Atacama Pathfinder EXperiment APEX
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Atmospheric transmission for ALMA band 5

In the next figure the atmospheric transmission in the ALMA band 5 frequency range (163-211 GHz) is shown. As can be seen in the figure, around 183 GHz there is quite strong absorption due to the JKa,Kc=31,3-22,0 para water line. At the Chajnantor Plateau the transmission at 183 GHz is better than 0.3 if pwv < 0.5 mm. In 2012 this happened about 15% of the total time when the pwv was measured. This means that at Chajnantor, the 183 GHz water line is accessible for galactic observations for some 50 days every year. Naturally, outside the water line, at frequencies below 174 GHz or above 192 GHz, the atmospheric transmission is similar to that in the frequency range of APEX-1 which is the most commonly used heterodyne receiver at APEX.