Hi,
I recently image NGC 6744 and comet Siding Springs at ISO 6400 and was surprised at the result.
I chose M17 and surroundings as the test.
Well I'm even more surprised, it's come up well.
This is 28 x 2 minute subs at 6400, I did 5 x 2 minute ISO 800 for the core of the nebula.
Surprising lack of noise.
I did use the cooler box as it was reasonably warm, which has effected the stars due to the weight of the cooler which has flexed the coma corrector.
I know it's this now as I've recently beefed up the focuser and there's no flex there.
Anyway I'll be using ISO 6400 more in future.
8" F/4 astrograph HEQ pro 5 mount. Orion mini guider.
Unmodded canon 1100D, baader type III coma corrector.
28 x 2 minute subs, ISO 6400 and . Stacked in DSS, processed in Photo shop Cs2..
Cheers,
Justin.