That also gives me a useful guide on what to aim for. Roughly speaking, assuming no reducers were used with the Sky 90, that's a f/5.6 optics. When I tried H-alpha imaging those 4 minute exposures at f/2 would have been equivalent to 32 minutes at f/5.6. So putting aside sensor sensitivities I did go a bit deeper. I think I can afford to back off, both stopping down and maybe using shorter exposures to control the negative aspects of my setup.
I think I'll have to re-read Greg's book again as it has been a while since the last time I looked at it. I vaguely remember the question of exposure duration, aperture and "deepness" raised but can't recall the conclusion.
Bob, I hope you do have a chance this year to have a go around Cygnus. Seems to be so much there to explore. But if you do end up missing it, there's always Orion to look forward to
