Nice images Hisanori, your images have a quite unique colour cast to them, and you are getting some sharpening artifacts on the lower parts of the planet underneath the rings. I apply a different sharpening process between the planet and the rings in Photoshop, maybe this might help?
I like the ice packs, I'm able to point my OTA at the ground and rest the ice packs on the back surface, but tape works too
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Andrew