Panels a and b: Histogram of pixel values (black) of the RP blue (left) and red (right) contrast images shown in the middle row of Figure 1. The fit of the sum of two Gaussians to the histogram of contrast values is shown in orange, while the individual Gaussian functions are shown in green and blue. Also shown in red is the 100-point running mean of the histogram. The insets show enlargements of the central parts to highlight the differences between the various quantities discussed in Section 3. In particular, the dashed vertical lines show the location of the central Gaussian function for this particular image (red) as well as the mean (green) and the 1σ interval (orange) from all RP images, respectively. The black dotted vertical line denotes zero contrast. Panels c–f: Temporal variation of the QS level determined with the methods by Preminger et al. (2002, panels c and d) and by Nesme-Ribes et al. (1996, panels e and f) for all RP blue (left) and red (right) observations. The dashed red line marks the zero contrast level. Also shown are PDFs of the uncertainty of the QS bias estimations due to the pixel-by-pixel errors of the image processing as evaluated with a Monte-Carlo simulation (see Sect. 3 for more information). The PDFs are displayed in bins of 1 year and 10⁻⁵ in contrast and they are colour coded as indicated by the colour bars.