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Hi Orion,
I can't tell you why they don't sell a 1 x field flattener but your Orion non-reducing flattener should work fine. They are designed for telescopes from 400-660mm. The GT81 is 480mm.
I have to say, the Orion ED80 is a nice instrument, and William Optics make nice glass too, I own a William Optics M70 and an ED80. I just don't know if, aesthetics aside, it is much of an upgrade going from the ED80 to the GT81 unless you got a bad example of the ED80. Aperture will be the same, focal ratio is a bit faster, the William Optics is probably better optical quality but not a huge leap in quality. Maybe you should think about something else for an OTA or if you buy a 0.8x reducer for the ED80, it will give you the same aperture, focal ratio and focal length as the GT 81 for a fraction of the upgrade price.
Just a thought!
Good luck whatever you decide.
Joe
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