A variable shear lateral shearing interferometer consisting of two holographically produced crossed diffraction gratings is used to test nonrotationally symmetric wavefronts having aberrations greater than 100 wavelengths and slope variations of more than 400 wavelengths/diameter. Comparisons are made with results of Twyman-Green interferometric tests for wavefront aberrations of up to thirty wavelengths. The results indicate that small wavefront aberrations can be measured as accurately with the lateral-shear interferometer as with the Twyman-Green interferometer and that aberrations that cannot be measured at all with a Twyman-Green interferometer can be measured to about 1% accuracy or better.

Wavefront shearing interferometers have inherent advantages over more conventional interferometers because they do not need a separate reference wavefront. However, the fringe patterns are less directly related to the wavefront shape. In this paper, a method is described that uses data obtained from two lateral shear interferograms sheared in orthogonal directions to describe a wavefront of any arbitrary shape. Analysis of the data defines the wavefront on a regular grid, using a least squares criterion to match the measured data to the reconstructed shear data. Because each point on the final wavefront is involved in at most four measurements, the matrices tend to have many zero elements, making them easily solvable by simple numerical techniques, even for several hundred points. An error analysis indicates that the accuracy of the final results can be as good as the accuracy of the measured data. The procedure is described, and results of a typical analysis are shown.

The properties of two-conic reflecting aplanats are analyzed and discussed on the basis of third order aberration theory. Techniques for designing infinite conjugate two mirror aplanats and computing their image properties are developed. The secondary mirror alignment characteristics of Ritchey-Chrétien and aplanatic Gregorian telescopes are examined and neutral point locations defined. Design configurations corrected for a third Seidel aberration (astigmatism, image curvature, or distortion) are identified and their properties discussed. The properties of Ritchey-Chrétien and aplanatic Gregorian telescopes are compared.