Large Angle Optical Access in a Sub-Kelvin Cryostat

The development of lens-antenna-coupled aluminum-based microwave kinetic inductance detectors (MKIDs) and on-chip spectrometers needs a dedicated cryogenic setup to measure the beam patterns of the lens-antenna system over a large angular throughput and broad frequency range. This requires a careful design since the MKID has to be cooled to temperatures below $300\text{mK}$ to operate effectively. We developed such a cryostat with a large opening angle $\theta =\pm 37.8^{\circ }$ and an optical access with a low-pass edge at $950\text{GHz}$ . The system is based upon a commercial pulse tube cooled 3 K system with a ${}^4\text{He}$ - ${}^3\text{He}$ sorption cooler to allow base temperatures below $300\text{mK}$ . A careful study of the spectral and geometric throughput was performed to minimize thermal loading on the cold stage, allowing a base temperature of $265\text{mK}$ . Radio-transparent multi-layer-insulation was employed as a recent development in filter technology to efficiently block near-infrared radiation.