Zernike polynomials and Classical Aberrations

Aberration Comment
Defocus Defocus is measured with respect to the focal plane of the Shack Hartmann wavefront sensor. Our wavefront sensor analysis software Sensoft gives precise indications for correcting the defocus.
Tilt Tilt is measured with respect to the center of the reference frame.
Coma This is the on-axis coma, and includes the coma due to the shape of the optical elements as well as due to their misalignment. The correction achieved using the diagnostics from Sensoft will correct the coma due to both effects. See how coma originates due to the misalignment of optical elements.
Spherical aberration Can arise due to the use of the wrong focal plane (or, equivalently, the wrong spacing between the elements). It can also be due to the wrong shapes of the optical elements (conic coefficients). Sensoft suggests by how much the focal plane should be shifted to correct the observed SA. Temperature effects can also play a role.
Astigmatism Astigmatism can be inherent in the optics. It can also be due to support problems, as any imbalance in the forces applied by the supports will result in astigmatism. To be used in conjunction with the wavefront (or mirror surface) plot to identify which supports are not correctly adjusted. The orientation of the optical elements can be changed relative to each other to minimize this aberration.
Triangular Coma If this has a high value, look for problems with the 3 fixed points.
Quadratic Astigmatism A high value indicates the presence of support problems with 4-fold symmetry (like the 4 lateral fixed points).