I spent an hour or so on the phone with Lang discussing his ambitious project to build a model of all the WISE imaging using as a very strong prior the SDSS imaging catalog. The results are beautiful, and will help enormously in SDSS-IV eBOSS targeting. We talked about various issues with doing enormous least-square fits like these: The idea is to believe exactly everything we know about the SDSS catalog, the SDSS PSF, and the WISE PSF, and then just find the set of amplitudes (brightnesses), one per catalog entry, that best explains all the WISE pixels. This method makes use of all the WISE epochs but without co-adding them. It also deals as correctly as possible with overlapping sources, since the fit to all amplitudes is simultaneous. It is very beautiful and is—under brutal assumptions—the Right Thing To Do (tm).

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