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Since May 2012 new in the program I have the tubes with one or more flat sides  to mount the tube directly without clamps. This design offers the highest strength for the astrophotographers, rail and OAZ has to be mounted directly on the flat side. The flat side can be designed so that the focuser is very close to the optical axis  to reduce obstruction and still reaches a gigantic stiffness because of the wall thickness up to 50mm at the edges of the flat site.
Visually, this tube work very well on ALT / AZ mounts, for example also the Alt AZ EQ6.
Because of the high strength of the flat side, the rest of the tube can be designed very lightweight.
The flat side is of course possible with all of the core layers or even without core layer.