The analysis and optimization of the spatially synchronous fringe detection method (SSFD) to remove angular misalignment in echelle mosaic assembly

Abstract

The grating mosaic provides an aperture large enough to meet the resolution requirements of the astronomical spectrograph operating above the R > 100,000. The interferometry technique is deployed to maintain the accurate alignment of the mosaic. To separate the compensative errors of the mosaic and avoid the weak light detection of zero-order diffraction, the spatially synchronous fringe detection method (SSFD) is proposed. The fringe pattern of the misalignment is simulated and the alignment sequence is analyzed and optimized. An experimental mosaic of two 25 mm × 50 mm. 31.6 grooves/mm echelles are aligned to verify this method. The peak-valley(PV) and root-mean-square (RMS) values of the measured diffraction wavefront errors are 0.423 λ and 0.034 λ, respectively. The proposed method facilitates the mosaicking of all echelle gratings.