Thank you David and Doug.
David i cant belief that not many people have used the Nikon for astronomy while it is the easiest camera to modify (only 12 screws to undo).
Doug The Nikon's use debayering incamera, even on the raws you take so it takes some noise out (and very dim stars
) so it makes almost no difference between ISO800 and 1000, except that it makes the picture a bit lighter.
Will try ISO1600 when i get my next clear night and do the HH again to see what the difference is.