Didn't take Jupiter for a few days, so try again tonight. I did try to use f/30 at first, but found that it couldn't really reach focus, I blamed the seeing and went back to f/20, and it could reach focus easily, I found it not as bad as f/20, so I wonder why. Anyway, I took several f/20 clips.
Going back to f/30, the problem came again. I wonder why 50% more of image scale would be that far worse. I checked the CCD, remove the dust, turn the barlow, and I finally found why. The NGF-S reached the end of its focus travel. It could reach very near the optimal focus, but it's a few hairs away. I turned the C5 focus knob, and the fine tube again with the NGF-S in the other direction, wow, it's sharp again.
It's so confusing that since I could even make out the Jovian satellites when the focus travel is just a few hair away, and my near parfocal eyepiece also told me that it's close to focus indeed. Luckily, I got it found before it's too late. So, I've a few f/30 clips too.
ALl taken with 1/15s, f/20 clips at 550 gain and the f/30 clip at ~800. Transparency dropped today to around 5/10, and seeing was 3-4/10, I estimated it at 2/10 before that, but it's actually 3-4/10, not very bad indeed.
2147 (GMT+8), f/20:-
2149 (GMT+8), f/20, alternative color style:-
2150 (GMT+8), f/20, 150% resized, this one is sharpest of the batch, so I resized it to 150%:-
2203 (GMT+8), f/30:-
2205 (GMT+8), f/30:-
2207 (GMT+8), f/30:-
2208 (GMT+8), f/30:-
Additional Remarks: on the next night I found that the NGF-S cannot fully sit in its SCT thread adapter in a C5 OTA, there was a gap there since the focuser has a fat base and so preventing the focuser from sitting all the way in. This could cause collimation problem.