Flat Field Calibration issues - Canon 6Da

That looks really strange indeed. Exactly how are you taking you flats, do you use special software (like SGP) to take them, what are your settings etc. I'll have a look at your data as well.

@vincent-mod

AV mode with BYEOS. I put the flats in the Dropbox link.

@vincent-mod

Triple layer of diffusion fabric stretched over lens with rubber band. Lens pressed against flat white laptop screen. Same f/4 but lower ISO which shouldn’t matter.

Ah ha! I had the exact same problem. Let me guess, you have a light pollution filter screwed onto the end of your lens. The problem is created by stray light. I corrected it by making a lens hood out of black felt and extended quite a bit beyond the end of the lens - acts as a good dew shield too, when getting the lights. I take my flats by pointing to a dusk sky and one needs a lot of light baffle to prevent stray light. If you are using the t-shirt method, it may have something to do with where you put the t-shirt and if you have a filter or not. I found with the t-shirt method, you cannot have a dew shield and probably not a LP filter at the end of the lens.

I am not using any filters  Best they can cause reflections on bright stars  but I am using the lens hood.  For flats I have to remove the hood. I’ll try and do some tests without using the lens hood. Good point thanks 

Ok, I'll get to your data tomorrow. Flats do indeed need to be taken in the exact same conditions as your lights so that hood difference might affect it. Keeping the focus exactly the same is important too, I can imagine the lens focusring shifting a little bit easily.

Good point on the focus, I’ll tape the focus next time. 

ps. I had a look at your data and it looks like the flats are working actually, I'm now suspecting stray light maybe in your lights. And this is more likely as I look closer, trying without flats and then to get the gradient gone, APP clearly is unable to do so as there is no normal gradient. This is almost always the case with light coming into your lens from e.d. a light somewhere in a corner or so.

Hi Robert @huerbsch (& @vincent-mod),

Were you able to solve your flat issue in the mean time?

I would think your issue is caused by either:

1. stray light entering your light and your flats in different amounts...
2. not shooting the flats with the exact same optical train as the lights. The circle illumination pattern actually might have to do with the lens hood being there or not in the optical train and/or the pressing as mentioned : "Lens pressed against flat white laptop screen"

Kind regards,

Mabula

Thanks everyone, I will try your recommendations and report back.