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In the Newtonian and Refractor etc the focal length is fixed. The plate scale therefore doesn't vary....you fit all the dangle dolly stuff just inside the fixed position of the prime focus.
With a SCT and some Mak's the effective focal length varies with the main mirror position up to 10-12% ie an f10 system can end up f11 with all the "bits" attached. (This also impacts on the effect of a reducer on an SCT, the attached Spreadsheet shows the result....)
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