Quote:
Originally Posted by SkyViking
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  |
That's very interesting (and good
) to hear Rolf. I need to tweak focus less with the AG12 (CF Tube) than I did with the Starfire (Al Tube)...night before last I did a 2hr image run up to the zenith (object in the perfect spot = didn't have to move the dome slit
!!!) with no noticeable change in focus ~ 1120mmFL at F3.8, subs just got slightly sharper as the object climbed toward the zenith and the seeing improved
.
Mike