The "final touch" on Paul's 16" F4.5 telescope is a solar/sub-aperture
mask for his telescope. Here, the mask is "closed." The
mask can be rotated such that the opening is "between" the
spider vanes -- an "unobstructed" telescope!

 

The top of the mask rotates to reveal one of 2 openings.
This view shows the solar filter.

The mask is rotated into the "sub-aperture" position.
The diameter of the opening is 6", the focal length of the telescope is 72" --
so, an F12 -- perfect for lunar/planetary views.