Optical filter for x-ray astronomy CCDs

Abstract Charge coupled device (CCD) detectors are sensitive from the X-ray to the NIR (near infrared) region. For X-ray and soft X-ray observations in astronomy, an optical filter has to be placed in front of the CCD to suppress the UV (ultraviolet) and VIS (visible) radiation of stars by more than 6 orders of magnitude. However, this filter must remain highly transparent at photon energies above 100 eV. A prototype filter was developed for a satellite borne CCD X-ray imaging detector. This unsupported filter has an effective size of 30 mm × 10 mm and is composed of multilayered thin films of parylene N, aluminum and carbon with mass densities of 25, 30 and 25 μg/cm 2 , respectively. The fabrication technique used for the filter and measurements of its optical and mechanical properties are presented.