Quantifying photometric observing conditions on Paranal using an IR camera

TL;DR

This paper demonstrates that an IR camera combined with a fluctuation analysis can effectively quantify and monitor photometric observing conditions by detecting thin clouds and assessing sky brightness over Paranal, supporting VLT operations.

Contribution

The study introduces a novel method using IR camera data and fluctuation analysis to quantify sky conditions and detect thin clouds, enhancing observational planning at Paranal.

Findings

IR camera and fluctuation analysis can detect very thin clouds

Approximately 60% of nights are cloud-free at Paranal

Method is robust against variations caused by water vapor

Abstract

A Low Humidity and Temperature Profiling (LHATPRO) microwave radiometer, manufactured by Radiometer Physics GmbH (RPG), is used to monitor sky conditions over ESO's Paranal observatory in support of VLT science operations. In addition to measuring precipitable water vapour (PWV) the instrument also contains an IR camera measuring sky brightness temperature at 10.5 {\mu}m. Due to its extended operating range down to -100 {\deg}C it is capable of detecting very cold and very thin, even sub-visual, cirrus clouds. We present a set of instrument flux calibration values as compared with a detrended fluctuation analysis (DFA) of the IR camera zenith-looking sky brightness data measured above Paranal taken over the past two years. We show that it is possible to quantify photometric observing conditions and that the method is highly sensitive to the presence of even very thin clouds but robust against variations of sky brightness caused by effects other than clouds such as variations of precipitable water vapour. Hence it can be used to determine photometric conditions for science operations. About 60 % of nights are free of clouds on Paranal. More work will be required to classify the clouds using this technique. For the future this approach might become part of VLT science operations for evaluating nightly sky conditions.