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Artificial Nulling
When a paraboloidal mirror is tested at its center of curvature, the measured wavefront will have a non-zero Z8 component (principle spherical aberration):
- Z8 = D4 / (384 R3)
- where D = diameter, R = radius of curvature, = wavelength
FringeXP uses this information to remove the desired correction from the data
Notice the strong dependence on the diameter – a small error in defining the pupil can make a significant error in accuracy
- Example: 300 mm f/5 mirror, if D/D = 1% then Z8 = 1/20
- (unfortunately, we’re no better off with Foucault )
Artificial nulling has accuracy issues for large, fast mirrors.
Notes: