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Rubie Mccloughan
Having said that, another option is to just take a bunch of igrams at different tilts (different angles of fringes) which will move the dust around and hopefully get averaged out. DFTF is pretty good at ignoring dust but I think it caused that dip shape in the center of Dale's wavefront (and not something actually on the mirror). When Dale wants a super high resolution map of his mirror he does like 100 igrams! With the mirror at 8 different rotations (so when you back rotate the wavefront the dust errors also rotate around) and with about 12 igrams at each rotation with at least 4 different angled tilts (left, right, up, down for example).
I would rotate the test plate 90 degrees and repeat the test. But with maybe 50 fringes instead of 6. At least 2 more igrams - one at the main rotation and one rotated 90 degrees. To see if the astig rotates 90 also. DFTF has lots of simple features to be able to subtract off any consistent astig that isn't from the test plate.
Spatial filter does help a lot. A few years ago, when the Interferometry Group was still ay Yahoo, one member presented his igrams that ware absolutely pristine looking. He had a compact version of a spatial filter attached to his Bath. Those who have access to Yahoo (I haven't used in a long time) and know how to navigate, should be able to find it. Even Google Images might some of his work.
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