The construction and optical performance of Fabry-Perot eta- lons for spaceflight are considered. Optical mounts with sufficient mechanical firmness to handle intense vibration loads cause significant distortion of etalon plates, degrading the etalon resolving power and peak transmittance. A mount is described that combines mechanical robustness with very low plate distortion. An etalon mounted in this manner survived spaceflight-level vibration testing, in two axes, without damage or degradation. The defect finesse of this etalon, measured after the vibration tests, was 46, at a wavelength of 532 nm, about three times the values achieved with previous Fabry-Perot etalons for spaceflight.