We have recently developed a novel magneto-optical system that enables real time imaging at rates of ~ 70,000 frames per second. The system is also capable of very fast field ramping, at rates exceeding 3000 T/s. This new system has been exploited in the study of flux avalanches, focusing on dendritic flux patterns in YBCO films. We report on first observation of dendritic avalanches in these films triggered by rapid field ramping and discuss effects of different substrates on dendritic formation. The ability to routinely generate dendritic avalanches allows the first study of the experimental conditions for the instability, in particular the thresholds temperatures and ramp rates for the appearance of dendrites. The results will be discussed in the framework of recent thermo-magnetic instabilities theories.