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Very nice results - your guiding setup is working pretty well to let you go that long with very little elongation.
I was playing with my own D200 on the weekend and did a little experimenting with best iso numbers too. I found if you leave the long exposure noise reduction on, and set the 'high iso' noise reduction to normal, it works best at about iso800, if you disable the Nikon 'star-eater' by switching the camera off as soon as you close the shutter; that forces it to save the image without taking the dark frame.
I was able to get the attached with 5 x 5 minute subs, 5 x 5 minute darks and processed in deepsky stacker with a tiny bit of horsing around in photoshop. You can see plenty of glow from the amplifiers in the corner of the light frames, but the dark subtraction does remove most of that. ISO 1600 is too much for the D200, but 800 seems like a good compromise.
I mucked the focus up when I tightened the loose adapter on the drawtube without considering that it moved the camera so don't look too closely, the earlier images of the night were much sharper... Scope was a 4" f5 APO and polar alignment was only via the alignment scope, no drift aligning. I was experimenting too, the sky was very dark, and there was a 25" Dob 5m away...
cheers,
Andrew
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