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I also have a 9.25" SCT and struggled with focus issues. I used to use a Bah mask on a bright star. However, what I didnt realise was that slewing to the object and even imaging over long periods can change the focus on a SCT. If you try to minimise the distance between where you focus and the image, and also refocus as the night goes on - you will iron out some of the SCT focusing issues.
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