Ultra-Light Liquid Helium Dewar for Balloon-Borne Observatories: First Flight and Technology Development from BOBCAT

The Balloon-Borne Cryogenic Telescope Testbed (BOBCAT) project is a multi-phase technology development project from NASA GSFC's Observational Cosmology Laboratory to create a cryogenic balloon platform for 3-to-5-meter telescopes at submillimeter wavelengths. A spectroscopic survey using cold optics at balloon altitudes would improve mapping speed by a factor of 100,000 compared to current platforms such as SOFIA. Launching such a large telescope on conventional zero-pressure balloons requires significant weight reductions for the dewar. We meet this challenge by separating the tasks of cryogen storage and instrument cooling: launching the payload warm with an ultra-light bucket dewar venting on assent, creating vacuum in the ultra-light dewar walls at float, and pumping cryogen from a storage dewar into the ultra-light dewar. The BOBCAT1 payload launched in August 2019 from NASA's CSBF facility in Ft. Sumner NM and demonstrated controlled cryogen transfer at a float altitude of 130,000 ft. The BOBCAT2 payload, scheduled for launch in 2020, implements the ultra-light dewar technology and will attempt to show comparable boiloff rates of liquid helium with weight savings five times that of the conventional bucket dewar.