Hi Fred,
Have you seen Richard Crisp's articles?
http://www.narrowbandimaging.com/home_page.htm
in particular this article regarding the 16803 chip (I'm sure he has tested the 6303 as well):
http://www.narrowbandimaging.com/ima...port_crisp.pdf
The graph on page 15 tells the story. Heavy going but worth understanding. Basically, you want to go past -25C for narrowband imaging if you can. As an example have a look at the pink curved line, which is a plot of noise versus temperature for a 10 minute exposure. At -25C it hasn't dipped below the bold dashed horizontal line (the 100% line) which means that the exposure is dark noise limited. It's not until you cool to -33C that the line drops below the bold dashed line and is no longer limited by dark noise. If you were planning for 10 minute subs I would be aiming for -35C with this chip.
In all cases he has assumed the sky background brightness is 0, problably a valid assumption at f9 and observing from a dark sky with narrowband filters.
Terry