<scp>HubPUG</scp>: proper motions for local group dwarfs observed with <i>HST</i> utilizing <i>Gaia</i> as a reference frame

We present the method behind HubPUG, a software tool built for recovering systemic proper motions (PMs) of Hubble Space Telescope (HST) fields with two epochs observations by utilizing stars observed Gaia as foreground frame reference. HST PM experiments have typically relied on use distant background galaxies or quasi-stellar objects (QSOs) stationary sources against which to measure PMs. Without consistent profiles, are more difficult centroid, but benefit on-aggregate from their large numbers. QSOs, though they can be fit stellar point-spread functions, sparse, most containing none. Historically, references PMs would been because individual own. However, has now provided positions and over 1.4 billion stars, much likely well-imaged in around targets versus sources, predictable require less observing time per-image good signal-to-noise. This technique allows us utilize power too faint observe itself. recovered Milky Way satellites Sculptor Draco comparable uncertainties HST-only Gaia-only measurements, limited primarily current capabilities data. also show promise this M31 new measurement Andromeda VII.