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What brand is it?
I have meade ones (F/6.3 and F/3.3), I think they are advertised as "focal reducer/field flatenner". I don't understand the field flatenner bit, as by reducing the focal length they bring the image in smaller hence increasing vinyetting and edge distortion, from what I have experienced. Set up in the right position of the image train it's not a problem (no distortion of vinyetting visible).
Roger.
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