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Something I do with focusing and live view is to prefocus somewhere else in the sky to get it sharp before pointing to my target. Star only I'm talking about here. Live view isn't precisely "live" either, there is a slight lag between when you adjust focus and when you notice it on the screen. So take it slow, focus on a bright star and let go so it settles then adjust a tiny bit one way or the other, again slow and let it settle each time. Do this while watching the live view screen and carefully look for stars appearing or disappearing on the screen. these are just at the edge of magnitude the screen can display. As you move slightly out of focus they become slightly diffuse (out of focus) and vanish from the screen, but when you get the focus spot on perfect they suddenly appear on the screen. That's when you have focus spot on. Be careful with zoom lenses, especially heavy ones as the weight of the glass on some can slowly "wind in" the lens, losing zoom/focus as you tilt upwards. Also dont forget to try stopping down the aperture from say f1.8 to f4 to help sharpen up the stars too. lenses are never sharpest wide open.
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