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In my experience with a William Optics 80ED F7 refractor, the 2” WO diagonal has a longer light path and allows me to come to focus with a WO 2” extension tube. If I insert my WO 1 ¼” diagonal, I need to use a 3” extension tube to reach focus, due to the shorter light path through the 1 ¼” diagonal.
Some WO ‘scopes, such as the 66mm F6 Petzval, are designed for a WO 2” diagonal with an SCT screw thread.
Cheers
Dennis
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