first of all, let me praise your nicely executed secondary wire spider!
As you mentioned (and this is a general issue with building portable telescopes) the designer has to compromise between size of the UTA and spider stiffness.
I was curious, how stiff is your spider against torques caused by the weight of the secondary/holder assembly? Can you actually see the spot of a laser collimator shift slightly when the scope moves from zenith to horizon?
Thats a good question.
I do see the laser collimator spot shift as I move from zenith to horizon (from centre of paper circle to edge of the hole), but I don't think it is the wire spider at fault. Perhaps a small component of the movement may be the spider, but if so, it is in any case dominated by a shift associated with the load shifting on the trusses, and that in turn most likely because Merope's UTA is a little light, not quite as rigid as it would be if I was to build it again.
I spent a good hour today trying to identify the source:-
1. The focuser could flex but was easily ruled out because the likely shift was in the reverse direction.
2. The secondary mirror could be shifted a little by hand in any direction, but it was difficult to find a convincing relationship.
3. I clamped the altitude on the horizon and loaded/unloaded the trusses by lifting and weighting the UTA. This reproduced the motion seen in the laser. However the action of loading the UTA could potentially distort the UTA and thereby alter tension on the wire spider.
4. I flexed the truss tubes sideways by grabbing two adjacent tubes in the middle and pressing them together and pulling apart. This had an interesting effect. If the two tubes were joined at the UTA there was absolutely no effect. However if the two tubes were joined at the LTA (and thus separate at the UTA) then a significant shift occurred. Again this points me to flexure in the UTA, which in turn may alter tension on the spider.
5. One of the spider strings had a lower looser sounding note than the others, so I tightened it, and of course then had to tighten all of the strings to bring it back to collimation. The string was in a position that I felt made it a likely culprit. However the consequence was that the magnitude of the shift was not improved, but if anything now very slightly increased (the laser spot now touched the edge of the paper ring at horizon). Again, this probably points to flexure in the UTA (although I haven't figured out how yet).
Bottom line: yes Merope has some flexure that affects the spider, but it isn't the wire spider itself at fault, except to say that a wire spider is intrinsically more exposed to flexure in the UTA than a solid vaned spider because (unless you use a single ring) it is loaded at more points. In other words, the design, construction and stiffness of the UTA is more critical if you use a wire spider.
Be aware too that the UTA is a critical component of the truss system, and it will flex as the load shifts on the truss system. I've used 25mm aluminium trusses for Merope and consider them to be borderline adequate, and the UTA to be on the light side. In particular the red tube material is only attached with thin foamed double sided tape and performs very little by way of being a structural element; that was a mistake. I should have epoxied it into place, and I should have laminated the 9mm thin ply rings with carbon.
Edited by Oberon, 15 February 2017 - 01:57 AM.