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I have a C11 and the CN-212. They both have similar FL. One is F/10 the other F/12.9. I have pushed the CN-212 to 8.2m FL. It depends on the quality of the optics. Mirrors and barlow. Visually it would be hard. With a camera it's much easier. The smaller the aperture the less affected by seeing it is and quicker to reach temperature equilibrium. I think you could push your C8 to 4m or more quite comfortably. You need a quality barlow with no light scattering or chromatic aberrations. The hardest thing with increased FL is tracking so your mount smoothness, stability and how stiff it is.
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