Effects of the XIFU thermal filters on the focal plane spatial uniformity

Abstract

The XIFU detector, on board the ESA space mission Athena, must be shielded by specific filters from incoming ultraviolet and visible radiation due to bright astrophysical sources and infrared radiation heat load from the instrument, which would degrade its energy resolution. The current design for all five filters is a thin polyimide membrane, about 45 nm thick, coated with an aluminum layer of 30 nm. Filters are further equipped with a honeycomb Au/Ag-plated stainless steel mesh, or a niobium mesh for THF0, with a 5-mm pitch between cells, cell bars 40 to 50 μm wide, and a total blocking factor of about 2%. The mesh serves multiple purposes, such as structural strength, vibrational rigidity, radiofrequency attenuation and improved thermal conductivity. Filter uniformity requirements are not still defined. In order to perform this analysis, aimed at deriving requirements on the thickness uniformity of the filter membranes and on deriving design and/or operational constraints to prevent shadowing by the meshes, we have assumed that the impact of filters onto the image spatial uniformity at the focal plane shall be less than the uncertainty associated to calibration accuracy. The calibration accuracy requirement on the total XIFU QE currently identified in the calibration plan is the lowest between 4% absolute (0.04) in a given energy bin and 3% relative to the QE in that bin, over the full-energy range of interest, or a fixed value of 0.01 when QE is < 0.33. We use as input the Athena telescope PSF at the filter distances from the focal plane, obtained with a ray-tracing Monte Carlo code that follows incident photons from the mirror pupil up to the focal plane. Then, we employ an algorithm based on cross-correlation to study the effect on the filter stack transmission uniformity of checkerboard test patterns, meant to simulate thickness variations in the filter membrane at different spatial scales. We also study the shadowing effect of the filter meshes. The results of the study originate a new set of uniformity requirements for the filters.