Quote:
Originally Posted by strongmanmike
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 |
My experience is identical Mike, the FWHM decreases steadily as the object rises towards the Zenith.
At least we can say any focus shift is so little that FWHM doesn't increase
When it comes to truss versus tube OTA's I believe trusses are a lot more rigid. I would think even a CF tube sags a little bit under gravity, and in that case that's perhaps what causes focus shift rather than any temperature induced changes?
Of course a truss OTA will also sag to a degree, but I think it is less because of its stiffness. And in particular a Serrurier truss is designed with two truss segments to cancel out any sagging. While my OTA is not optimised with different truss tube thicknesses for each segment, I imagine it still benefits from this principle to some extent. Hence why it appears to need no focus adjustment throughout an imaging session.
But when that is said, even a fast Newtonian is more forgiving than the more complex optical designs. I don't know how this homemade OTA would fare if it housed a Cassegrain type system.
