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I purchased a "Carey Mask" it is a disk with holes that you place in front of the objective or end of the scope which shows a star with a diffraction in the shape of an x... when focus is perfect the x is balanced and sharp.
Google Bintel and focuser mask that should show you..
You could make one.
In years past we would use two holes in a disk ..say 1 inch dia... the holes would line up when focus was achieved...so you could make one of those in a couple of minutes..the one I have you could make in an afternoon or less.
Alex
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