Active distance stabilization of large bodies with picometer repeatability

The paper presents control design and results of a leading experiment in the distance stabilization of large bodies, emulating optical mirrors, with picometer repeatability. The experiment, called COSI (Control Optics Structure Interaction), was funded by the European Space Agency (ESA) in view of future space telescopes needing picoradian precision over time scales >1 s. Distance stabilization is achieved by actively controlling the optical length of Fabry-Perot cavities in the vacuum. The first experiments stabilized three 0.5 m distances between two 7 kg plates with a residual control error better than 3 pm (1σ), in presence of severe environment noise and artificial micrometer distance variations, thus fully demonstrating feasibility of COSI concept and technology