Sky intensity fluctuations on time scales from tenths to tens of seconds (u′, g′ and r′ photometric bands). A section of the light curve of G 185-32 (18 May 2005), with a duration of 1 min, is represented in the upper panels, while the reference star is shown in the central panels. The average counts of each integration (8.8 ms) are reported. The lower panels show the intensity ratio (target’s counts divided by reference star’s counts). This figure shows that on time scales longer than about 1 s it is essential to have a reference star in order to reduce the sky fluctuations. However, for shorter time scales, the light curves of the target and the reference star are not coherent anymore, as we can see from the small panels representing a short section (only 2 s) of the same light curves. Note that the u′ band has a different vertical scale in all panels.