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Practise!!
Another idea is to use a Ronchi grating on a flip mirror. Set the CCD to your "best" Hartmann focus and then put a Ronchi grating on the guide port of the flip mirror, adjust until you get the gray null image of a star and lock in place. If the focus is spot on, all you need to do the next time is set a bright star to null with the Ronchi, flip to the CCD and away you go!
If you find the focus is still "soft" adjust it slightly and note the view in the Ronchi ie three wide bands etc. When it's 100% then re-adjust the Ronchi screen ( without moving the focusser) back to a null. Then you have a great reference for the future....
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