De la conception instrumentale
à l’exploitation des observables


Andrea Bianco

The dispersing element is one of the key optical element in modern spectrographs for astronomical application. Indeed, it defines the dispersion, resolution of the spectrograph. Moreover, it is probably the less efficient component in the instrumentation ; therefore, it is crucial to have a dispersing element with high diffraction efficiency.
In this framework, Volume Phase Holographic Gratings (VPHGs) have become the reference technology in the last years especially for low and medium resolution spectrographs in the 0.3 – 2.5 microm spectral range thanks to their very high efficiency (more than 90%), large dispersion and size, easy customization. They have also been used as cross-disperser in echelle high resolution spectrographs such as ESPRESSO@VLT. For these reasons, spectrographs like MUSE and FORS2 @ VLT, HERMES @ AAT, etc. In the near future, MOONS@VLT, WEAVE@WHT, 4MOST@VISTA will be equipped with such gratings, as well as MOS spectrographs for the EELT (Harmoni).
The standard technology that makes possible the realization of VPHGs is complex since it requires both holographic exposure and chemical developing process. To overcome this limitation and to pave the way to other non-conventional applications of VPHGs, we are studying new materials and processes based on self-developing photopolymers.
By exploiting such new technology, we developed conventional VPHG based dispersing elements for small facilities and medium facilities and we are studying new configurations based on multiplexed VPHGs that allow for the recording of different spectral ranges or a very wide spectral (0.35 – 1 m) range in one shot. In this way, it is possible to boost the overall performances of the spectrograph, saving observing time or improving the S/N of the recorded spectra.
Finally, the main outcomes of the workshop : “Dispersing elements for astronomy : new trends and possibilities” held in Milan (October 9-11, 2017) will be reported showing other emerging technologies such as lithographic gratings and making a comparison with the VPHG technology.