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Thanks Alex and Martin for your suggestions
Alex it could be dew but I think it looks like the optical train/ coma corrector. as per Martin.
I use flats. I use a tracing pad and 1 layer of thin white cardboard and 2 layers of Tshirt . 0.5sec exposure gets 1/3 across the histogram but I don't have access to ADU. This gets rid of my dust dougnuts but not this vignetting. I have tried the vignetting tab in the wipe function of startools and it looked different but not better.
I also do dark flats and biases. maybe I don't need the dark flats.
I will also try Siril background extraction as suggested by Glenn. I wonder if I can use this with Startools.
Cheers, Rob.
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