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The dispersing element is a key element in modern astronomical spectrographs, since it has to provide the required dispersion and resolution with the largest possible efficiency. As a dispersing element, the diffraction grating covers most of the applications, sometimes coupled with prisms both as GRISM or immersed grating (especially for the infrared). Among the different kinds of diffraction gratings, Volume Phase Holographic Gratings (VPHGs) are extremely interesting thanks to some unique features:

  • the large peak diffraction efficiency (theoretically up to 100%) also at large dispersion (line density, G, up to 5000 l/mm);
  • the easy customisation that is often required in the astronomical spectrographs;
  • the robustness;
  • the possibility to make large size elements.

Consequently, VPHGs have become the baseline in the modern astronomical spectrographs working in the visible and near infrared spectral region at low and moderate resolutions. And spectrographs are common workhorse instruments in virtually every observatory. Furthermore, large size telescopes are equipped with several instruments of that type that offer different resolving power and / or wavelength range coverages.