Top panel: results of the analyzed stellar proper motions, when observing at b = 0 and varying galactic longitudes, with a S/N ≤ 5 for Gaia (marked with crosses) and a S/N ≤ 0.5 for a combination of Gaia and a proposed Gaia successor mission (marked with diamonds), both for G ≤ 20 mag and G ≤ 18 mag (shown with black and red lines, respectively). Bottom panel: results of the analyzed stellar proper motions, when observing at b = 0 and varying galactic latitudes, with a S/N ≤ 5 for Gaia and a S/N ≤ 0.5 for a combination of Gaia and a proposed Gaia successor mission, both for G ≤ 20 mag and G ≤ 18 mag (with same use of crosses, diamonds, black, and red lines as in the top panel). Black and red dashed lines represent the number of QSOs at G = 20 and G = 18 mag, respectively, from the BOSS catalog for comparison.