Quote:
Originally Posted by alistairsam
Hi Brent,
I do admire your build, a few questions.
have you noticed expansion with long imaging runs?
how did you ensure all truss tubes have their holes perfectly aligned as the distance between upper and lower holes across all tubes would need to be precisely identical to ensure alignment along the optical axis.
finally, how did you ensure the rotating upper cage does not shift around the optical axis when rotated?
11kg is far better than the 20 plus kgs of the gso 12" F4 OTA.
edit: Read Rolf's thread again, got answers to a lot of my questions.
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Rolf is the guru with all the technical stuff for sure.
I haven't imaged yet, still building the rest of the system so can't answer that.
The build is more tolerant of minor differences than you think. I measured and cut each set of trusses ( top & bottom ) then checked and filed them to within a half mil or so.
I then crushed the ends and drilled the holes on the drill press with an end stop centring jig, bit of alum channel bolted to the drill press table.
Ditto with jigging all the little brackets. The old adage, measure twice cut once is useful. Only loused up one length.
Got it all together but it was too long so pulled all the bottom trusses and cut another 50 mm off and redrilled to acheive the sweet spot.
The very minor differences all balance out in the randomness of assembly and the mirror cell and diag take care of the rest.
Rotating UTA was a bit of a fluke. I got two almost perfect ply circles with the router which required almost no sanding and it went from there. I was extremely careful with the spacer tubes length etc to ensure the Rotator assembly stayed extremely flat, no strain. Also the diag is generously sized and can take some amount of deviation in centring. Still refining the guides, just replaced them with much lower profile clips but again can't emphasise the importance of being able to make accurately consistent components and the drill press and jigs are the answer there.
NZ temperatures don't fluctuate wildly enough to show any length problems in expansion but they would be extremely minor and the design means they would be along the axis anyway, please adjust your focus.
As I said it very robust and stable not to mention light. With my DSLR, finder and knick nacks attached it still comes in under 15 kg.
If I ever build something bigger it will be a serrurier design, the shorter balanced axis makes it much more user friendly than the long single unit truss designs and more rigid. Hmmm, a 16" Serrurier .....