Great choice, Howard. So far I have seen 1 (dust lanes), 3 (HII regions, or as I call the large ones that I have seen, star clouds), and 4 (spiral arm spurs - the eastern one clearly).
As far as dust lanes, I found helpful the extreme dark adaptation that is achieved naturally as clouds pass over a dark location. This is assuming that you are tracking M51 and the transparency is good once the clouds pass. The attached sketch may look strange but it is quite precise as far as the appearance of the dust lanes that night. Both arms indeed appeared cleanly split along the middle, except where dense star clouds made them appear thicker. It was an unusual view and not my best view of M51 in general, but it has so far been my best view of the dust lanes. I just had to track M51 as the clouds were passing and be prepared to observe immediately when they cleared. I almost never observe unless it is completely clear, but this and a few similar experiences made me interested in further experimentation with extreme dark adaptation.
16", up to 225x, blue zone site, 2400 ft. I did not measure it on that night but 21.7-21.9 by SQM-L is typical there (Cherry Springs, PA).
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