In some cases it may not be possible to obtain adequately defocussed images or, the optical system my have some large high order aberrations, such as a severely turned edge on one of the mirrors. Under these circumstances the reliability of wavefronts generated from ef will be compromised. However, by choosing on of the alternative data reduction procedures one may maximize the accuracy of the wavefront under adverse conditions.
The highorder sequence attempts to correct a larger number of Zernike polynomials in the wavefront. This sequence will allow retrieval of accurate wavefronts in the presence of strong high order aberrations. This sequence should be avoided unless the data quality is very high since the reconstructed wavefront can diverge.
The rolled sequence may give better results with extra-focal images which are affected by a rolled edge on the primary or secondary. A rolled edge is only a problem if it causes the edges of the extra-focal images to be within the caustic region. Unfortunately this is often the case, since the curvature on a rolled edge may be many times higher than that of the rest of the mirror. The best solution is to increase the extra-focal distance to make sure the whole image is outside of the caustic region. If this is not possible, the rolled sequence may give more accurate results.