Simons Observatory: on-sky performance of radio-transparent multi-layer insulation using Styroace-II styrofoam

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.