The use of gratings as optical elements is proving to be a powerful technique in the metrology of unusual optical surfaces such as cylindrical X-ray optics, cone-shaped optics for laser ring resonators, and highly aspherized conventional optics. However, the tremendous flexibility for the optical designer is obtained at the cost of unusually tight tolerances in the areas of grating fabrication, interferometer alignment, and interferometer environment. In this paper, a review is given of the use of gratings in optical metrology, followed by derivations and discussion of these various tight tolerances.
"Systems Considerations for Diffractive Null Correctors (DNC's)", Proc. SPIE 0560, Diffraction Phenomena in Optical Engineering Applications, (10 May 1986); doi: 10.1117/12.949619; https://doi.org/10.1117/12.949619