I think that the issue is not ICLONE but how Space Engine creates the images for the Skybox. I have a program called VUE and it can create different image formats including HDR. When I import this image to Iclone, the image is very sharp. Look at the image below which the background is 360 deg HDR and even if you use a 105 mm lens, it still looks good. I am going to use the Skybox output or a regular video screen until I can figure how to make the cylindrical version look sharp. 

I found something very interesting.
There is an option called 'sync sky' in the visual tab.

I am not quiet sure about the exact purpose of the 'image' checkbox there.
But it has a substantial effect.
I would say it is the root cause of that issue.

Here's an image with the 'sync sky / image' option checked:

You can see the color artifacts.

And here is the result of unchecking that option:


It seems that some details get lost / invisible), but also the color gradients / artifacts.

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I realize those are artifacts, but MAN! It looks like you are on a ringed planet and that makes it look kind of awesome!