An X-ray entrance window will be placed between the telescope and the detector. The window is transparent to X-rays and opaque to the optical background, functioning at the same time as an optical filter and a part of the cooler shield. The window consists of two similar filters, both fabricated of a polyimide substrate (0.3 microns) supported by a hexagonal polyimide grid, and coated on both sides with aluminium (30 nm), where the aluminium acts as the optical filter. The total thickness of each filter is 18 microns, and the total thickness of Al for the window system is 120 nm. In addition, the transmission is affected by the thin contact layer (Au/Pd; thickness 30 nm) on the detector.
The thickness of aluminium is determined so that the UV, optical and IR flux from the source and background will be reduced to a negligible level for any reasonable source brightness. The filter attenuates the flux by at least 10-9 from IR to UV; thus the noise from, e.g. Sirius (-1.4 mag) would be less than 0.01 eV. For a zero magnitude star the respective noise would still be only about 0.3 eV.