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2. Observations

The observations were carried out using the 90 cm Dutch telescope and its CCD camera at ESO La Silla, Chile, between the tex2html_wrap_inline832 of April and tex2html_wrap_inline834 of May, 1994. The ESO No. 29 CCD chip was installed in the camera. It is a chip of the type TEK 512CB, thinned and AR coated for improved quantum efficiency. Its dimensions are tex2html_wrap_inline836 pixels of tex2html_wrap_inline838 microns each. Its linearity goes up to 265000 tex2html_wrap_inline840/pixel remaining always better than 0.5%, which makes it an excellent tool for observing relatively bright targets, as the binaries of our sample.

We have selected a fixed window tex2html_wrap_inline842 pixels wide on the CCD chip and we centered all double stars observed and the photometric standard stars used in this window before starting the exposures. We could save a lot of time reading out only this part of the CCD after each exposure. This improved significantly the efficiency of our observations. The window remained the same all nights. On the other hand, we used the whole surface of the CCD chip for the exposures aimed at its astrometric calibration.

According to the schedule followed, all visual binaries presented here were observed in only one filter during each night. Observing a target in different filters in the same night with CCDs is a time consuming method: as the focal length of the telescope depends on the filter properties, it is obvious that the instrument must be refocused whenever a different filter is used. On the contrary, using the same filter during the whole night increases significantly the efficiency of the observations.

Eight exposures per double star and the extinction star were taken in V in the night of April 26 and four in the night of April 30. The duration of the exposures was as long as possible, at least two seconds and typically around five seconds. Eight exposures per double star and the extinction star were taken in B in the night of April 27 and four in the night of May 1. The duration of the exposures was at least three seconds and typically around six seconds. Finally, only three exposures per double star and the extinction star were taken in U in the night of April 28, since the duration of the exposures was between fifty and ninety seconds. The lower efficiency of the observations during the last three nights is due to the installation and tests of the new data acquisition system of the CCD camera, which is based on a new camera controller and a DEC Alpha workstation computer. We should note that at the 29th of April a lot of time was lost due to major problems of this newly installed data aquisition system of the telescope. In addition, the night was not photometric.

We preferred to use some of the stars in the central region of M 16 open cluster for the estimation of the scale of the CCD. For the calculation of the instrumental position angle of the CCD camera we used traces of stars in M 16 as well, taken on separate CCD frames.

During the first night (of April 25 to 26) 18 photometric standard stars from the E regions were observed, in all filters, for the calculation of the transformation coefficients into the UBV system. Five exposures per filter were taken.

The atmospheric conditions under which the observations were carried out were very good. During all nights seeing at the 90 cm Dutch telescope was varying from 1.2 to 1.5 arcseconds but at the NTT (new technology telescope at ESO La Silla) was usually much better, while the temperature was fluctuating from tex2html_wrap_inline852. Furthermore, the data reduction showed that all nights used for photometric observations were indeed photometric.

By the end of the observations more than 1500 CCD frames were taken. The data reduction was performed by using the MIDAS image processing software, while in some cases, we also used DAOPHOT. It is evident that a bias offset has been subtracted and a flat-field correction has also been performed.

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