Characterization of two ultraviolet-blue volume-phase holographic gratings based on dichromated gelatin and photopolymer recording materials

Volume-phase holographic gratings (VPHGs) are widely used in astronomical spectrographs due to their adaptability and high diffraction efficiency. Most VPHGs in operation use dichromated gelatin as a recording material, the performance of which is sensitive to the coating and development process, especially in the near-ultraviolet (UV). In this letter, we present the characterization of two UV-blue VPHG prototypes for the BlueMUSE integral field spectrograph on the Very Large Telescope, based on dichromated gelatin and the Bayfol (R) HX photopolymer film as recording materials. Our measurements show that both prototypes meet the required diffraction efficiency and exhibit similar performance with a wavelength-average exceeding 70% in the 350 to 580 nm range. Deviations from theoretical models increase toward 350 nm, consistent with previous studies on similar gratings. We also report similar performances in terms of spatial uniformity and grating-to-grating consistency. Likewise, no significant differences in wavefront error or scattered light are observed between the prototypes.