THE TREATMENT OF SKYNOISE IN VHE GAMMA-RAY ASTRONOMY DATA

Ground-based imaging Cerenkov telescopes usually compare ON source ima ges with OFF source images in order to determine whether Very High Ene rgy (VHE) gamma rays have been detected. Skynoise, comprised of starli ght, sky-glow and artificial sources, is present as a background in al l Cerenkov images. The effect of skynoise differences between ON sourc e and OFF source data can easily give rise to false claims of VHE gamm a ray detection. Early imaging Cerenkov telescopes used hardware paddi ng in an effort to minimise the effect of skynoise differences, howeve r Cawley has shown more recently that software padding is a preferable technique. In this paper, we show that for situations in which a sign ificant ON and OFF source skynoise difference exists, a modified softw are padding algorithm may be needed. In this implementation of softwar e padding, the scaling of skynoise reduction (or cleaning) thresholds with skynoise for camera pixels has important implications. A first pr inciples approach to the skynoise behaviour in camera pixels prompted the design of modified software padding techniques, based on the asymm etric nature of skynoise fluctuations around camera pixel pedestal lev els. 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 gamma-ray cuts to the data. The applica bility of modified software padding to imaging systems in general is a lso briefly discussed. (C) 1997 Elsevier Science B.V.