ransac

I worked on astrometric tweak today—the code that does precise astrometric calibration following rough calibration by our blind system at astrometry.net. Mierle convinced us at the astrometry.net meeting that we should be using the ransac algorithm for tweak, and I have become even more convinced since then. Ransac is much better than the astronomer's usual tool of iterative sigma-clipping, especially when (as Mierle advocates) the inlier/outlier decision is made by fitting a model to the residuals that consists of a gaussian core of inliers and a flat distribution of outliers, superimposed.