The Blue Multi Unit Spectroscopic Explorer (BlueMUSE) on the VLT: characterization of two VPHG prototypes based on dichromated gelatin and photopolymer recording materials

TL;DR

This paper characterizes two UV-blue volume-phase holographic gratings for the BlueMUSE spectrograph, demonstrating they meet efficiency requirements and show consistent performance, with detailed analysis of materials and optical properties.

Contribution

It presents the first detailed comparison of dichromated gelatin and photopolymer VPHGs for BlueMUSE, including efficiency, uniformity, and wavefront error assessments.

Findings

Both prototypes exceed 70% diffraction efficiency in 350-580 nm range.

Performance deviations increase below 350 nm, aligning with previous studies.

No significant differences in wavefront error or scattered light between prototypes.

Abstract

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, whose performance is sensitive to the coating and development process, especially in the near-UV. In this letter, we present the characterization of two UV-blue VPHG prototypes for the BlueMUSE integral field spectrograph on the VLT, based on dichromated gelatin and the Bayfol$\circledR$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 350-580 nm range. Deviations from theoretical models increase towards 350 nm, consistently with previous studies on similar gratings. We also report similar performances in terms spatial uniformity and grating-to-grating consistency. Likewise, no significant differences in wavefront error or scattered light are observed between the prototypes.