Spatially synchronous fringe detection: a method to facilitate the alignment of an echelle grating mosaic by separating the compensative angular errors

Abstract

Alignment of mosaic gratings is traditionally supported by two interferometric verifications: on the zero order to verify the grating surfaces and on the blaze to verify the groove direction. In the case of low frequency echelle grating an interferometric measurement on the zero order is hardly feasible due to extremely low contrast of the fringes. The complete alignment has then to be carried out on high order (close to the blaze) where the two misalignment errors (the tip and rotation) show the same effect on the interferogram. The acquisition of a low and a high diffraction order image simultaneously, referred to as the spatially synchronous fringe detection method (SSFD), is used to analyze the misalignment. Iterative adjustment with the autocollimation configuration at the low and high order is used to separate the compensative errors of Δθ x and Δθ z . A prototype mosaic with two 110mm×220mm segments has been aligned with the support of this method. A numerical simulation of the alignment procedure as well as the error orientation analysis of this mosaic grating are presented. The mosaic grating with an accuracy of Δθ x <0.64µrad, Δθ y <1.13µrad, Δθ z <0.65µrad and a wavefront RMS error of 0.149λ has been completed. This method can greatly facilitate the alignment of an echelle mosaic for an astronomical spectrograph.