intra-pixel issues for astrometry; fiber collisions

[No posts for a while; a short mental-health staycation.]

At group meeting, at Fadely's request, I summarized the #LGAstat meeting. The group was interested in the comparisons of data with simulations, but thought my ideas about re-doing LMC infall inference were dumb! I will keep refining that idea. One question: Do sub-pixel flat issues screw up astrometric measurements at HST resolution? The local-group proper motions are very small, and the instrument is not hugely sampled, so intrapixel issues in the flat could be adding variance to the proper-motion estimator. Added variance generally leads to added kinetic energy and that leads to inferences that the satellite is at pericenter, which is what is being inferred!

Chang Hoon Hahn (NYU) told us about fiber collisions in SDSS-III and IV large-scale structure projects. This problem—That some close pairs of galaxies in the sample only obtain one redshift between them—sounds simple, but is far from it, when two-point (or higher order) statistics are of interest. He showed us the advances they have made by treating the missing redshift as being drawn from a fairly realistic pdf in redshift space. It looks promising, but there are still advances to be made. One interesting thing (that Phil Marshall should love) is that his current correction scheme is like a marginalization over the missing redshift, but marginalizing with a sampling approximation and using only one single sample as the integral approximation!

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