Tried this lens this month, and found that it would NOT auto-focus on Orion's belt, so tried max focus (past the infinity mark) FAIL. So tried the infinity mark. This worked. My best results were f1.4 at ISO 500 and 2-3 seconds, with remote release mirror up option.
My eyes could NOT tell in the LCD when true focus was achieved so I "bracketed" a SMALL bit on either side of truly in the middle of the infinity mark. Success.
Live view was not helpful. I was trying to avoid star trails. Photos will not upload tonight even when resized.