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: