The CMOS sensor of ASI 120mm is quite sensitive in creating Newton's ring. I found it's pretty terrible in the last imaging session. People suggested that Newton's ring could be removed by tilting but then it involves on more adapter and it's not really a good way indeed.
An alternative approach is to do a flat field which can remove the Newton's ring as well as any dust on the CCD. A flat field could be acquired by de-focusing the image a large bit, and I found it natural for the whole work flow. I normally do a prime focus shot, and then pump up the magnification by adding a barlows/powermate, and before I refocus with the barlows/powermate, I simply capture a flat field first!
First up is a mosaic of two shots, taken at 1151 and 1153 (GMT+8) respectively.
So, two shots were taken afterward by using the method above with a flat field. The first one is done very close to the flat field, so the Newton's rings were removed completely. It's taken at 1157 (GMT+8):-
Next one is taken at 1159 (GMT+8), I don't exactly know, but seems like the Newton's ring is not as cleanly removed as above.
There might be two reasons:
1. field rotation since I'm doing alt-az tracking with the Nexstar SE mount, I found it unlikely, however
2. the flat field was not done correctly, since the solar disc was not occupying the whole frame, so the flat field is only partial, some dark corner was not illuminated afterall
So, next time when I do a flat field, I shall move the camera to the center of the solar disc.