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2 The observations

2.1 Site and instrumentation

Our observations were carried out at the Kryonerion Observatory of the National Observatory of Athens, Greece, in four clear but not really photometric nights, between June 25 and 28, 1998.

We used the Photometrics CCD camera of the observatory, attached to the Cassegrain focus of its 1.2 meter telescope. Its back illuminated and thinned chip has $516 \times 516$ square pixels of $24\mu$ size. We refer to D. Sinachopoulos et al. (1998) for a detailed discussion of the CCD chip. One exposure per CCD frame was made, each having a duration of a few seconds. Four sequential CCD frames were taken per double star observed.

A Bessel V filter was used for all exposures. We observed targets with both components fainter than magnitude 8. Given the relatively small angular size of the pixel and seeing conditions, as they are discussed below, we did not need to use a density filter.

Most observations were made very close to meridian, in order to minimize the effect of atmospheric refraction.

The reduced data of our observations in the two nights of June 27 and 28 indicate that these two nights were partially photometric.

2.2 Seeing conditions

We monitored the dome seeing during our observations, defined as the full width at half maximum (FWHM) of a well exposed, and therefore symmetric, stellar image on a CCD frame. It was found $2.5\pm 0.5$ arcseconds in the first night, $1.8\pm 0.5$ in the second, $1.4\pm 0.3$ in the third, and $1.3\pm 0.3$ in the last one. Figure 1 presents seeing development through the four consecutive nights and Fig. 2 its distribution in the same period. Remark that the pixel size is 0.30 arcseconds.

Temperature changed only up to two degrees around the same mean value (19 degrees) each night.

\includegraphics [angle=-90,width=14cm]{ds8188f1.eps}\end{figure} Figure 1: Seeing (FWHM) in four nights
\includegraphics [angle=-90,width=7cm,clip]{}\end{figure} Figure 2: Seeing (FWHM) measurements

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