Dr Paul Leyland

That is excellent data, thank you!  Just what I was looking for.

I recognize the difficulty of extrapolation but for present purposes any number which is good to within a factor of two is easily precise enough. For instance, 10s for mag 11 tells me that 30s should give about mag 12 because a factor of 3 in flux collection is close enough to the 2.5 times lower flux from a target one magnitude fainter.  The precision value you quote is also encouraging. I chose 0.1mag as an upper limit because that corresponds to the best which can be reasonably expected without a camera.

Last night an interesting observation of the moon with an iPhone and eyepiece projection suggests that using the 25cm Dob with the DSLR at prime focus might be feasible. It might not be, but it is certainly worth trying.

BTW, have you come across Russ Laher’s Aperture Photometry Tool (APT)? It runs on any OS with Java installed and can told to use elliptical apertures — just the thing for poorly tracked images.