Relative frequency of observations in the various combinations of window class and gate, as a function of magnitude. The four blocks represent the four window classes (WC); within each WC the eight stripes represent (from top to bottom) gate number 4, 7, 8, 9, 10, 11, 12, and 0. The graph was constructed from a random 1% sample of the AF observations of the primary sources. The faint sources observed in WC0a at gate 0 are the Calibration Faint Stars, a small fraction of faint observations receiving full-pixel resolution windows for calibration purposes (Gaia Collaboration 2016a).