μ_b proper motion difference map (panel a) and associated uncertainty map (panel b), showing the difference between the mean μ_b maps of RC2 and RC1. Colour bars indicate the proper motion values for each pixel (panel a) and their associated uncertainties (panel b). Empty pixels represent regions with an insufficient number of stars to compute a value. The position of the supermassive black hole, Sgr A*, is marked by a black star in each panel, and the map sizes are indicated.