So in summary, strut (and truss) dobs sometimes suffer from collimation drift when moved from zenith to horizon, and therefore most people collimate at 45 degrees to minimise drift when altitude changes. It is easy to check to see if your dob suffers from collimation drift, just put a Cheshire or laser in the focuser and move the scope from zenith to horizon and watch the collimation target. If you see drift off of perfect collimation then you have some flex in the structure.
This is possibly caused by a few things: 1. top section sag, 2. top section rotation due to focuser weight, and 3. mirror movement. The first two are addressed by this mod. When we put heavy items like coma correctors, 2" EPs, etc in the focuser the weight on the top section is increased to the point where the section is placed under rotational forces or sag when moved to low altitudes. The string brace provides two triangular braces to the top section, pulling in opposite directions, which creates a virtual truss configuration. The string braces stop rotation and sag by securing the top section to the lower tube top ring.
Here are the string brace photos I promised. Based on the Dobstuff article (linked to in previous posts), and materials sourced from Bunnings and ebay for the Dyneema 2mm string. The turnbuckles have heat shield slide overs to black them out. The Dyneema is attached to the top bracket with 2mm wire thimbles (just to prevent cutting by the bracket), it can be tied to the turnbuckle hoop with a double figure eight knot. Tensioning is by feel (although you can get sailing standing rigging tension measuring devices - they are expensive). Just make them equal and pluck them like guitar strings to get them the same. Don't make them so tight that they pull the scope out of alignment.
For those curious about the yellow hoop (its black on the inside), it's electrical cable snake material and it's an idea I picked up from Arthur A to keep the shroud from sagging into the field of view.
