For cosmic microwave background (CMB) telescopes, high-density polyethylene (HDPE) is widely used as reimaging lens material on account of its extremely low loss and excellent mechanical properties. The impedance mismatch between free space and lens can cause non-negligible reflection loss and low image quality. An anti-reflection (AR) coating is essential to reduce such effect. Single or multiple layers porous polytetrafluoroethylene (PTFE) membranes are glued to the HDPE lens surface as AR coatings in many previous CMB experiments. However, it is difficult to get a uniform bonding across a curvature surface for a large-aperture CMB telescope. Due to the mismatch of the thermal expansion of the lens and AR coating materials, the AR coatings have delamination or separation problem during cryogenic cycling. Simulated dielectric AR (SDAR) coatings based on the theory of meta-surface are suitable for low frequency and large aperture CMB telescopes. The effective refractive index of SDAR coating is tunable by cutting holes or grooves into the lens material to form sub-wavelength structure. The SDAR coating is made directly from lens bulk material, which can naturally solve the thermal expansion mismatch problem. In this paper, we present the SDAR coating prototype machined on a 30 mm thick HDPE plate. The measurement results show that the SDAR coating can effectively eliminate standing waves with the performance improvement about -10 dB to -20 dB. And the transmittance exceeds 98% in the 30 GHz to 50 GHz band, which is about 10% higher than that without AR coating.
Publisher’s Note: This paper, originally published on 22 December 2020, was replaced with a corrected/revised version on 12 March 2021. If you downloaded the original PDF but are unable to access the revision, please contact SPIE Digital Library Customer Service for assistance.
AliCPT-1 is the first CMB degree scale polarimeter to be deployed to the Tibetan plateau at 5,250m asl. AliCPT-1 is a 95/150GHz 72cm aperture, two lens refracting telescope cooled down to 4K. Alumina lenses image the CMB on a 636mm wide focal plane. The modularized focal plane consists of dichroic polarization-sensitive Transition-Edge Sensors (TESes). Each module includes 1,704 optically active TESes fabricated on a 6in Silicon wafer. Each TES array is read out with a microwave multiplexing with a multiplexing factor up to 2,000. Such large factor has allowed to consider 10's of thousands of detectors in a practical way, enabling to design a receiver that can operate up to 19 TES arrays for a total of 32,300 TESes. AliCPT-1 leverages the technological advancements of AdvACT and BICEP-3. The cryostat receiver is currently under integration and testing. Here we present the AliCPT-1 receiver, underlying how the optimized design meets the experimental requirements.
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