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if the star was taking up 1/4 of the chip area at focuser-full-in, then it was roughly 10mm across. At f4, that means that the mirror needs to be about 40 mm closer to get focus. If you meant that the star was about 1/4 the linear size, then you need another 20+ mm.
try your 20 and 40mm spacers together. my GSO f4 needed about 55mm and there was not as much stuff in the back-focus zone.
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