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Astigmatism in the Bath Interferometer
The beam-to-beam separation in the Bath interferometer causes measurement-induced astigmatism. This problem is greatest for small, fast mirrors.
For mirrors tested at center of curvature
- OPD = D2 d2 / (16 R3) waves
- where D = mirror diameter, d = beam-to-beam separation,
- R = radius of curvature of mirror
- Example: 150 mm f/5, d=8 mm then OPD = 0.042 waves at 633 nm
Notes: