The UVscope instrument in the framework of ground-based cosmic ray fluorescence observatories.

UVscope is a stand-alone portable photon detector developed at IASF-PA as experimental support activity in the high-energy cosmic rays field. The apparatus is designed to measure the sky transparency and the diffuse night sky background light in the Ultraviolet wavelength range where the atmosphere fluorescence process mainly takes place. In particular, UVscope can measure the relative intensity of the main fluorescence lines with respect to the background light allowing its to explore for a possible correlation between their variations and the variations of the environmental conditions (atmospheric and geographical). Taking advantage of the comparative information coming from the fluorescence detectors and the monitoring facilities available oil site, feasibility and significance of UVscope are being tested at the Pierre Auger Southern Observatory; if this investigation phase will be successful; UVscope could be used as a relatively simple and complementary instrumentation at the Northern site of the Pierre Auger Observatory, currently under definition. The present version of the UVscope instrument, with angular aperture of 6.5 degrees, is based on a multi-anode photomultiplier: its absolute calibration with the Vega star and preliminary results from measurements "on field" are here reported.