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The spectral resolution of the PSPC in the carbon window below 0.28 keV is percent (FWHM). A further subdivision of the carbon band into two bands is possible, since the boron filter has little transmission between 0.188 keV and 0.28 keV. The boron filter itself consists of a carbon film with a coating of boron carbide. The whole structure is supported by a tungsten wire mesh with a thickness of m and 4 mm spacing. The filter itself has a diameter of 28 mm. Therefore, only the central portion of the PSPC field of view is covered; the rest of the field of view is open. In Figure 4.8 the transmission of the boron filter as a function of energy is shown.