There's a nicely written, "unofficial" explanation of these charts and their use in finding limiting magnitudes, at the Web site of Bill Arnett: http://www.seds.org/billa/lm/rjm.html If anyone's interested, it's somewhat easier to digest than the IMO material, AND it includes a pretty whole-sky graphic for looking up individual charts. Veikko Makela is credited by Bill on the page. Lew