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At a guess, the widest possible view would be the angle from the centre of the mirror to the edge of the tube, anything wider that this would cause vignetting of the image caused by the tube itself. My trig is a little rusty but an F5 8" would be something like 1000mm focal length, aperture is 200mm which gives a triangle from the centre of 1000 on the adjacent side, 100 on the opposite side. The angle would be the arctan of .1 which is about 5.7 degrees as a ball park figure, or about 11 Moons!
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