Beuermann K P, Bräuninger H, Trümper J
Appl Opt. 1978 Aug 1;17(15):2304-9. doi: 10.1364/AO.17.002304.
The aberrations of various types of curved constant-period transmission gratings are discussed, using Fermat's principle. We show that optimal performance is achieved with a grating assembled from individual facets which form a nonclosed surface. For this geometry, primary coma and astigmatism are proportional to the facet size and, hence, may be adapted to the angular resolution of a Wolter-type telescope.
