I finished the faint-source proper-motion paper. It still needs to be vetted by collaborators, but I am stoked. Here is the abstract:
The near future of astrophysics involves many large solid-angle, multi-epoch, multi-band imaging surveys. These surveys will, at their faint limits, have data on large numbers of sources that are too faint to detect at any individual epoch. Here we show that it is possible to measure in multi-epoch data not only the fluxes and positions, but also the parallaxes and proper motions of sources that are too faint to detect at any individual epoch. The method involves fitting a model of a moving point source simultaneously to all imaging, taking account of the noise and point-spread function in each image. By this method it is possible—in well-understood data—to measure the proper motion of a point source with an uncertainty (found after marginalizing over flux, mean position, and parallax) roughly equal to the minimum possible uncertainty given the information in the data, which is limited by the point-spread function, the distribution of observation times, and the total signal-to-noise in the combined data. We demonstrate our technique on artificial data and on multi-epoch Sloan Digital Sky Survey imaging of the SDSS Southern Stripe. With the SDSSSS data we show that with this technique it is possible to distinguish very red brown dwarfs from very high-redshift quasars and from resolved galaxies more than 1.6 mag fainter than by the traditional technique. Proper motions distinguish faint brown dwarfs from faint quasars with better fidelity than multi-band imaging alone; we present 16 new candidate brown dwarfs in the SDSSSS, identified on the basis of high proper motion. They are likely to be halo stars because none has a significantly measured parallax.