Carbon Fiber Composite Truss Tubes
|Among other things, the aerospace industry has produced a new material that that is very useful in telescope construction. This material, Carbon Fiber Wound Composite Tubes, make ideal truss tubes for telescopes. It has been found that, depending on the wind angle of the fiber, the tube can be made to either expand or contract with temperature. This variation in the Coefficient of Thermal Expansion (CTE) can be optimally tailored to give a zero CTE. Thus the telescope truss members can now be made such that as the temperature changes the distance between the primary mirror and a secondary mirror/camera can remain constant. In practical terms this means that once critical focus has been achieved at the start of an evening, the focus will remain unchanged throughout the night. For normal viewing this is of little practical value as we are always changing eyepieces and re-focusing. However, for an automated telescope that is taking images throughout the night it is of tremendous value. No longer is it necessary to use temperature-compensating focusers or schedule multiple re-focus steps during the night. With temperature changes on the order of 30-40 degrees F in the summer this is will keep the camera in critical focus all night long.|
|An example of what can be expected: Assume a 24-inch f/3 primary mirror with a CCD camera at prime focus. The truss tubes that hold the camera are about 72 inches long. Further we assume that the seeing is about 1 arc second. Now if we plot the apparent size of the star image over a 70-degree change in temperature for truss tubes made of steel, aluminum and carbon composite with a critical focus at 70 degrees, we can then see what the star images would look like as the temperature falls.|
|As can be seen the stars become very out of focus as the temperature falls. Admittedly this is probably a worst-case scenario but shows what can be accomplished with carbon fiber wound truss tubes.|
|Another advantage of this material is that it has a very high specific strength. This means that for a given weight it is much stronger than steel.|
1. Being a composite, the resin matrix must be protected from ultra-violet light to prevent it from de-composing. Applying an opaque paint coating to the outside of the tubes easily solves this.
2. Being tubes, in order to attach them to a structure they must have some sort of fixture that will prevent crushing the tube at the mounting points. Often a simple “plug” in the end will suffice.
3. Cost. The cost of these tubes ranges from $20 to $60 per foot. Painted and plugged. Thus for our example telescope above with eight 6' truss tubes the incremental cost over steel would be about $2400.