Thanks for sharing that Steve. This post illustrates why I all but gave up at home and went remote with Roboscopes. It's not the telescopes we have access to alone, it's the sky. It is absolutely shocking how much of a difference good skies make to integration and SNR.
I recently took some frames on P3 of NGC6559 and my Lum subs are actually slightly blown out at 120 seconds - 60 would have been better. it's difficult to describe just how fast F3.3 / 3.6 is under dark skies is - and I've been learning, quickly, that shorter is better.
Most piers will run unguided at 120seconds or less, which means the dithers are so much quicker. In addition, you get more total dithers which is absolute gold to SNR. More dither, more signal.
The noise of modern CMOS sensors are so low compared to CCD we just do not need the longer subs we used to.
As an example, take M51 - a very popular target and lets use Steve's ODK 16 he's used on M27 above. I have attached a picture of the angular size (green circle) used to calculate the surface brightness of 22.9 mag/arcsec2.
Using Steve's ODK16 at home in Bortle 7 ish skies, (without taking in to account dithers) and using 120s sub frames I would need to take 1,217 images or integrate for 40.5 hours to get an SNR of 20.
To get the same SNR at Roboscopes in average conditions (not peak), I'd need 14 hours of subs or 426 x 120s frames. Staggering difference.
Shooting at 300 seconds instead of 120 seconds does a few things. Using the same equipment, target and sky (Roboscopes) above and just changing the sub length means we get slightly more signal and only need 13.92 hours of data (instead of 14.17) to hit our SNR. So technically we are better off with long subs - but in the real world that is simply not true.
By the time you take in to account the hundreds of extra dithers we have been able to get and the fact that we have lost less frames due to clouds nuking 2mins of data at a time rather than 5mins at a time, you end up with a better image. Plus - it will be unguided so dithers recover much much quicker.
Sorry for the word salad, I typed this up quickly.