A tube framework for telescopes. It consists of eight struts arranged in triangles. The struts run between a ring which forms either the top or bottom end of the tube and an open box at the centre of gravity of the assembly. This box is attached to the declination axis of the mounting (or the altitude axis in an altazimuth mounting). The triangular framework ensures that strains and loads are carried evenly around the tube, whatever the orientation of the telescope, and that sag is identical at both the top and bottom of the tube. The system was devised by the American engineer Mark Serrurier (1904–88).