Image vignetting and gradient issues

[Celestron4]

I currently use a 6" SCT as my main imaging scope and am having some significant issues with vignetting and fairly nasty gradients. I've attached a couple of images as examples below - the first is an unprocessed single exposure and the second is the stacked and edited version. I know my flats could be better but still have a feeling that it is a limitation with the scope and may not be easily resolved. Does anyone know of any work arounds? 

Imaging scope - Celestron Nexstar Evolution 6

Mount: Celestron AVX

Camera: Canon EOS600D (+ Celestron SCT T-adapter)

Celestron f6.8 reducer/flattener

 

Edited November 13, 2022 by Celestron4

[ollypenrice]

I think you should begin by looking at your flats. The vignetting in the images is very easy to see - bright middle and dark corners. Your stretched flats should look like this and, if they don't, there is something wrong with them. Assuming they do look like this you could open them in a program which lets you mouse over the image and read off the ADU values. This will let you express the brightness of the corners as a percentage of the brightness of the centre. A fall-off of more than 25% is going to be a problem in terms of S/N ratio but the vignetting will still go.

The second image is inverse-vignetted in green. If this was calibrated with flats it seems that the green has over-corrected, giving that magenta inner part.

Could you post a stretched JPEG of your flats?

Work-around: I don't think Pixinsight's DBE would have much trouble flattening these but how much signal there is in the corners I don't know.

Are you using the reducer-flattener in the optical train? It's pretty much obligatory, I think.

Olly