  We investigate the thermal expansion of low thermal noise Fabry-Perot
cavities made of Low Thermal Expansion (LTE) glass spacers and Fused Silica
(FS) mirrors. The different thermal expansion of mirror and spacer deforms the
mirror. This deformation strongly contributes to the cavity's effective
Coefficient of Thermal Expansion (CTE), decreasing the zero crossing
temperature by about 20 K compared to an all-LTE glass cavity. Finite element
simulations and CTE measurements show that LTE rings optically contacted to the
back surface of the FS mirrors allow to tune the zero crossing temperature over
a range of 30 K.
