I don't have any direct experience with UV photography. Pretty much every lens is going to have a focus shift in the UV and IR, and the sense of the focus shift will usually be shorter focal length in the UV, meaning when you focus the lens in the visible the value on the focusing scale will be "too close" in the UV. Stopping down helps because stopping down gives you greater depth-of-field / depth-of-focus to compensate for focus errors. I would try adjusting your focus to shorten the distance between lens and camera by a small amount, and stopping down more than f/11.
The other thing is, clearly you should do a focus test with your UV setup and a static subject. If you try a yardstick at an oblique angle to the camera, you can measure the focus shift (like, lens focused in the visible at 1 meter on the center of the yardstick, but the part of the yardstick that comes out in focus was at 1.1 meters from the camera), and then you'll just know what the shift is for your particular lens.
Split images darken when the lens is slower than f/5.6 or so, for geometrical reasons.
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