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Fig. 11


Fringes from the three baselines. Top, 50   μm piston error on the N primary mirror. Bottom, fringes are centered: the three mirrors are cospheric within a few microns (not possible to have a better adjustment due to atmospheric turbulence). Fringes are subsampled on the pixels of the CCD for larger baselines (N/S, NE/SW) and they appear with a moire pattern. To avoid the moire, fringes are sufficiently stable to be magnified by a factor 10 (using a lens in front of the CCD) but the speckle image would become larger than the CCD. We have checked that the moire pattern does not disturb the accuracy needed to center the fringes (Fig. 12). The red arrows indicate the dominant direction of the oscillations of the fringes on the CCD.

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