----- Original Message -----
> I have some small spots on the pictures that appear to be always at the
> same area and are mainly visible on long exposures.

Try taking pictures of uniform background (e.g. clear sky), out of focus, with 
narrow aperture.

If you see these spots more strongly in these conditions, and consistently at 
the same location, they're probably dust on the sensor. It's actually not that 
hard to remove (either pay ~ 50$ in a shop, or buy a 20$ kit and do it 
yourself). Your favorite search engine is your friend.

> If I understand
> correctly - Canon has a way to collect dust data and somehow clean these
> areas (my guess is by approximating the pixels). Is there a way to do a
> similar thing with DT?

dt has "hot pixels" to discard and approximate _pixels_ that are way off 
(typically dead pixels), but a spot due to dust covers many pixels and 
darktable won't be able to remove that automatically.

One important property of dt is that the resulting image depends only on the 
RAW file and the content of the database (or the xmp file), and the "dust 
removal by keeping an image of where dust is" doesn't fit well in this model.

Obviously, you still have the « spot removal » tool, but you need to remove 
each spot by hand.

-- 
Matthieu Moy
http://www-verimag.imag.fr/~moy/
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