This could be light pollution which isn't coming from a sky gradient. So that would potentially be, internal reflections caused by a light somewhere near.. it's hard to pinpoint.

Thanks Vincent, that is very plausible under my Bortle 7 skies... I think I will stay at narrowband imaging, then I have hopefully no such effect,

CS,

Janos

That might be the cause indeed, are there any lights shining directly into the direction of your setup? Even from rather far away?

I don't have lights nearby the setup and I have a 25 cm long dew shield in front of the telescope, that prevents normally any stray light. What I suspect is that the sky background is too bright, even with light pollution filter. My scope is a TS 150mm f/2.8 Hypergraph, so very quick for these circumstances.

Oehh right, those kind of setups are indeed very sensitive for light reflections. Still I would say this looks like there is some kind of internal reflection going on, maybe it's a good idea to double-check the setup for specifically this? Could also be that internal parts are a bit too reflective.

I know this is an old thread but I recently tried taking Flats with a SW 150PDS in my darkened garage and had weird light patches in my flats. Seems the back end (mirror end) of my scope and I suspect any reflector, leaks stray light like a sieve and you don't need much.  So now I either do flats at night in the garage or wrap both ends of the scope in a blackout material or similar. I saw an immediate loss of stray light artifacts ensured. 

Chris

Posted by: @chrispeace

I know this is an old thread but I recently tried taking Flats with a SW 150PDS in my darkened garage and had weird light patches in my flats. Seems the back end (mirror end) of my scope and I suspect any reflector, leaks stray light like a sieve and you don't need much.  So now I either do flats at night in the garage or wrap both ends of the scope in a blackout material or similar. I saw an immediate loss of tray light artifacts ensured. 

Chris

Thanks Chris, I will try that as well. Until now I was not able to take proper flats with my imaging train,

CS,

Janos