I get the impression that Siril mirrors flat frames unnecessarily.
I took Lights as a sequence of shots in Ekos. I took Flats by setting the mount in home position. Photos were taken with a DSLR (Om-1). Then I used OSC_Preprocessing script.
Below I am pasting results made of five frames.
The first time the prcess was done on the images that came out of the camera. Note the reflected smudge above and below the nebula.
The second on the images I first mirrored outside Siril (to down mirror flip). The smudge remains but only in one place.
What is the problem? Could it be that this model of camera saved the exif data “upside down” when it was out of home position?
P.S.
Yes, and I will clear my sensor 
Thank you.
Does it mean Ekos is storing fits data differently than DSLR which I am using? Should I understand that if I do lights and flats using the software I will be in a safe place?
A bit lost here. I will need to figure out how to set flats frames in Ekos while not seeing histogram, eh.
Yes, it is generally a very good idea to use the same software (either Ekos or your DSLR internal software) to capture all the frames. Otherwise, you may possibly have have mismatched orientations as you have just found out. And on top of that, if your DSLR is not 16b, say 14b for instance, you may have different conventions on how the info is stored in a 16b container (whether it is zero-padded to the left or to the right). See the discussion there: https://www.cloudynights.com/topic/809087-siril-way-over-correcting-calibration/?p=11671653
Thank you. very much for this explanation.
