Introduction

Many astronomical observations require information on the cloud cover at the time of the observation (either day or night). Such information has not been simple or cheap to obtain or has been unreliable in the case of visual estimates made by observers. We were presented with a particular such requirement in connection with atmospheric fluorescence detectors which are currently used for the detection and analysis of the highest energy cosmic ray showers. Such detectors are currently employed by the High Resolution Fly's Eye (HiRes) (Bird et al. 1995) and will be used by the Pierre Auger Observatory (Boratov et al. 1997). What was required was a cloud detector with a substantial field of view which could be used to determine whether or not there was cloud in the viewing direction of a mirror collecting atmospheric fluorescence light. Such a cloud detector might be used with a limited number of others to cover the complete sky. The characteristic angular field of view of a detector would be of the order of twenty degrees.

Fluorescence detectors such as HiRes view the whole sky with many photomultiplier pixels which are sensitive to the nitrogen fluorescence light produced in the atmosphere by the passage of a cosmic ray cascade. Cascade images are thus obtained for distances up to some tens of kilometres. The analysis of the data requires a knowledge of whether or not any particular part of a cascade track is obscured by clouds. The fluorescence detectors operate continuously on nominally clear moonless nights and many mirrors may be involved, possibly at remote sites. The cloud detectors must therefore be robust, requiring no attention for perhaps years on end, and they must draw little current since they may be at a site which is solar powered with limited current resources.

We have taken an idea for a cloud detector for automated telescopes by Ashley and Jurcevic (1991) using an infra red pyroelectric element and developed it into a device based on a thermopile element which requires no mechanical chopping and draws little current.

б© Copyright Astronomical Society of Australia 1997