Optical characterization of microwave absorbers for straylight suppression in the LiteBIRD telescopes

LiteBIRD is an experiment dedicated to the measurement of the primordial B modes in the cosmic microwave background polarization. This signal is characterized by its faintness, and it is dominated by contamination sources such as galactic foregrounds and instrument systematics. Consequently, the optical system onboard the LiteBIRD satellite must meet several stringent constraints on optical ghosting, and in general, straylight mitigation. The use of microwave absorbing coatings on the 4K-cooled internal surfaces of the telescope enclosure, around the aperture stop and the forebaffle, is required to ensure straylight suppression. Therefore, a selection of materials with suitable optical and thermo-mechanical properties is required. For the LiteBIRD absorbers, a baseline material recipe was selected, based on a mixture of epoxy resins and conductive loading powders. The design features a 3-D textured broadband nonreflective surface, which has been developed to minimize reflections in the 89-448 GHz band. A variation of the design is also under study for the purpose of extending the straylight mitigation toward the lower frequency (40-448 GHz). This will be achieved taking into account the necessary design variations to accommodate the challenging constraints of the mass allocation for the telescope.