3 Reduction method

The acquired images are visually sorted, by eliminating those showing important distortions. The selected images are composited (shift and add) with the help of a program using the functions of MIPS (Buil et al. 1993, Mips V. 1.02). At this stage, we verify that the FWHM does not exceed a maximum of 1.2''. To make the measurements of composited images, we used a program of image reduction in C language, written for this purpose. This program was described in a former publication (Salaman et al. 1999). It calculates the position angle, the angular separation and the magnitude difference. In the few cases where the composited image does not show two components obviously separated, it is treated by the wavelet method (Wavelet function of the QMIPS32 program), whose principle consists in analysing the spatial frequencies of the image. The selective extraction of the higher frequencies allows one to separate the components.