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Doing the same for the two 2x barlows stacked, I came up with a focal length of 8123.8, giving a focal ratio of 32.5 (not 19.2 like it should be). Meaning the two 2x barlows stacked are actually working like a 6.7x barlow, not 4x. Probably why it hasn't been working very well! All starting to make sense now. The 3x barlow with an extender tube should be close to optimal, even as it was at f/19.7 is probably not too bad but a little bit more might be better.
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