards with simply a list of true correspondences (since that encodes all the information we have about astrometric distortions—any polynomial fit to that information is just an approximate description of the information you actually have, which is the correspondences themselves). So we came up with a few ways to nail (in a Bayesian way) the correspondences and hope to implement these in the next little while. (Note how vague I am being; that's because we aren't sure what will work or how well.)

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