Optimized fused silica used in new astronomical applications

Being able to survey the sky at ultraviolet and near-infrared wavelengths, astronomical spectrographs can study objects almost impossible to see in visible light alone. Very low light intensities of small objects require single-photon sensitivity. The unique optical properties of fused silica, e.g., highest transmission, best homogeneity, and lowest absorption, enable next-generation observations, space-bound as well as earth-bound. However light absorption in the range >800 nm limits the performance of IR spectrographs when using standard fused silica. Metallic impurities and OH groups embedded in the fused silica matrix need to be reduced to achieve best operation. The MOONS camera system at ESO's Very Large Telescope observes the universe in the NIR range using low OH synthetic fused silica lenses up to a diameter of 900 mm. In the UV spectrum of 160-350 nm, standard fused silica performs in earth-bound systems extremely well. However, in space-bound applications, radiation damage reduces the transmissivity over time. Radiation hard fused silica windows for the PLATO M3 mission of the European Space Agency (ESA) protect the sensitive telescope from damage due to solar protons. Key optical instruments require calibration of the light source to normalize spectroscopic measurements. A radiation resistant fused silica diffusor with Lambertian scattering behavior supports ESA's Copernicus program in the Sentinel 5 satellite.