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Gooday Jim,
With a SCT with its generous back focus, you can use either thick or thin.
With a Newt with limited back focus usually this requires a thin OAG especially if a coma corrector is used.
In your situation does the field flattener have to be a fixed distance from the CCD chip like a coma corrector?
This would determine how thick or thin the OAG should be.
You should install the OAG in between the flattener and the CCD chip.
You want the OAG pick up prism as close to the main camera as practical.
Other wise you will need an extension tube to get the guide cam to find focus.
Cheers
Bill
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