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Mmm, I kinda understand what you're getting at. To me, it seems that the second 'objective' is 20mm f/10 and takes the parallel rays from the etalon and focuses them 200mm later. The f/10 ratio remains the same, so is it that the 40mm objective has previously concentrated the 40mm aperture into 20mm space that the 400mm focal length is still valid?
Cheers,
Jason.
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