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Fig. 7


Position (top) and its derivative (velocity at bottom) of the guiding focal image projected on an axis perpendicular to the fringes of the short baseline. The red and blue curves correspond to periods with small and large amplitude oscillations obtained on oAnd (data selected among the recorded images between 22h57 and 23h00) and γAnd (data selected in the first minute of the recorded images between 02h52 and 02h57). The unit of the vertical position axis is one arcsecond (1 pixel = 0.133′′ on the guiding camera). The error bars are the median seeing at the moment of the observation. The error bars of the bottom graph are the standard deviation (σ) of the velocity points. The mean velocities are 0.2 and 0.1 fringe/ ms for guiding with large and small oscillations, respectively. One pixel of the guiding camera equals 6 fringes for the 5 m baseline. It is expressed in fringe/ ms, but for guiding with small amplitude oscillations, the fringes move probably less than 0.1 fringe/ ms because this value is affected by the atmospheric tip-tilt.

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