Illustration of the DDF cadence issues. Upper panel: example of the artificial light curve for the first year of observing with DDF cadence in i band (blue circles) and in r band, contaminated by CIV line. Green points represent the net contamination for ϵ_i = 1.0 (see Eq. (2)). The delay is calculated between the green and the blue points. We adopt standard values of parameters from Table 2, z = 3.276. Middle panel: Histogram of the time separation between the consecutive observation dates in r band in the selected DDF field during the whole ten years. Lower panel: same for the i band.