Optical filters for the EPIC CCD camera on board the XMM astronomy satellite

We have developed optical filters for ESA's x-ray astronomy project XMM (x-ray multi mirror mission). Specific CCDs will be used as detectors in the focal plane on board the observatory. Since these detectors are sensitive from the x- ray to the NIR (near infrared) spectral range, x-ray observations require optical filters, which combine a high transparency for photon energies in the soft x-ray region and a high opacity for UV (ultraviolet) and VIS (visible) radiation. With respect to the mission goal three types of flight model filters were designed having different spectral transmittance functions. We report on one of these types, a so-called 'thick' filter, which has been realized to be used as flight filter on board the observatory. This filter attenuates radiation below 10 eV by more than 7 orders of magnitude. It has an effective aperture of 73 mm without any support structure. A 0.35 micrometer thick polypropylene carrier foil is coated with metallic films of Al and Sn. We describe transmission measurements in the soft x-ray photon energy range to determine the thickness of the individual layers and present the optical performance data of the filter.