Near-field reflection measurements of lightweight absorbers in the millimeter-wavelengths for space missions

The development of millimeter-wave absorbers for astronomical instruments is an important area of research regarding stray light suppression. Millimeter-wave absorbers for space missions should be selected based not only on their broadband performance at oblique incidence but also on their polarization characteristics, weight, and radiation tolerance. <i>LiteBIRD</i> is a space telescope designed to test inflationary theories by observing the polarization of the cosmic microwave background (CMB). The Low-Frequency Telescope (LFT) on board <i>LiteBIRD</i> needs to know the far side lobes at the −56 dB level, so surrounding it with millimeter-wave absorbers suppresses stray light. Because even the smallest stray light reflection may cause non-negligible far side lobes in the millimeter-wave absorber inside the telescope, it is necessary to accurately measure the reflectance of the millimeter-wave absorber not only specular reflection but also diffuse reflection. We performed near-field reflection measurements of a new lightweight absorber for two polarizations with a 30 degree angle of incidence in the frequency range between 70 GHz and 110 GHz. Measurements showed a reflectance of -16 dB for TE and -19 dB for TM, lower than the industry standard Eccosorb AN-72. In addition, we evaluated radiation tolerance and confirmed that the difference in reflectance before and after exposure was within 3 dB.