Carbon fibre and aluminium bars in combination

[Jessun]

Aluminium expands about 20 times more than carbon fibre for every degree of temperature rise.

So on designs when a carbon fibre tube has a couple of sturdy losmandy bars bolted to it end to end, then what happens?

I know on some expensive brands the aluminium bar (just one fitted usually, and not full length) is mechanically allowed to expand at a different rate so the CF OTA 'floats' above it so to speak, but for lower end brands, is there a similar solution? I'm thinking of derivates of the popular GSO RC's in particular here.

If not done properly, could the bars actually warp the CF tube and affect collimation?? Is that where they put two on?

Would this material combo with its associated mechanical work - or lack thereof - make an OAG a necessity?

Just pondering here... Any thoughts?

[Lars]

Some time ago there was a thread at Cloudy Nights where a member measured the lenght of his CF OTA and dovetail with a micrormeter at different temperatures. The result was that with the dovetail attached, the dovetail actually managed to shrink the CF OTA almost as much as a dovetail alone when the temperature dropped. The suggestion was to use tube rings, not absolutely firmly attached, so the CF tube wouldn't be affected by the aluminium parts.

/Lars

[perfrej]

I have asked myself this a number of times... It is, from a technical stand-point, true in all mixtures of materials, from railway tracks that bend in the sun to carbon fibre scope tubes. I'd say it should be a problem. Not unsolvable by any means, but nonetheless a problem. It should bend the carbon tube. I'd make sure that:

1. The rings (must be rings) are felt coated

2. There is one bar on the top and one on the bottom (identical) so that the stretch and contract are symmetrical around the tube

This way the extension should move the rings and not bend the tube.

Two cents' worth, eh?

/per