VPHG@INAF: toward large size and diffraction efficiency (Conference Presentation)

Volume Phase Holographic Gratings (VPHGs) have introduced, in recent years, a significant improvement in low and medium resolution spectrographs with an apparent throughput step up. They are still considered the baseline for the DE in low and medium resolution spectrographs. At INAF-Osservatorio Astronomico di Brera, we have been developing VPHGs based on an innovative approach for some years. Almost 15 produced VPHGs are operating in spectroscopic facilities and some prototypes are under production for several spectrographs. VPHGs up to 190 mm in diameter can be manufactured in a wide range of line density. Moreover, innovative configurations, such as multiorder grating and stacked grating, have been developed to increase the spectral range and resolution at the same time. The next steps will be the improvement of the VPHG performances especially in the diffraction efficiency across the bandwidth and the transparency in the UV. Concerning the size of the diffraction elements, a new facility is under building that will be capable to produce VPHGs up to 450 mm in diameter.