I am holding one of the six cannonball rollers assemblies I used for the roll off roof structure.  Cannonball track is basically a piece of pipe with a slot cut down its length.  The rollers fit inside the pipe.  These rollers are originally designed to hold structures from the ceiling.  I thought they would work inverted.  That is, I mount the track on top of my walls, and attach the rollers to the underside of the roof.  There are some problems with this.  Preliminarily, it seems the roof wants to roll not on the rollers, but on the head of the support bolt (seen here just below and to the right of my thumb).  The roof still rolls, but not as freely as I would like.  Still, the rollers give the roof added protection from being blown off in a high wind (a problem that I really worried about).  With the rollers in the tracks, the only way the roof can be blown off is if it takes the tracking with it (unlikely).

Here are two additional views of the rail assemblies.  The left shows the cannonball track and attaching hardware.  Right shows the completed roof assembly from inside the observatory, and the hole through which the outside rail will travel.