Quote:
Originally Posted by RyanJones
But that doesn’t change the optical length of the mirrors it just moves the focal point further out of the tube.
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Yes, and to enable the tip of the light-cone from the primary-mirror to reach a camera's sensor. An eyepiece can sink down into a draw-tube, but a camera cannot, therefore you have to move the primary-mirror forward a bit.
Granted, this case does seem to differ, and I couldn't find an article by "Alchemy" online. Is "Alchemy" an individual, or a publication? Did the OP use a focal-reducer of some sort, or did they have the primary-mirror re-figured? I doubt the latter.