Quote:
Originally Posted by Satchmo
At this point you can move to a knife edge , which will allow you to refocus the apparent shape to pick the best work line to work towards a plane wavefont in the null , which is not really a strategy available with a ronchi grating , which just displays the progression of relative slopes from centre to edge .
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Yes, but the knife edge test is much harder to combine with a Ross null lens.
The Ross null is very sensitive to misalignment and the light source offset needs to be kept very small, especially for fast mirrors.
In fact, it is mainly the residual misalignment, that shows up as astigmatism, is what mainly limits the usefulness of the test in the final stages of figuring.
However, the Ross null is very useful for evaluating the smoothness of the surface.