Introduction

The two frequency bands of CHAMP+, shown superimposed on the zenith atmospheric transmission
Located at 5107 m altitude on Llano de Chajnantor in the Chilean High Andes, on what is considered one of the world’s outstanding sites for submillimeter astronomy, the Atacama Pathfinder Experiment (APEX) offers unique atmospheric conditions: from the ALMA site characterization and excluding the Bolivian winter, the 50 and 25% quartile columns of precipitable water (PWV) are 1.0 and 0.6 mm, respectively. The latter corresponds to zenith transmission of the atmosphere in its submm windows of ~50% (see Figure).
To make best use of these unique conditions, CHAMP+, a dual-color heterodyne array for spectroscopy in the 450 and 350 μm atmospheric windows has been built, in collaboration with SRON-G (providing the SIS-mixers) and JPL. The instrument was successfully commissioned in summer 2007.
As a Principal Investigator instrument CHAMP+ is available to the APEX com­mu­nities in collaboration with MPIfR only (contact: Rolf Güsten).