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Graeme,
As Wavy said, the field flattener corrects curvature which is a function of focal length not diameter. Both scopes you mention have 600mm focal length, the Orion flattener is designed to work on 400-600mm focal lengths so the flattener should work on both scopes.
The SW120 being an achromatic objective (two colours focus simultaneously) won't produce image quality comparable to the ED80 which is an apochromatic objective (three colours focus simultaneously). The flattener only flattens field curvature, it does not correct for the problems that will be introduced by the achromatic objective.
Joe
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