Proper motions in NGC6681 globular cluster

Proper motions from HST/ACS for a sample of its stars are already available, and this allows us to construct a distortion-free reference at the epoch of GeMS observations that is used to measure and correct the temporally changing distortions for each GeMS exposure.

In this way, we are able to compare the corrected GeMS images with a first-epoch of HST/ACS images to recover the relative proper motion of the Sagittarius dwarf spheroidal galaxy with respect to NGC 6681. We find this to be (pm_{alpha}_cos{delta}, pm_{delta}_)=(4.09,-3.41)mas/yr, which matches previous HST/ACS measurements with a very good accuracy of 0.03mas/yr and with a comparable precision (r.m.s of 0.43mas/yr). This study successfully demonstrates that high-quality proper motions can be measured for quite large fields of view (85arcsecx85arcsec) with MCAO-assisted, ground-based cameras and provides a first, successful test of the performances of GeMS on multi-epoch data.