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Hi Rod,
The test does contain artefacts as it is based on a small number of interferograms. If we take that into account then yes, most likely the central area up to 35m radius is about 1/16 of a wave on the wave front. If we include all the "noise" then 1/10 wave peak to valey should be read.
This was not an easy testplate to make, even though it is spherical, simply because it is f/2.6 and hard to see the full surface with a classical test aparatus. Something like an 80mm diameter would had been a better size, much easier to make.
For the secondary, if you can take good on axis images of the fringes then DFTF has no problem analizing them. You will need a good Fizeau interferometer so that the fringes are produced and seen in parallel light.
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