For very long exposures, you may want to think about using a "darkframe" as well. You should be able to google plenty of info on the subject. But basically what you want to do is take a picture with the same settings (ISO, exposure, etc.) as your picture but with the lens cap on. Optimally, it should also be taken at the same air temperature and camera temperature as your original.
You can then subtract the noise from your darkframe from the original. In Gimp load both pictures as a separate layer. Put the darkframe above the original in the layer list. Then set the layer mode to "Subtract".
On both my SX20 and a530 I get a patch of reddish noise bleeding down from the top left corner on long exposures. It looks like you may have the same problem. Subtracting a darkframe gets rid of that nicely.
If you plan on doing a lot of astrophotography, you may want to look into Registax and DeepSkyStacker. They're both free.
