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Fig. 7.

image

Scintillation noise (σscint) as a function of airmass. The top panel shows the scatter of the instrumental R- and I-band brightness of a photometric starndard star (SA 113-475) observed with an exposure time of 23 msec at four different airmasses (X = 1.26, 1.7, 2.2, 2.75; small circles) along with the mean values at each airmass (large circles). In the bottom panel the standard deviation of the brightness of the source, in each airmass, is plotted against the airmass with the linear fits to the data provided (see the text for more details).

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