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Fig. 7


Picture of the metrology table. A large white hood (on the left in the picture) protects the table from the rain. To protect the optics from humidity and wind, a second black hood is also used (open on this photo to see the optics) when we are making observations. It has several holes to allow laser beams to pass through. A green laser is used as a vertical reference (a principle schema is presented Fig. 9) in order to align the metrology gondola. The laser distance meter measures the altitude of the metrology gondola to a precision of  ≈ 1   mm. As described in Figs. 3 and 4, we use the supercontinuum laser to cospherize the primary mirrors; a camera above the 600   mm field mirror near a pupil plane records the white fringed return image (only visible when the primary segments are aligned).

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