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 300mK to operate effectively. We developed such a cryostat with a large opening angle theta = +/- 37.8 degrees and an optical access with a low-pass edge at 950 GHz. The system is based upon a commercial pulse tube cooled 3 K system with a He-4-He-3 sorption cooler to allow base temperatures below 300 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 mK. Radio-transparent multi-layer-insulation was employed as a recent development in filter technology to efficiently block near-infrared radiation.