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Thanks Wavytone for your suggestions.
I agree that test plates would be the preferred way but I have no idea how to test a 6" sphere with a 7m radius and prefer to avoid the extra work if possible.
However I may do that for the second surface.
I do own a precision 8" flat that will allow me to test the lens in auto collimation with a Ronchi or Ross Null test, but that would show me the combined errors of both surfaces.
So a combination of one test plate and auto collimation with the flat, should do the trick.
Another possibility I'm thinking of is to use the Bath interferometer on both sides of the lens and compare the result to what is expected from ray tracing. The reflecting (back) surface will dominate the seen error and I should be able to correct it iteratively.
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