HeyJP

Solved: Wow, unanswered after 3 years. I was looking for this same info and found this in 2024. Then I figured it out and thought I’d answer for the next generation. the backfocus on the Hyperstar is measured from the flange (shoulder) from which the male threads project out. Don’t count the thread length. Total backfocus must be within .5mm of the target 39.8mm. The backfocus consists of three components (only 2 if no filter tray). Camera: most ZWO cameras have 17.5mm backfocus from camera base to sensor. Filter tray: The Starizona filter tray is 17.5mm thick (0 if you don’t use it!) Adapter: the adapter will wither be 4.8mm if you use filter tray or 22.3mm without the tray. With tray: 17.5 + 17.5 + 4.8 = 39.8mm no tray: 17.5 + 0 + 22.3 = 39.8mm there ya go! Jim in Boulder