Zeroth-Order Efficiency Optimization of the GI Grating for the CSST Slitless Spectrograph

Abstract Redshift measurement uncertainty by direct reference of zeroth-order images of the CSST slitless spectrograph GI gratings is evaluated. Total optical throughput ratio of zeroth-order image with respect to its first order science photons is much lower for GI gratings compared with GU or GV gratings. Potential optimization of the GI gratings are investigated to enhance zeroth-order throughput without compromising their first-order scientific performance. Rigorous Coupled-Wave Analysis (RCWA) shows that groove profiles optimization could boost a gain factor of 2.5 in GI band zeroth-order image photons while preserving its first-order efficiency above 55%. Superior balance is achieved between centroid precision and spectral efficiency. Point source Signal-to-Noise Ratio (SNR) of 20-22 magnitude in I band demonstrate significant boost in zeroth-order image, yielding a centroid precision better than 0.353 pixels at 22 mag. A novel optimization strategy of grating parameters is reported as well.