Read noise is far less at 1600 ISO than at lower for a twelve bit DSLR. If you want to map very faint stuff to your available dynamic range then use 1600 ISO.
You will lose dynamic range at high ISO settings. Bright stars will saturate but at least you have the faint data.
If you need a high dynamic range then 400 ISO is a good compromise.
That is why I am working on my HDR process.
There is no correct answer it all depends on the objects dynamic range and your optical train including camera.
All data for this image was taken at 1600 ISO. 11MB
http://d1355990.i49.quadrahosting.co...MCmosenhc2.jpg
See here
http://www.iceinspace.com.au/forum/s...ad.php?t=50390
Bert