Peter,
not sure what the actual reducer/flattener is you are considering, but guess it is an SCT style??
If so, it should be OK on the C8, but I'd be hesitant to use it on the refractors, by all means try it, but don't expect miracles.
There will be a specific distance, the figure of about 75mm rings a bell, but you may have to experiment as well.
Gary
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