was that for my Sub at 120 seconds ?
Bob seems to think thats OK ?
It is on the high end of what I find acceptable if I am trying or concerned about preserving star color. If I plan on substituting stars with different exposures or with my broadband filters then I may go higher. The problem going longer is that SXT sometimes does not remove the clipped stars from the image and then it is a lot more difficult to add them back in from another source.
IMHO the metric to look at is pixels clipped. It is far more consistant with our dark skies. Mean or median ADU was problematic for me when I first got started in imaging. Once I move to clipped, my exposures and images improved.
Robin Glover of SharpCap fame has what is regarded as one of the best presentations on the subject. It is the work that Jon Rista and others have used that I have followed in setting my exposures. Here is a link below. This is a very good presentation and well worth watching.
Again IMHO trying to do the math to know if you are 4x or 10x read noise is not for those who took math four decades ago and don't use it now. As such this often quoted metric is a lot harder justify in the beginners forum.
Probably the easiest way to know if you are overexposed is to look at the sub in the linear un-stretched state. If it looks black with only a few hints of stars you are not overexposed. You may even be able to go higher, especially if the second check shows only 100 pixels clipped. You image from this post passes this test with flying colors, so much so I thought PixIsnsight was not giving me the correct answer. I opened it in Voyager to confirm my results.
That's bad advice. Underexposing will lower the dynamic range for the object you are inaging by bringing the faint detail into the read noise bits. That's bad. As I pointed out, once you're not swamping noise, it takes forever to get enough exposure to bring it back out.
Best advice, as Greg, Gilmour and I pointed out, is to expose so the faint details are above the noise floor. Sometimes that means overexposing the stars a little, which you can correct by combining with short exposures for stars only. A far better way to image than what you suggest.
The 2600mc especially with the gain 100 has incredibly low read noise. It really does not take that long of an exposure to swamp it. This gives us a lot of latitude in exposures that were nearly identically on the dim details.