I was using a:
Canon 1D Mark2
70-200 f2.8 L IS
4 sec exposure @ ISO 1000
I think the extra lens speed helps. I also cleaned up the image with noise ninja which also helps.
If you look at the second comet image on my website wwww.magiclight.co.nz under landscapes. I used a different technique here. From my astrophotography days. I took 4 images, exposures ranging from 30 sec to 4minutes. I then stacked and averaged them using Registax. This enables me to record the fainter tail without blowing out the highlights in the comet head. The camera was piggy backed on a tracking telescope.
Yes, icebergs - amazing! I hung out the door of a chopper to take these.