Quote:
Originally Posted by clive milne
Another advantage often attributed to the combination of carbon fibre and zero expansion optics as their sole domain (albeit erroneously) is stability of focus. You can actually achieve the same result with plate glass optics and OTA's incorporating highly expansive materials (like aluminium) if you design the system such that the errors induced by the collective cancel out to zero.... have a look at the cal poly 18" as an example of an intelligent telescope design employing basic materials on the whole and using exotics where there is a performance/$ advantage.
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You make an excellent point. Carbon trusses are everywhere these days but I don't really see a need for them in typical amateur instruments. I think they are more of a gimmick because they look nice.
When I did my 12.5" f/4 Serrurier truss I did consider using carbon truss tubes but could not justify the extra expense for no apparent gain. It would also have made my OTA heavier because I would have had to add proper fittings to each tube end.
Instead I built the OTA from birch plywood and aluminium tubes. And I can confirm what you say, there is literally
zero focus shift throughout the night. I carefully inspect each subframe afterwards, so can say that with certainty.
If that still holds for, say, a 20" made of the same materials I don't know. But at this scale any expansion is evidently within the focus tolerance and not measurable.
The materials for my OTA (excl. optics and focuser) cost around $300 so that's decent value for money
