2 Image acquisition

As we did for the measurements in 1997 (Salaman et al. 1999), the images were acquired with a resultant focal length of 15.349 m obtained thanks to a Barlow lens 2$\times$.

This focal length was checked on wide pairs measured by the Hipparcos satellite. The CCD camera was a Hi-SIS22, already used for the measurements of binaries with the equatorials of the Nice Observatory (Gili & Couteau 1997). This camera consists of a $768 \times 512$ square pixels detector. The side of the pixel is $9~\mu$m and its field on the sky is 0.12''. The theoretical resolving power of the instrument (1.22 $\lambda/D$) is 0.33'' (at the 0.68 $\mu$m wavelength, the highest sensitivity of CCD sensors). The program of acquisition was QMIPS32 (Buil et al. 1997; QuickMips32 V. 1.8). For each binary, 7 to 20 images were recorded with integration times ranging from 0.05 to 1 s. The atmospheric conditions were not very good. The seeing, expressed by the FWHM of the long exposure stellar image intensity profile, often exceeded 1'' which only allowed acquisitions of images about every 2 nights. During our two missions in 1998, from May $14^{\rm th}$ to $26^{\rm th}$ and from September 14$^{\rm th}$ to 27$^{\rm th}$, the images of 65 binaries were acquired.