I definately agree with every word your saying clive but under the given loads if your axially deforming your truss system... wow.. unless your using ultra thin walled sections (>1mm) which most don't so ide say unless your trying for the most ultra light sections this is almost a moot point in my opinion. I have been working on a truss
façade on a 27 story apartment tower that is apart of a steel framed roof, where in the loads are massively bigger than that of a telescope and axial deformation is negligble even under ultimate wind conditions of close to 70m/sec (just to put some perspective on the situation)
So while FEA is very valid we might be talking single digit micron values and this i challange even the most experienced telescope builders to measure during a imaging session and quantify. The main reason why i would go 4 truss's as apposed to 3 is for the principle of resolution and stability of the top OTA section, it forms to the circular tube better and not cut into the actual tube and support on nodal points is better than three why else do cars have 4 wheels instead of 3? (unless the cat burgler swindled your 4th rim... little bugger).
For instance my SW 10" has a OD of 298mm and the mirror is 254mm giving a clearance of 22mm around the outside so the boundary layers of those truss tubes that only clear the actual mirror by 5-10mm will start effecting the defraction and give you spiroulus artifacts being thrown around. You could just get smart and pull cold air down the tube like most high end imaging set ups already do this prevents dew on even a moderately dewy night, effectively the draw down of the tube mixes the are and doesn't allow the ambient air to cool down sufficiently enough when in the local environment of the vane. Job sorted.
I think at the end of the day we are looking for stability and with the loads in the rhelm of Kg not kN and seeing being the limiting value 99% of the time we are well under that cap there for it doesn't matter.
As for the spider vanes yeah they increase the diffraction and to say that the thickness isn't a key player i think is odd. the boundary layer is only so thick. A key point noted by VC 200 L owners is the spider vanes create diamond stars. I have modified about 9 sets of these vanes to take them from 4mm down to 1.5mm and hence deleated the diamond stars back to round stars.
Further more I love my diffraction spikes on the stars when you get it right the rainbow effect is just stunning IMO. So when I upgraded my vanes from 0.8mm flimsy SW vanes to 1.5mm SS my diffraction spikes increased dramatically I don't believe that the boundary layer has done this. With the paint and what not it semi insulates it but i do agree that the thermal loss they will be cooler and hence change the refractive index locally around them though once again, my point before about drawing the air and not allowing it to be cooled moots that point. One other point about the spiders being supported by wires instead of a solid vane while this is fine for an observatory mounted telescope, a telescope like mine that rides shotgun on the back seat on trips out into the yonda down dirt roads and what not I feel far more comfortable with a solid set of vanes holding it all together.
Another point is the actual depth of the vane and this in my opinion is the real winner coming to wire V solid is off axis light rays where they impact the primary mirror the actual depth of the vane as a projected width is huge compared to a slice of wire.
I guess to sumerise what im saying is people are gunning for this level of perfection and cost benifit becomes more cost than benifit and this is by far the most fundamental engineering principle wouldn't you agree?
Brendan
PS. one day I may be lucky enough to have my setup where the skies no longer become the limiting resolution cap and I may start pushing for the extra pfffft but upto now my experiences have lead to my conclusions above. I am no a expert in the physics of optical design but I am a practical person with the ability to see things, learn new techniques rapidly and fix things i have no knowledge about so don't let my little avatar fool you into a young bull headed fella with no idea on the world.
