Focuser mounting

I've seen a lot of discussions about people make truss dobs where they try to get the focuser as close to the inside diameter of their upper ring as possible. Some talk about using 1/4" plywood or 1/8" aluminum so the focuser can be right on the edge of the inside diameter.

Now I understand why you would want to do this from a vignetting perspective, but isn't it a problem that the focuser draw tube will extend into the light path when the focuser is racked in?

If you were going to add a filter slide wouldn't that also be a problem?

Getting your focuser as close as possible to the secondary has the benefits of making the fully illuminated field of view as large as possible and making the diagonal as small as possible.

And it causes the focuser tube to extend into the light path (unless you have a Clement focuser), makes using a filter slide problematic, and makes baffling more difficult.

Since diagonals come in discrete sizes, I love the convenience of a filter slide, and I notice a loss of contrast due to improper baffling more than a slight loss in field illumination, I chose to build my telescope with a slightly longer focuser-to-secondary distance.

It also lowers the EP height a couple inches.

Hope this helps a little,
Kevin