Highly stable but lightweight structural materials are essential for the realization of spaceborne optical instruments,
for example telescopes. In terms of optical performance, usually tight tolerances on the absolute spacing
between telescope mirrors have to be maintained from integration on ground to operation in final orbit. Furthermore,
a certain stability of the telescope structure must typically be ensured in the measurement band. Particular
challenging requirements have to be met for the LISA Mission (Laser Interferometer Space Antenna), where the
spacing between primary and secondary mirror must be stable to a few picometers. Only few materials offer sufficient
thermal stability to provide such performance. Candidates are for example Zerodur and Carbon-Fiber
Reinforced Plastic (CFRP), where the latter is preferred in terms of mechanical stiffness and robustness. We are
currently investigating the suitability of CFRP with respect to the LISA requirements by characterization of its
dimensional stability with heterodyne laser interferometry. The special, highly symmetric interferometer setup
offers a noise level of 2 pm/√Hz at 0.1Hz and above, and therefore represents a unique tool for this purpose.
Various procedures for the determination of the coefficient of thermal expansion (CTE) have been investigated,
both on a test sample with negative CTE, as well as on a CFRP tube specifically tuned to provide a theoretical
zero expansion in the axial dimension.