Consistent, repeatable parallel alignment of accessories within a drawtube or focuser is important for imaging and for collimating, and it has become even more critical with the popularity of ultra-fast focal ratios, larger image chips, and precision collimating tools.
There is a misconception that the problem of the cylindrical slip-fit system we use for eyepieces and other optical accessories is centering. There are adapters and holders on the market that are advertised as "centering" or "self-centering", but even if an accessory is pushed off-center by a millimeter it will not introduce a significant problem if the accessory and holder axis remain parallel. In contrast, angular misalignment between the accessory and the drawtube axis of even a few arc minutes can be detrimental, especially for collimation and imaging. The bottom line is that centering is nice, but parallelism is golden.
The Parallizer™ 2 inch-to-1.25 inch adapter has a patented design that insures dead parallel alignment between a 2” drawtube and a 1.25” accessory. It works by clamping each cylindrical surface between two parallel straight edges, in order to secure stable alignment. It is like dropping a cylinder into a V-block : totally wiggle-free. The straight edges can be seen in the photo looking down the 1.25” barrel.
Because the inner and outer straight edges are parallel with each other to high accuracy, precision alignment between the drawtube and the accessory is assured. The adapter's non-marring clamp screw is set at 45 degrees, pushing the accessory down against the adapter flange when tightened. The Parallizer has zero in-focus loss, and doesn’t require a clearance slot in a drawtube to seat fully. It is threaded for 48mm (2") filters.
The Parallizer has also been written up and can be seen in the New Product Showcase on Page 40 of the September 2011 Sky & Telescope magazine, and in the New Products column on Page 32 of the July-August 2011 Astronomy Technology Today magazine.