Mick,
Sorry about delay in reply......I have been away.
I am not sure what you mean by a "backboard". The cloth shroud and foam dew shield prevent stray light from entering the focuser. If I operate without the shroud and/or dew shield then I would have to fit a 200mm diameter baffle on the UTA opposite the focuser. I have found the black rip stop nylon shroud wooks well except when a car drives close by and a little glare is then noticeable.
The "truss" is made of "Plus 450" archery bowstring triangles, held under 10kg tension by 16mm aluminium tubes in spring loaded sockets.
Cheers,
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