NIGHTTIME IMAGE QUALITY AT ROGUE-DE-LOS-MUCHACHOS OBSERVATORY

Nine months' seeing statistics at the Observatorio del Rogue de los Mu chachos (ORM) are presented. Measurements were taken with a differenti al image motion monitor on top of a 5-m high tower. From the whole cam paign the mean seeing is 0.76'', the median value is 0.64'' and 7% of the time the seeing is better than 0.3''. The best seeing measured is 0.17''. A seasonal variation is noticeable, better seeing conditions a ppearing during summer, which corresponds to the occurrence of a well- defined inversion layer due to the high prevalence of trade winds. Dur ing the summer, 50% of the time seeing is better than 0.54'', value wh ich drops to 0.46'' during June, with a narrow distribution (rms = 0.1 7''). These figures place the ORM among, or better than, the best astr onomical sites (Paranal and La Silla, Chile) where seeing statistics a re available. When inspecting individual nights, it sporadically happe ns that seeing deteriorates abruptly and slowly recovers, with a 1.2-h our exponential decay time. This phenomenon is interpreted in terms of meteorological instabilities (gravity waves) which trigger a turbulen t regime.