Help with large ring in images [Deep Sky] Acquisition techniques · Kieran Gunessee · ... · 2 · 152 · 1

Hello,

I have recently started to get this large ring in my images (see below). I believe my flat frames are causing it but I don't know why.



I have cleaned my optics thouroughly (filters, coma corrector, mirrors), yet the ring persists - it's the same before and after cleaning. Flat frames calibrate various dust spots and the like, but it keeps this large ring. I also can't see this on the flat frames directly.

To test, I stacked my light frames with no darks (ring still present), then no flats, which didn't show the ring, hence my conclusion that it is related to the flats.

I'm using a Skywatcher 130PDS, ZWO533MM Pro, Optolong LRGB Filters and a ZWO Filter wheel.

To create my flats, I simply use the 'white t-shirt' method, by putting a white t-shirt over the end of the scope and placing a diffuse light panel on top. This has worked well for years, but has just started giving me this issue.

Weirdly, this ring doesn't show on my narrowband images, only LRGB.

Any help would be appreciated!

Thank you,
Kieran

Either a light leak during either flat taking session or during light frame captures or somehow something has changed in the imaging train. Way more likely the former than the latter. If the FW doesn't get in the same exact position for each filter change this is also a possible cause. You don't see the effect with NB filters because there is hardly any background.