Cryoscope Pathfinder: wide-field optical performance validation in the lab

Year: 2024 DOI: 10.1117/12.3020678
Cryoscope Pathfinder is a 16 deg² field-of-view (FoV) infrared telescope, operating in the photometric Kdark bandpass (2.35-2.5 μm). With a plate scale of 7.1′′ per 18 μm pixel on a 2048×2048 Teledyne H2RG detector array, Cryoscope Pathfinder will be the widest and deepest near-infrared imager of its kind in the K-band. A spherical primary mirror is utilized but unlike classical Schmidt designs, Cryoscope employs two fused silica meniscus lenses located on either side of the entrance pupil whereas a Schmidt telescope uses two nearly flat aspheric plates of different glass material located at a distance equal to twice the focal length from the vertex of the primary mirror. An achromatic doublet tuned to the passband delivers a flat focal plane even at large FoV. The convex meniscus element can support atmospheric pressure, allowing the entire optical path to be evacuated and cooled to 80 K to reduce thermal self-emission in a design delivering two orders of magnitude greater FoV than existing ground-based infrared telescopes. We report room temperature performance measurements, which confirm that manufacturing and alignment errors do not significantly compromise the excellent wide-field performance predicted by optical models. Interferometry in a warm bench-test setup has demonstrated on- and off-axis Strehl of 0.95 in the operating bandpass of the telescope, with co-alignment errors within ±0.2 mm. Mechanical assembly of the cryostat is now underway. Cold tests will then assess the performance of the system under vacuum and measure the thermal self-emission by capping the entrance aperture with a narcissus mirror. On-sky tests at Caltech will then be used to demonstrate focus control and PSF image quality using bright star standards over the full FoV. Cryoscope Pathfinder will be deployed to Dome C, Antarctica in December 2024 where it will benefit from infrared sky brightnesses more than 30 times darker than at temperate latitudes.