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Hmmm... I wouldn't have thought that vignetting would have been serious at F4.
A KAF 3200 chip for example has a diagonal of 18mm.
The refractive element of the Orion AO has a clear aperture of 40mm.
An aperture stop of this diameter placed 80mm from the focal plane will
deliver a fully illuminated field of 20mm. At 90mm this will be reduced to
17.5mm. The shadow of the OAG pick off prism might be more noticeable
but I would be surprised if it was a deal breaker after flat fielding.
caveat... I could be wrong.
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