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I recently went through this exercise in order to calculate the BF of a TAK TAO-67 for use with my TEC180. The TEC FF BF is too short for my use. I used CCDI and found the results harder to ascertain than I expected. And the results seemed to vary from night to night a bit. I suggest that you find an excellent star field with nothing too bright. Find an exposure that doesn't saturate a lot of stars, but is short enough to not show any tracking errors. Then take at least several exposures for each different length and average the results. I plotted my results and could see a fairly linear progression in FWHM values towards the best position. It took me several nights of work using a micrometer. I could detect a .5mm change in position. Good luck!
Peter
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