Effects of Proton Irradiation on Glass Filter Substrates for the Rosetta Mission

The Optical, Spectroscopic, and Infrared Remote Imaging System (OSIRIS) instrument on board the Rosetta spacecraft, a mission of the European Space Agency to comet P /Wirtanen, includes two cameras for acquiring images of the comet. A set of interference filters deposited upon glass and fused-silica substrates will be added to the cameras for wavelength tuning. For this mission of more than 10 /years in an interplanetary environment, the requirement of preserving the optical characteristics of the filters is a critical one. We checked the variation in the transmission of some filter substrates after proton irradiation that simulated the solar wind. To produce a situation that is representative of the interplanetary environment, we irradiated proton fluences at three energies: 1.5 * 10^11 protons /cm^2 at 4 MeV, 1.9 * 10^10 protons /cm^2 at 8 MeV, and 7.1 * 109 protons /cm^2 at 18 MeV. Seven substrates were tested: three Suprasil-1; three colored glasses, namely, OG590, KG3, and RG9; and one quartz. In addition, two interference filters were checked. The results obtained show that Suprasil-1 is rather insensitive to this irradiation, whereas very small reductions in transmission, of the order of a few percent, occur for colored glasses. The transmission of these filters was remeasured 2 years after the irradiation, and showed a general decrease in the transmission reduction.