Higher dispersion and efficiency Bragg gratings for optical spectroscopy

Abstract

Massively multiplexed spectroscopic stellar surveys such as MSE present enormous challenges in the spectrograph design. The combination of high multiplex, large telescope aperture, high resolution (R~40,000) and natural seeing implies that multiple spectrographs with large beam sizes, large grating angles, and fast camera speeds are required, with high cost and risk. An attractive option to reduce the beam size is to use Bragg-type gratings at much higher angles than hitherto considered. As well as reducing the spectrograph size and cost, this also allows the possibility of very high efficiency due to a close match of s and p-polarization Bragg efficiency peaks. The grating itself could be a VPH grating, but Surface Relief (SR) gratings offer an increasingly attractive alternative, with higher maximum line density and better bandwidth. In either case, the grating needs to be immersed within large prisms to get the light to and from the grating at the required angles. We present grating designs and nominal spectrograph designs showing the efficiency gains and size reductions such gratings might allow for the MSE high resolution spectrograph.

Maunakea spectroscopic explorer advancing from conceptual design

“…With respect to our work plan which was presented at the last SPIE conference in 2016, we have made substantial progress: The international design team has completed the enclosure and telescope structure conceptual designs, selected a multiplexing configuration, developed the LMR and HR [16,17] spectrograph designs, and established a fibre testing facility [18] to characterize the fibre transmission system performance. In conjunction, the PO has established the science calibration methodology [19] , system budgets [20] , and observatory operations concept [21,22] .…”

“…Despite concerns in IE losses in focal ratio degradation (FRD) and defocus from the Sphinx spine tilt, the difference in the system-level IE [9] between the two systems after all losses are accounted is negligible. The point spread function variations due to spine tilt will be corrected by the science calibration system using our prescribed calibration plan [17] . 3 Injection Efficiency is defined as the fraction of flux from an astronomical point source that enters a fibre compared to the flux delivered to the focal surface.…”

Mauna Kea Spectroscopic Explorer (MSE): a preliminary design of multi-object high resolution spectrograph

“…The design with improved surface relief technology theoretically gives fantastic diffraction performance, obtaining an average efficiency of high than 90% over the working window. But it is actually in need of more technical confirmation than the VPH grating technology (for details see the reference [13]). The exact grating technology adopted by the MSE HR depends on the further study to confirm the relevant feasibility in manufacture and cost in the ongoing design phase.…”

Improvements in VPHGs for astronomy based on photopolymers

“…Notably, other technologies are fortunately entering the field, in particular binary lithographic gratings that provide a valuable contribution especially for high dispersion DEs. Such gratings can be produced by using lithographic [10][11][12] or holographic 13,14 approach and they show an apparent versatility and large gratings can be produced. Notwithstanding these possibilities, they cannot be considered the final solution.…”