Top: median standard deviation per orbit, σorb, estimated from test UBr data (see Sect. 5.2) for stars in the Sco I, Cyg II, Cru/Car I, and Cyg/Lyr I fields, obtained in stare (open circles) and chopping mode (filled circles). Colours used for stars in the four fields are explained in the inset. The continuous lines are theoretical relations for average CCD temperature, average aperture, and average number of points per orbit, calculated from Eq. (13). The colours are the same as used for plotting symbols. The dashed line is the theoretical line with the flat-field noise of 0.4 mmag per point added. This line is shown only for the Cru/Car I field. Bottom: the ratio of σorb and , the theoretical values of standard deviation per orbit, calculated for individual sizes of optimal aperture. Symbols are the same as in the top panel.
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