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Ric,
A 5 or 10 minute shot
guided on the nucleus would probably be a good choice at f/2.8. Have
you tested your lens at f/2.8? If it is acceptable, you can shoot wide
open; however, you might want to consider stopping it down 1-stop to
f/4.0. You would have to double your exposures though. This will
improve the stars at the corners, and might remove potential vignetting.
Keep in mind, these exposures assume you have started your shots after
astronomical twilight ends. What would be the reason for the additional
exposure guided on a star? Any foreground subject is going to be
significantly blurred due to guiding.
Wade
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