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

TL;DR

This study analyzes the long-term stability of tilt-to-length coupling in LISA Pathfinder, revealing slow drifts correlated with temperature changes and optical distortions over 100 days.

Contribution

It provides the first detailed analysis of long-term tilt-to-length coupling stability and its temperature dependence in LISA Pathfinder.

Findings

Coupling coefficients drifted by 1 μm/rad and 6×10^{-6} over 100 days.

Temperature changes caused coupling variations of 8 μm/rad/K and 30×10^{-6}/K.

Optical baseplate bending during cooldown contributed to coupling changes.

Abstract

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.