Left: standardized CC derived from our method (red, extrapolated to the range of values of the whole disc), measured CC for the QS (orange) with 1σ uncertainty (black), the whole background (blue), calibration wedge measurements and fit (green rhombuses and line, respectively) of Ar observation taken on 20/07/1948. Shown also is the slope of the derived CC. Right: distribution of densities for the QS (blue), AR (red) and whole disc (black). The horizontal dashed line in both panels denotes the highest value of the QS CLV.