In this foufault test, the knife edge is slowly moved into the beam (twice) in the center of the mirror's radius. This mirror was made to be able to do null-tests on parabolic mirrors, so it needs to be of high quality. It is also the reason why this mirror contains a central hole. The knife is moved with steps of 2.3 micron.

Apart from a slightly turned down edge in the center, you can see no obvious correction zones in the mirror. The black square square shape that you make out is caused by the mirror stand behind the mirror.

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