[Jeff]

Thanks Dunk, Pat and Dylan. I hope somebody gets some small benefit from my stumbles ... and eventual enlightenment. Despite being an engineer and computer geek, it still takes me a while to "learn my way" through new equipment and techniques sometimes.

Hi Eden. I too initially tried Gain 0 and Offset 125, but found that I was going nowhere near utilising the full dynamic range of the sensor (i.e. spread of brightness values). I settled on Gain 22 and Offset 120 (my testing with a light box suggested I could go to values of around 24/123, then pegged these back a little). I found from other on IIS and Cloudy Nights forum threads that others have settled on similar settings.

I found that by capturing subs across the full dynamic range, fine details such as "dust" around nebulas became much more evident, even before stretching calibrated/stacked images during processing.

I agree that the "Fast" capture mode has more read noise and is best suited to framing and focus before an imaging run. The mistake I made was thinking I could also use it to experiment with Gain/Offset settings before an imaging run.

So for me, settings of Gain 22 and Offset 120 suited me for most targets through an ED80 at f6 (with 0.8 reducer/flattener), typically with 5 minutes subs. When capturing fainter targets like the Leo Triplet galaxies (through a GSO RC8 scope), I found that getting a good histogram at 5 minute subs involved bumping the settings up to Gain 63 and Offset 140. At lower settings, the captured subs were faint with bunched histograms - requiring longer exposure times and revealing less details during processing.

Ah .... the joys of tinkering.