gogo nous a donc écrit :

Hi Norbert,
sorry, I should have said that this "sunspot" is unfortunately a big
dust speck on my toucam sensor (I really should clean that soon). So,
the highest contrast zone I can see on this is really the lighter
patch I choose before...

OK, a bit cleaning seems in fact necessary )
Have you tried to take a large area (512 pixels) on the edge of the sun,
with a 50/50 share between the sun surface and the sky ? That's usually not
the best option, but I haven't any other idea.
Otherwise, may be you could use Registax 2.

--
Norbert. (no X for the answer)
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