I have found out how to get rid of (most) Newtonian Rings today in FireCapture!
The manual says that you should de-focus the image and take a flat frame – in my experience this worked but only partially – the answer is the amount of de-focusing. I was dramatically de-focusing – I have discovered I need to de-focus enough to blur the background details of the image with the telescope pointed at centre of the sun but not to go too far with the focuser otherwise for some reason part of the Newtonian rings will not be removed.
So flat frames should be taken as follows:
(1) Point telescope at centre of sun – so that whole image is filled with sun.
(2) Using manual control on Lakeside electronic focuser on Sky Watcher Equinox 100mm OTA, de-focus just enough to blur the background details – today I de-focused from an in-focus setting of 1600 on the focuser to about 2000. Newtonian Rings will still be visible together with any other errors on Quark filter.
(3) Take flat frame.
(4) Immediately Rings and errors disappear.
(5) Re-focus the focuser.