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Vibration will totally kill the pleasure in using it if you make it so long ...I'd recommend no longer than a 60" tube for that diameter. Modern eyepieces even basic Plossles work really well at F8. Aside from that at F26 at the knife edge it will be very hard to see what your surface looks like especially with the extra air currents involved in the light path at the longer Radius of Curvature.
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