The Atacama Large Millimeter Array (ALMA), an international astronomy facility, is a partnership of Europe, North America and East Asia in cooperation with the Republic of Chile.
ALMA is a sub-millimeter wave interferometer consisting of 66 antennas located on the Chajnantor plateau in the Atacama Desert of northern Chile at 5.000 m altitude. In each antenna, a cryogenically cooled instrument is installed, the Front End System, which is the first element in a complex chain of signal receiving, conversion, processing and recording.
The operating temperature of the Front End cryostat is as low as 4 K and the noise temperature is one of the crucial parameters that describe the performance of a high sensitivity cooled receiver. The noise performance of a receiver can be characterized by performing a relative measurement, using a thermal calibration load. High emissivity absorbing materials immersed in liquid nitrogen (LN2) is used as a reference radiometric temperature to calibrate the ALMA receiver.
On the ALMA Operating Service Facility (OSF) an LN2 generator from Stirling Cryogenics has been installed to provide the required liquid nitrogen for the calibration activities. The LN2 system is part of the Infrastructure and mandatory for the successful astronomical observations.