Development of the Half-Scaled Reflector of the LiteBIRD Low-Frequency Telescope

LiteBIRD is a space mission aimed to measure the polarization signal of the cosmic microwave background (CMB). One of the telescopes of LiteBIRD is the low-frequency telescope which has two aluminum reflectors. The reflectors are designed with thin surfaces to minimize the weight of the reflectors. Due to the thinness of the surfaces, there is a potential risk of deformation due to machining and thermal stresses. We need to establish the fabrication methodology to achieve the required surface accuracy and maintain it throughout the operation period. This paper describes the fabrication and prototyping of the half-scaled reflector and the evaluation of the surface accuracy as a demonstration. We confirmed that we can achieve an accuracy of the reflector surface of less than 10  \(\upmu\) $\mathrm{\mu }$ m RMS. Additionally, the deformation after the thermal cycle test was less than 2  \(\upmu\) $\mathrm{\mu }$ m RMS. These meet our requirements for CMB observation.