The ratio PREMOS-A:PREMOS-B (thick black circles with error bars, labelled 1–6), as plotted in Figures 2b and 3a, with hourly average ratios when there is overlap for PREMOS-A:PMO6-V/SOHO and PREMOS-A:DIARAD/SOHO (yellow and blue dots, respectively). Six-hourly averages for PREMOS-A:DIARAD/SOHO (dark-grey circles) are given for the period between the grey vertical lines labelled “Heater on” and “Heater off”. PREMOS-B:DIARAD (red dots) is plotted after 3.746 days of PREMOS-B exposure. Exposure time is applicable to PREMOS-A or PREMOS-B, depending on which is the numerator. All ratios are shifted by the value indicated in the legend. Thick grey circles with error bars represent an exposure time estimate based on Lyman-α (see Sect. 4).