New GRISMs for AFOSC based on volume phase holographic gratings in photopolymers

摘要

Volume Phase Holographic Gratings (VPHG) can provide an improvement of diffraction efficiency and functionality in already available astronomical instrumentation. Here, we present the design, manufacturing and testing of two GRISMs mounted on the AFOSC camera (at the 1.8 m Asiago telescope) based on VPHGs. Such diffraction gratings have been written on a new solid and green sensitive photopolymer material produced by Bayer MaterialScience AG that show interesting performances (refractive index modulation, homogeneity, stability). The GRISMs have been designed according to the specific requests of astronomers. One GRISM consists in a very low dispersion VPHG (285 l/mm) that covers the range 500 - 1000 nm and suitable for observations of supernovae. The second one is a 600 l/mm VPHG for the Ha region. Both gratings show peak efficiency close to 90% and same diffraction efficiency is shown by the corresponding GRISMs. This high device's performances means that the coupling losses are very low, also thanks to the matching of the refractive index between substrates and prisms. Some observations have been finally carried out and the gain in terms of efficiency and signal to noise ratio have been calculated in order to compare the photopolymeric VPHGs with the classic GRISMs already mounted and used in the AFOSC spectrometer.