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Correction!
I've just rechecked on MAPUG, Doc G quotes-
"
Focal reducers are designed, optically, to work at a fixed and specified distance from the film or CCD imager. They are best corrected for this one distance. For the Meade 0.63 focal reducer this distance is 87 mm from the rear of the lens (96 mm from the exit pupil) and for the Optec MAXfield 0.33 it is 29.5 mm. (from the rear of the lens structure) In practice the MAXfield distance of 29.5 mm is measured from the rear element of the reducer but is actually about 35 mm from the nodal point. The MAXfield is a very thick optical system. Note that all measurements are slightly approximate since the the focal reducers are thick lenses and do not have their nodal point marked on the lens mount. In this discussion, I am using the simple thin lens formulas so approximate locations for the lenses and images are sufficient. Additionally, the 0.63 reducers have considerable variation allowed for their placement. The 0.63 reducers work well from about 80 to 110 mm but with slightly different reduction ratios."
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