Magnetorheological finishing of volume phase holographic gratings for high fidelity spectrograph wavefront performance

Volume Phase Holographic Gratings (VPHGs) have become a staple of astronomical spectrograph design over the last several decades. VPHGs commonly employ dichromated gelatin (DCG) as the index modulation medium which provides high efficiency, low stray light, robust environmental protection, and high index modulation – thereby enabling wide spectral bandwidths and angular tuning. However, wavefront performance can be a compromising factor in system design, even when gratings are polished after assembly using traditional polishing techniques. The final diffracted wavefront of a DCG-VPHG is a combination of several compounding factors: substrate homogeneity, surface flatness, holographically recorded wavefront, processing techniques, bowing of the glass during the capping process, and imprinted adhesive wavefront errors, some of which can be mitigated through careful process control. To address the remaining gap in wavefront error relative to other components, we propose custom magnetorheological finishing (MRF). In this paper, we demonstrate the wavefront improvement observed by applying custom MRF to a large diffraction grating (15 x 15 cm). The performance enhancements to be shown promise to improve grating wavefront performance considerably, thus improving overall system performance. The grating used in this study is part of a technology demonstration for the future HARMONI Spectrograph for ESO’s Extremely Large Telescope (ELT). The grating for this wavefront improvement project has been produced as a collaboration between University of Oxford researchers, Wasatch Photonics, and QED Technologies.