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 time spans. In this work, we investigate the long-term stability of the coupling coefficients of this noise. We show that they drifted slowly (by <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"><a:mrow><a:mn>1</a:mn><a:mtext> </a:mtext><a:mtext> </a:mtext><a:mi>μm</a:mi><a:mo>/</a:mo><a:mi>rad</a:mi></a:mrow></a:math> and <c:math xmlns:c="http://www.w3.org/1998/Math/MathML" display="inline"><c:mn>6</c:mn><c:mo>×</c:mo><c:msup><c:mn>10</c:mn><c:mrow><c:mo>−</c:mo><c:mn>6</c:mn></c:mrow></c:msup></c:math> in 100 days) and were strongly correlated to temperature changes within the satellite (<e:math xmlns:e="http://www.w3.org/1998/Math/MathML" display="inline"><e:mrow><e:mn>8</e:mn><e:mtext> </e:mtext><e:mtext> </e:mtext><e:mi>μm</e:mi><e:mo>/</e:mo><e:mi>rad</e:mi><e:mo>/</e:mo><e:mi mathvariant="normal">K</e:mi></e:mrow></e:math> and <h:math xmlns:h="http://www.w3.org/1998/Math/MathML" display="inline"><h:mrow><h:mn>30</h:mn><h:mo>×</h:mo><h:msup><h:mrow><h:mn>10</h:mn></h:mrow><h:mrow><h:mo>−</h:mo><h:mn>6</h:mn></h:mrow></h:msup><h:mo>/</h:mo><h:mi mathvariant="normal">K</h:mi></h:mrow></h:math>). Based on analytical tilt-to-length coupling models, we attribute the temperature-driven coupling changes to rotations of the test masses and small distortions in the optical setup. Particularly, our findings lead to the conclusion that LISA Pathfinder’s optical baseplate was bent during the cooldown experiment, which started in late 2016 and lasted several months. Published by the American Physical Society 2024