Last Wednesday we reset the HEPI length actuator which is used for tidal correction back to zero. The accumulated value at this point was 0.40 mm. The apparent tilt seen by the optical lever was –7.1 µrad in pitch and +5.9 µrad in yaw. However, most of this apparent tilt is due to longitude-to-angle coupling in the optical levers; for ETMX E1200836 lists 20.9 µrad/mm in pitch and 11.9  µrad/mm in yaw. With 0.40 mm we expect to see 8.4 µrad in pitch and 4.8 µrad in yaw.

After resetting the tidal actuator the cavity was locked and realigned. The misalignment was mostly in yaw where we adjusted by -1.6 µrad which is also seen in the optical lever. This was required for both ETMX and TMSX. The pitch alignment was minor with 0.5 µrad for ETMX and nothing for TMSX.

Summarizing, the observed longitude-to-angle coupling is within 15% of what we expect and the ETMX HEPI length-to-yaw coupling is about –4.0 µrad/mm.