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You want signal to be significantly higher than the various noises in the CCD.
So that means the emphasis is on the noise levels of the camera from various sources. Usually dark current (thermal noise) is the largest hence the need for strong cooling. Electronics quality also plays a large part.
I have used both 5, 10 and 15 minutes as standard sub lengths.
Another factor is how accurate your tracking is. No good doing 15 minute subs if they all have elongated stars. So that comes into it.
Another factor is weather. Again, its not productive doing 15 minute subs if there is patchy cloud around. 5 minutes may have a higher success rate.
So with those things in mind I usually settle on 10 minutes as a good compromise between tracking results, weather, camera noise, depth of signal.
If the tracking is good I use 15 minutes especially on faint targets like galaxies.
On cameras with small well depth and with fast scopes I use 5 minutes to control bright star sizes and that worked well for me.
So I think there are several factors that need to be balanced out when working out ideal exposure length.
What you don't want is 15 different exposure lengths you use as that makes managing a darks library impractical.
Greg.
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