If I'm understanding correctly, you used the Bath in conjunction with the Ross Null lens ? If so that makes sense that you got round fringes of just about equal spacing. I was puzzled at first and thought that you were testing a spherical mirror since a f/4 parabola without the Ross Null would show a wild fringe pattern. You might want to try a Biconvex lens ( equal radii on both sides) instead of the Plano-convex lens in the Bath, since it will have less abberation. You also need a good micrometer stage to adjust the positions. That will easily allow you to reduce teh number of fringes. Bath's are a fun device and having the ablity to do interferometry for next to nothing is a huge plus.
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Re: Foucault tester questions
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