Abstract

The water vapour in the atmosphere is the main factor affecting ground-based astronomical observations at IR and submillimetre/millimetre wavelengths. Precipitable water vapour (PWV) forecasts over astronomical sites are usually provided at the zenith. However, better observational scheduling could be carried out if PWV forecasts were provided along lines of sight from an observing position. In this study, we implemented a methodology to provide PWV forecasts along all lines of sight at the APEX observatory from a regional numerical weather model. Results show that the best observing conditions are located between -10 degrees and 150 degrees of azimuth, from the zenith to 50 degrees elevation and approximately between 00 and 10 LT (4 and 14 UTC). The Chajnantor region shows a large homogeneity over the sky during each observing time, with half the time showing standard deviation (SD) values below 0.26 mm and less than 15 per cent of the time showing SD values larger than 0.5 mm, when considering all azimuth and elevation angles from the zenith to 15 degrees elevation. The PWV along lines of sight over this region may differ from the PWV calculated at the zenith. Thus, PWV forecasts along lines of sight should be better used in everyday operations at observatories rather than regular PWV visualizations at the zenith. This will allow better observational planning in specific directions, depending on the objects that want to be observed. In addition, site testing studies can be benefited from using this numerical methodology when searching the best sites for IR and submillimetre/millimetre observations.