We had a very interesting and long discussion on exactly this theme in the forum last winter and discussed the results of tests (the "gummy bear nebula"
) and real life images concerning the best ISO/exposure time balance. Ist really worth reading it again though some pictures and tables are obviously missing because of dead links:
How to avoid wasting photons with too-high ISOs (?) - Astrophotography - PentaxForums.com
What I learned for a setup with limited exposure time since then is:
1) maximum integration time is what counts most, no matter what the ISO is. This means longest possible exposure time and as many shots to stack as you can get. Go to (but don't exceed) the reasonable limits of your tracking device and shoot as many pictures as you can. Shorter exposure time means more images, so less noise but also less dark data.
2) With that maximized exposure time reduce ISO so low that the brightest stars do not burn out.
3) Use flat frames or your processing will be very limited when it comes to dark nebulas. Use offset/bias and dark frames. I sometimes skip the dark frames because at least at cool temperatures and lower ISOs the Pentaxes have no problem with amp glow or so.
I think that Christophers project requires a relatively low ISO (100-400 ?) to handle the huge dynamic range within the DSOs contained in the field of view. 60 (90) x 60 s @ ISO 200-400 and F3.5 would result in nice data for processing the darker nebulas - I am sure! Going much higher in ISO will burn M42 and the bright stars completely with that aperture and exposure time. For M42 and the brighter stars another stack being shot with ISO 100 would be good to be blended in to gain back the burnt center of M42 and bright stars.
I would not use the lens wide open because there are so many stars in the field that the imperfections (fringes, halos, coma, diffraction) of each make the processing very hard in the end.
