Another critical component for LISA is the point-ahead angle actuator mechanism (PAAM) which must be able to move ±0.4 mrad while not disturbing the optical pathlength by more than 1 pm/√Hz. The latter requirement is particularly difficult to verify, and we have built a test setup to do so on behalf of ESA. The PAAM in question is part of a three-mirror cavity, the resonance frequency of which is compared to that of a known stable cavity. We were able to show that at least one, possibly two, industrial actuators fulfill the requirements.
A long-standing open question in the LISA design with huge consequences for the spacecraft design is the so-called reciprocity of the back-link fiber, i.e. the equality to pm level of the two optical pathlengths through a single-mode fiber when used in both directions. We achieved a noise level of 5 pm/√Hz, which now allows to go ahead with the planned LISA design. Many tricks and stabilization loops were required to achieve this performance, which may have some consequences for the LISA baseline design.