I took many wonderful pictures with the standard 10 secs exposures both in AltAz and Eq mode (in the latter case using a Skywatcher tripod and Eq head). I made the polar alignment withinh the 0.3, 0.3 limits (green check), and I tried the 30 secs exposure on a galaxy (M51). The first 10 minutes I got only the first 2 frames, and the next 18 were all discarded. I made a similar try yesterday: same result. Anyone has suggestions? Should I take care of a perfect polar alignment 0.0 , 0.0? Should I calibrate something more? Could it be the building vibration (or light wind) since I live at 7th floor (rooftop)? The tripod quality and stability is excellent. The rooftop is solid (see picture). Thank you so much for your help.