Yep, I understand. It's probably a nit, but if the spider is offset on a Newtonian to properly decenter the diagonal, that's going to offset the Bahtinov as well, if it's mounted on the secondary stem. This slight offset won't interfere with the Bahtinov diffraction behavior, but the mask may be offset relative to the tube ID and cause mechanical interference. The folks I've discussed this with so far are just going to center and mount the masks directly on the end of the Newtonian tube and not on the secondary stem.
Of course this decentering doesn't apply to a refractor, Cassegrain or Gregorian. Again, the mask in post 2626350 is the best design, as it serves both obstructed and unobstructed pupils equally well.
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