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Hi Stefan,
These will be hot pixels (or dead pixels), due to the temperature and the long exposure.
DSLRs are especially noisy when the temp is 15 deg C or more. The longer you keep the shutter open, the more noise like this will occur. You can normally remove it by turning on the In Camera Noise Reduction (ICNR) which takes an internal dark frame after each light frame.
Alternatively, you can leave this off in the camera and take your own dark frames and stack these together to subtract them from the stacked light frames. If you take say 5 x light frames (exposures), then take 5 dark frames straight after (lens cap on) so you have the same amount of noise on each dark.
Cheers
Chris
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