The treatment of skynoise in VHE gamma ray astronomy data

Abstract Ground-based imaging Cerenkov telescopes usually compare ON source images with OFF source images in order to determine whether Very High Energy (VHE) gamma rays have been detected. Skynoise, comprised of starlight, sky-glow and artificial sources, is present as a background in all Cerenkov images. The effect of skynoise differences between ON source and OFF source data can easily give rise to false claims of VHE gamma ray detection. Early imaging Cerenkov telescopes used hardware padding in an effort to minimise the effect of skynoise differences, however Cawley has shown more recently that software padding is a preferable technique. In this paper, we show that for situations in which a significant ON and OFF source skynoise difference exists, a modified software padding algorithm may be needed. In this implementation of software padding, the scaling of skynoise reduction (or cleaning) thresholds with skynoise for camera pixels has important implications. A first principles approach to the skynoise behaviour in camera pixels prompted the design of modified software padding techniques, based on the asymmetric nature of skynoise fluctuations around camera pixel pedestal levels. Examples are presented using data from the University of Adelaide's 37 pixel imaging detector. For such a low resolution imaging system, modifications to software padding are necessary to avoid misleading ON-OFF excesses after applying γ-ray cuts to the data. The applicability of modified software padding to imaging systems in general is also briefly discussed.