Sensitivity of two-channel Ronchi test to grating misalignment

The Ronchi test is performed by placing a periodic grating at or near the focus of an optical system. Like most geometric tests, the Ronchi test measures wavefront slope instead of the contour of the wavefront like conventional interferometry. A two-channel Ronchi test with a horizontal and a vertical grating can be used to reconstruct a non-symmetric test surface. This paper investigates the errors introduced by different rotational and translation misalignments of the gratings in the two-channel Ronchi test relative to one another. The Ronchigrams are analyzed with a modal technique based on Zernike polynomials. The severity of each misalignment is dependent upon the frequency of the gratings used and on the f/# of the system.