I'm going to need to think about what you've said. Using the CFZ formula from here (www.goldastro.com/goldfocus/ncfz.php), it seems you should not be able to tell any difference in 20um of change if already well positioned in the CFZ. Your CFZ size for 1, 1.5, 2, and 3 arcsecs seeing (10% tolerance) is 42um, 63um, 84um, and 126um respectively. I guess the real question is how did you arrive at 2.8um / step for your system? We're both using EAF. My Celestron focuser thread pitch is 750um/rev (0.13um/step). You've calculated 21x coarser threads (seems hard to believe, but I couldn't find the thread pitch for your focuser to confirm). Were you forced to not use the 10/1 fine focus on your setup, or have I screwed up somewhere in the calc?

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