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I think there is a common misapprehension that capturing subs longer than the sky limited exposure is bad. I thought that myself until I did some more reading and thinking on the topic.
What happens when you go longer than the sky limit is that you no longer get a significant benefit from reducing the impact of read noise. That doesn't mean that you don't get an improvement in S/N. The longer exposure still gives you better S/N for both the wanted signal from the object and the unwanted signal from the sky glow.
The disadvantage of longer subs is that you'll lose more exposure time if you have to throw one of them away (due to a problem like a guiding issue) and that rejection may be less effective with fewer subs.
In the case where the sky limited exposure is short compared to the time needed to set up and download a sub (e.g. a bright sky glow with a slow camera like my STL11K) it does make sense to expose longer than the sky limit. The longer subs still improve S/N and the dead time waiting for the camera to download is reduced.
Cheers,
Rick.
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