Curious Gradient? Can anyone help?

[Stub Mandrel]

I've got a very odd pattern appearing and I can't get rid of it with flats.

Here's an example of a stacked image with a severe stretch. there's a round patch left and above centre roughly half the image height in diameter. I'm guessing the offset is a collimation issue.

I have processed data with the identical setup using flats taken both before and after imaging and using an on-scope flat field and the open sky at zenith around sunset.

The 'patch' doesn't show on flats (or darks or dark flats)

In the real world, it's fairly easy to lose with good processing as it usually only shows up on the dark background, but it does make life harder.

 

I've tried processing without darks, dark flats and flats and with no frames. Here's a stack with no control fames and aside from the vignetting you can just make out the offending shape within the brighter area:

 

So it seems this is an artefact that appears on subs but not on flats?

The scope is a 150PL and the camera is an ASI1600 with EFW and 1.125" filters and nosepiece - my thought is it could be stray light from behind the mirror (unlikely) or off-axis light  getting into the tube (more likely).

But does anyone have any ideas how I could solve this?

[silentrunning]

It wasn't dew was it? I have had something in the past that looked like that and it turned out to be dew.

[Stub Mandrel]

25 minutes ago, silentrunning said:

It wasn't dew was it? I have had something in the past that looked like that and it turned out to be dew.

I dont think so, the shape is consistent between sessions and it doesn't affect sharpness.