Stange donut in images

[Albert van Duin]

I tested my new Samyang 135mm T2.2 lens with a Baader modified Canon 5D MKII. 30, 30s exposures at allmost full opening and 400 ISO.
Please find attached a non calibrated and calibrated stack (dark, bias, flat), both treated with ABE in PI to flatten the background.
In both images you can see a donut, a bright one in the uncalibrated and a dark one in the ucalibrated version.
The uncalibrated one was over-stretched to show the donut.

When I look at the master flat, over-stretched, There is no donut.

What could be the cause of this? Maybe light entering the viewfinder of the camera?
Also the sky was not completely dark, twilight keeps going here this time of year and the moon was still up on the other side of the sky.
I also had a protective UV-filter mounted on the lens.

Any insights are welcome.
Thanks!

Albert

 

[Yannis Doukakis]

It looks too sharp to be light bleed from viewfinder.
Did you take your flats with the lens focused to infinity?

 

[Albert van Duin]

Thanks for your response Yannis,

I will try it next time with the viewfinder covered and see if anything changes.
The moon was shining when the lights were made and it was shining on the viewfinder side of the camera.

I'm quite sure focus did not move between lights and flats.
But since the donut is also there without flat-calibration, I guess it must be in the lights.

Albert