Atacama Large Millimeter Array, Chile

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.