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2 Observations

Data were recorded during three observing runs in December 1995, January 1997 and June 1997 at the TBL. We had quite poor weather conditions with very short coherence times of a few milliseconds only and a FWHM seeing often larger than 2 arcsec. A total of 33 double stars and 15 multiple stars was observed; details are given in Table 1.
 
Table 1: Table of measurements. $\lambda$ denotes the wavelength and $\Delta\lambda$ the bandwidth. The column labelled Mult. gives the number of stars in the system (multiplicity). The column labelled Obj. give the name of the measured components of the multiple star. Predicted values of $\rho$ and $\theta$ are computed from the latest available orbits. For orbits prior to 1982 (followed by a $\star$), orbital elements were found in the catalogue of Worley & Heintz, 1983. For $\epsilon$ Hya the angular separation ABxC was computed from AB, AC and $\Delta m_{AB}$as the distance between C and the photocenter of AB. The columns labelled $\Delta\rho$and $\Delta\theta$ give the residuals in $\rho$ and $\theta$
\begin{table}
\includegraphics []{tab1a.eps}
\end{table}


 
Table 1: Table of measurements (continued). For observations made with the PAPA detector, a $\dagger$ has been added in superscript to the exposure time (usually 2 ms). (*) $\nu$ Cyg was in fact discovered as double and then not used as reference star in the processing of BU 151
\begin{table}
\includegraphics []{tab1b.eps}
\end{table}

The focal instrumentation is the speckle camera of the Aperture Synthesis group of Observatoire Midi-Pyrénées (OMP). For the major part of the nights it was coupled with the ICCD detector of Nice University. This instrumentation is described in Aristidi et al. (1997b) and Prieur et al. (1998). During the runs in January 1995 and January 1997, the images were recorded on video tape for further processing. In June they were also sent to a PC hosting a Matrox Genesis digitizer board equipped with a digital signal processor (C80) which enables near real-time processing. As an example the power spectrum for a 128$\times$128 image size is computed at a rate of about 20 frames per second. The use of a reference star for usual calibration of the telescope+atmosphere transfer function was sometimes avoided by computing the cross-correlation between time-separated images and subtracting it from the autocorrelation (Worden et al. 1977). Though at a lower speed (9 frames/s) the system is also programmed to compute the cross correlation between the images and their square in order to find the absolute position angles of binaries (Aristidi et al. 1997a). This system does not include yet the classification of images according to the seeing that is used in the data processing as described in Aristidi et al. (1997b). Bright stars ($m_V\mathrel{\mathchoice
{\vcenter{\offinterlineskip\halign{\hfil$\displaystyle ... ) were re-processed from the video tape for magnitude difference determination using probability imaging (Carbillet et al. 1998a). This provides also the absolute position angle (PA) and is useful to confirm the PA computed by the cross-correlation technique (Aristidi et al. 1997a).

Problem of saturation is took into account the following way: before recording frames, the video digitizer displays saturated pixels, allowing the observer to adjust the gain and the offset of the detector to reduce the saturation. Moreover, at processing time (probability imaging), pixels at the maximum intensity level are rejected from the calculation of the magnitude difference.

The PAPA camera was used during part of these observations but because of technical testing and bad weather conditions it could only lead to measurements on the night of 23/06/97. It is actually a new version of the original camera described in Papaliolios et al. (1982, 1985). Modifications have been jointly implemented by P. Nisenson (Harvard Center for Astrophysics), D. Gezari (NASA) and L. Koechlin (OMP) in the last five years. The current version has a new binary mask setup and a refurbished image intensifier. A problem in the regulation of the high voltage power supply caused a strong geometric distorsion and a variation of the overall scale which imposed quasi permanent scale calibrations during the night. A small "hole'' at the center of the autocorrelation function was also noticed, due to a lack of detectivity of the electronics after each photo-event detection. A whole reduction procedure had to be set up in order to correct for these defects:


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