Top: average counts per u′-band observation (averaged over 1–3 background-corrected exposures) of the three reference stars. These are measured with a 10″ radius aperture. There is a loss of ∼9% in measured counts around the sunblock period (approximately 2020.2–2020.4) due to air masses as high as ∼5. Bottom: three stellar light curves normalised to reference star three. These are obtained by differential photometry. To correct for the zero point, we added the SDSS magnitude for reference star three. We subtracted 0.8 magnitudes from reference star three in this plot for readability.