Hi Marc
I think the're meant to go between the telescope back plate and the OAG. If you have a crayford type focuser it will go between the telescope back and the focuser.
There is a basic design for these reducer, correctors that assumes they are a certain distance from the focal point to get the best correction. A few years ago when CCD chips were quite small it probably did not matter but with the large chips we have now position may be a lot more critical for flattening and comma correction.
Barry
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