  The Balloon-Borne Cryogenic Telescope Testbed (BOBCAT) is a stratospheric
balloon payload to develop technology for a future cryogenic suborbital
observatory. A series of flights are intended to establish ultra-light dewar
performance and open-aperture observing techniques for large (3--5 meter
diameter) cryogenic telescopes at infrared wavelengths. An initial flight in
2019 demonstrated bulk transfer of liquid nitrogen and liquid helium at
stratospheric altitudes. An 827 kg payload carried 14 liters of liquid nitrogen
(LN2) and 268 liters of liquid helium (LHe) in pressurized storage dewars to an
altitude of 39.7 km. Once at float altitude, liquid nitrogen transfer cooled a
separate, unpressurized bucket dewar to a temperature of 65 K, followed by the
transfer of 32 liters of liquid helium from the storage dewar into the bucket
dewar. Calorimetric tests measured the total heat leak to the LHe bath within
bucket dewar. A subsequent flight will replace the receiving bucket dewar with
an ultra-light dewar of similar size to compare the performance of the
ultra-light design to conventional superinsulated dewars.
