Tilt-to-length coupling in LISA Pathfinder: long-term stability

The tilt-to-length coupling during the LISA Pathfinder mission has been numerically and analytically modeled for particular timespans. In this work, we investigate the long-term stability of the coupling coefficients of this noise. We show that they drifted slowly (by 1\,\(\mu\)m/rad and 6\(\times10^{-6}\) in 100 days) and strongly correlated to temperature changes within the satellite (8\,\(\mu\)m/rad/K and 30\(\times10^{-6}\)/K). Based on analytical TTL coupling models, we attribute the temperature-driven coupling changes to rotations of the test masses and small distortions in the optical setup. Particularly, we show that LISA Pathfinder's optical baseplate was bent during the cooldown experiment, which started in late 2016 and lasted several months.