Quote:
Originally Posted by bmitchell82
As a general rule of thumb you want at least four triangles up and four down.
As for the secondary steer well clear of wire bad joo joos for AP, if it was good RCOS and the like would use it.... for visual its fine though because a bit of movement isn't noticeable!
If your not a big fan of diffraction spikes use something around the 0.8mm stainless
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Hi Brendon, if I may, I would like to discuss a couple of points you raised here^
On the subject of trusses, contrary to popular belief, kilogram for kilogram a truss that uses three triangles is actually stronger than the traditional 4 sided design. If you dig through the old atm archives you should find a discussion between myself, Tom Krajci, Richard Schwartz (and a few others) including FEA results. I think the reason that this is so is that for any given direction of applied force, you will have 2/3rds of the truss elements resisting deformation. With a traditional truss, you only have 1/2 the elements working when you apply a force to any given side.
I think in the real world though, this is splitting hairs because extension and compression of the truss elements is so minor in the scheme of things. The vast majority of the flexure is a result of other factors such as the truss connectors and deformation in the element where the dec axis joins the OTA. I have always thought that the single best change you could make to the traditional design is to have the dec axis join the OTA at a node (or nodes) of the truss instead of half way between them. That way, all elements are acting under compression or tension.
On the subject of wire spiders, they can be made to be sufficiently rigid if you apply the offset design which in effect translates the forces into 1st order instead of second order... a very important engineering principle.
As far as spider diffraction goes, there is in fact two principles at play here:
The first one is obviously that diffraction is proportional to the thickness of the vanes, the secondary effect that most people are not aware of is that the spider vanes are generally a couple of degrees below ambient temperature due to radiation of thermal energy into the (relatively cooler) night sky. In this situation, you will have a boundary layer of air surrounding the spider vanes that has a variation of the refractive index (of the air) Wire spiders have an advantage in as much as air jacket is smaller and has a lower delt T than a traditional solid vane.
In practice, the contribution to the diffraction spikes is actually dominated by the boundary layer, and not the vane width, believe it or not.
The one advantage I see in traditional solid vane spiders is that they are cheap as chips and easy to employ.
Regards,
~clive.