2. Observations and reduction
The observations were obtained on August 17, 1990 at the "Turret Dome" of the Pic du Midi Observatory (France) in the quiet sun at the disk center. They consist in a 6 h 40 min time series (between 7:37 and 14:17 U.T.) of photosphere images in the CH band at 4308 Angs. (10 Angs. bandpass), where Network Bright Points are visible and may be used to delineate the supergranule cells. The exposure time on 1454 microfilm (35 mm) was 1/400 s. The time step of 45 seconds, which is the time delay used in the LCT algorithm, gives 533 frames for the entire sequence. The plate digitalization was performed with a MAMA microdensitomer (from the Institut National des Sciences de l'Univers in Paris). The digitization step was 40 microns, corresponding to a pixel size of . The final field of view, after alignment by cross-correlation, was . The images were not filtered to remove the 5 min oscillation.
Bogart et al. (1988) showed that good results can be obtained on large scale surface flows using the Local Correlation Technique (LCT) with a spatial resolution better than . Our data have a resolution lying between to during the entire sequence, which is good enough for flow computation. Horizontal velocities were determined by measuring the local displacements of the intensity field with gaussian masks of various sizes ( and , see below). The vectors of all the displacement fields were also averaged in different temporal windows (7 h , 1 h , 40 min, 20 min).
In Sect. 4, we used the 3-Hour time sequence previously analysed by Muller et al. (1992) for purpose of comparison.
© European Southern Observatory (ESO) 1998
Online publication: January 27, 1998