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With a 7mm you would be pushing the refractors capabilities in all but the best seeing conditions. And surprisingly clusters and nebula do better with larger EPs, not smaller. Again though your 70mm will struggle to get enough light from any other than the brightest. Try M42, the Great Neb in Orion. Middle group of Orions Sword. That is almost naked eye visible in good skies as a faint smudge and in 50mm Binocs is quite viewable.
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