[Rodstar (Rod)]

A problem that truss dobsonians the world over that use a wooden split block system to house the truss poles is that the blocks can be very sensitive to environmental changes. In particular, in warmer weather, wooden blocks can expand, meaning that the insertion hole for the poles becomes tighter. In extended warmer weather, it can become a very tight fit to squeeze the poles in. Another problem some have is that the blocks actually split (this has not happened with SDM’s blocks because of their hardy construction).

On a recent week long stay in Coonabarabran, I experienced a major problem with block expansion, and spent a number of hours battling to ease one of the truss poles in so that I could observe.

Peter’s fix, which is now to become standard on all SDM scopes, is to use split blocks made of Delrin. Delrin is an industrial plastic which is entirely unchanged by heat and cold (Peter even tried using a blow torch on it, and it was completely unaffected).

The Delrin blocks are a dream to use in practice, the poles sliding in with an almost lubricated feel. Although the black of the delrin material markedly differs from the appearance and finish of the marine ply of the Mary Rose, the new split blocks have an industrial beauty that does not detract at all from the overall appearance of the scope. More importantly, I now have spilt blocks that will last a lifetime without warm weather bloat!

Peter kindly re-machined the poles so they look shiney and new, and more importantly, so that they fit into the delrin blocks perfectly.