Vignetting ring, flat and severe DBE

[alhoest]

Hi!

Out of a test picture of a new optic,
Upper left: I have some vignetting. But it is worse than vignetting; it looks like a dark ring

Lower left: the flat

Upper right: the flat applied on the stack. An awful vignetting/ring remains.

Lower right: to be able to remove this remaining ring, I needed some strong DBE action. I couldn't find any post in the forum for reference. Finally I came to define many many samples: about 70 samples per row. A ring pattern still remains.

Was it the right way to proceed?
Has anyone any hint about my vignetting-ring issue?

Thanks
Alex

TS APO 65Q
QSI583
IDAS LPS P2 filter

[astropixel]

The ring looks too defined to be vignetting. Perhaps external light and shadow cast by the rim of the scope while taking flats, possibly? It may be one or two subframes only, that can be weeded out.

[kolec]

Hi

Try clear sky flat. No T-shirt , or dome flat

kolec

[Phil Leigh]

That flat looks very underexposed to me...

[alhoest]

The flat was taken with a led panel laid over the scope. The panel size was DIN A3, well oversized and maybe a little bent over the dew shield.
The exposure was about 35000ADU.
I'm afraid I should do more trials...
Alex

[Phil Leigh]

LED panel... "white" LED's by any chance?... or do you mean an EL panel?

Are you shooting colour? - if so, there is a good chance that the red channel will be far too low with the combination of "white" led's (that have almost no red in them) and the IDAS filter. This can also happen with an EL Panel which is too "blue".

If the level in any of the RGB channels of the flats is too low it can seriously mess up the operation of the flats - that is when I have seen rings such as yours.