Laboratory and on-sky evaluations of astronomical gratings with non-conventional designs

Abstract

Within the astronomical field, Volume Phase Holographic Gratings (VPHGs) cover nowadays a relevant position as dispersing elements (DE) because each observation could take advantage of specific devices with design and features tailored for achieving the best performances. The manufacturing of highly efficient and reliable VPHGs require holographic materials where it is possible to precisely control the parameters that define the throughput of the device (namely both the refractive index modulation and the film thickness), this is especially true for complex and novel optical designs, where the realization tolerances have to be strictly fulfilled to achieve the theoretical expectations. Moreover, in the design phase, it is crucial to take into account scattering effects and absorption losses to predict with accuracy the final behavior of the optical element. For this application, the most promising materials are the photopolymers because, beside the ability to provide the tuning feature, they bring also advantages such as self-developing, high refractive index modulation and ease of use thanks to their simple thin structure, which is insensitive from the external environment. Thanks to the advantages made available by photopolymeric materials, in this paper we propose innovative solutions for designing stacked spectral-multiplexed VPHGs and transmission dispersing elements (GRISMs) that can cover more than one octave in spectral range by using more than one diffraction order, providing huge advantages for the astronomers in terms of spectral resolution R or time allocation of the instrument for performing the observations. In this context, we also give hints for the process optimization that is crucial for achieving the results reported.