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Wavefront sensing

Some somewhat confusing results were obtained. Last summer a significant astigmatism was noticed in the NOT primary mirror, and it was found that this had probably been introduced after the aluminisation of the mirrors in 2002. The astigmatism was corrected to pre-aluminisation levels by applying suitable corrections to M1 in the active optics system. In March 2004 a couple of hours of WFS measurements were made. As before, only significant variations in aberration with zenith distance was found for spherical aberration and especially astigmatism. However, the variation in astigmatism had a negative dependence on zenith distance instead of the positive dependence found before.

Part of these differences might be caused by a different alignment of the mask used for the measurements. Some tests were made in early April to try and verify the results but the data are inconclusive and further more extensive measurement are planned.

We are currently also looking into the possibility of getting assistance in analyzing the WFS data from an optical engineer at the CUO.

Thomas Augusteijn 2004-05-18