Wide-band transmission response of the filters for the X-IFU detector of the Athena Observatory

Optical blocking filters are fundamental components to protect detectors onboard astrophysics space missions. They must be transparent enough in the energy range of interest, while shielding the detectors from unwanted out-of-band radiation. To accomplish this purpose, both the selection of the materials involved and their thicknesses are crucial. Large size and very thin polyimide filters are the baseline for shielding the two detectors on board Athena, a large mission selected by ESA to investigate the hot and energetic Universe. The two detectors, namely the X-ray Integral Field Unit (X-IFU), based on microcalorimeters, and the Wide Field Imager (WFI), based on DepFET active pixels, are designed to be sensitive in the 0.2 - 12keV energy range. The current design of the Athena filters consists of a set of 45 nm thick polyimide membranes for the X-IFU thermal shielding and 150 nm thick polyimide membranes for the WFI and X-IFU filter wheel UV/Vis blocking, all filters coated with an aluminum layer of thickness in the range 30 - 70 nm. To drive the flight filter design, the technology readiness assessment, and the qualification and calibration activities, both the in-band and out-of-band response must be accurately known. In this work, the transmission measurements performed from soft X-rays to medium infrared of X-IFU filter are reported as a wide band absolute transmission curve.