Sonostar 6-Star hubs are designed to be used with 1/2" Schedule 40 PVC tubing. Please note that there is no 1/2" dimension on these hubs. PVC pipe is measured by the inside diameter of the pipe so that the volume of air or liquid can be measured.
The hubs are made of high-strength white PVC, with pre-molded holes for bolt fasteners placed 1/2" from the end of each arm. There is a 10-degree curvature molded into each arm from the hub center, which gives the hubs an 'inside' and an 'outside' when putting the dome together.