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Kerber, Florian; Querel, Richard R.; Neureiter, Bianca and Hanuschik, Reinhard (2016): Through thick and thin: Quantitative classification of photometric observing conditions on Paranal. Observatory Operations: Strategies, Processes, and Systems, Edinburgh, United Kingdom, 27 June-1 July 2016. Peck, Alison B. (ed.) : In: Observatory Operations: Strategies, Processes, and Systems VI : 27 June-1 July 2016, Edinburgh, United Kingdom, 99101S Bellingham, Washington: SPIE.

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A Low Humidity and Temperature Profiling (LHATPRO) microwave radiometer is used to monitor sky conditions over ESO's Paranal observatory. It provides measurements of precipitable water vapour (PWV) at 183 GHz, which are being used in Service Mode for scheduling observations that can take advantage of favourable conditions for infrared (IR) observations. The instrument also contains an IR camera measuring sky brightness temperature at 10.5 mu m. It is capable of detecting cold and thin, even sub-visual, cirrus clouds. We present a diagnostic diagram that, based on a sophisticated time series analysis of these IR sky brightness data, allows for the automatic and quantitative classification of photometric observing conditions over Paranal. The method is highly sensitive to the presence of even very thin clouds but robust against other causes of sky brightness variations. The diagram has been validated across the complete range of conditions that occur over Paranal and we find that the automated process provides correct classification at the 95% level. We plan to develop our method into an operational tool for routine use in support of ESO Science Operations.

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