The Simons Observatory: On-sky performance of radio-transparent multi-layer insulation (RT-MLI) using Styroace-II Styrofoam

TL;DR

This paper addresses radiative loading on cryogenic CMB detectors and demonstrates a practical RT-MLI solution using Styroace-II foam to suppress ambient IR while maintaining high in-band transmission. The authors implement a compact 24-layer RT-MLI filter at 40 K on the SATs and evaluate performance with on-sky measurements and Jupiter-beam end-to-end analyses. Key results show IR rejection exceeding 90% with transmitted IR power below 12 W, and in-band transmission consistent with at least 95% through the RT-MLI stack, corroborated by laboratory measurements. The findings indicate a cost-effective, scalable approach for large-diameter cryogenic mm-wave instruments, with implications for deployment at higher observing frequencies after further evaluation.

Abstract

We present the on-sky performance of a Radio-Transparent Multi-Layer Insulation filter (RT-MLI) that uses Styroace-II styrofoam to reject ambient thermal radiation from entering a 0.42 m diameter aperture to a sub-100 mK bolometric detector array cooled by a dilution-refrigerator. We find that greater than 90% of the expected incident infra-red (IR) radiation is rejected, resulting in $<$12 W of measured transmitted power. Transmitted power in the detector passbands is consistent with a lower bound of 95%. We address filter design and placement, thermal loading, and mm-wave transmission.

Figures (2)

• Figure 1: Top: The 24 Styroace-II layers (46 mm thick) shown in the filter holder clamp, flipped upside down onto its sky-facing side. The top side of the small L-brackets shown bolt to the top of the CHWP assembly (Section \ref{['sec:design']}). Not shown is the solid aluminum cylinder that aligns the stack above the optics (Figure \ref{['fig:rt-mlicad']}). The thick piece of foam below the metal holder is not part of the filter stack nor the SAT optics. Bottom: The RT-MLI filter stack installed in the SAT. The flange on the periphery of the photograph is for the ambient vacuum plate that holds the window assembly. The visible MLI blanketing appears red due to reflection of light in the local environment.
• Figure 2: Cross section of the SAT input optics in both the metal mesh and RT-MLI filter configurations. The red plate at the top of the figure forms the primary ambient temperature vacuum seal. The square icons show approximate locations for the 40K alumina filter and CHWP cavity thermometers (see Table \ref{['table:irblocking']}) used to evaluate the RT-MLI IR rejection performance. The purple rectangles show the heaters that were used for laboratory thermal load curves to simulate radiative loading from the window, and in the on-sky RT-MLI configuration regulate the CHWP assembly temperature to $<$1 K fluctuations over 12 hours to address possible in-band systematics. In reality there are four heaters distributed evenly around the circumference of the CHWP assembly, but only two are shown. Thermometer and heater locations are consistent across both filter configurations. The MLI used for thermal insulation throughout the cryostat is not shown. Temperatures listed on the left of the figure are approximate.