this morning, we tried to find the origin of the 20Hz oscillation.

- we switched OFF the laser Smaract motors controller => no change

- then, we addionally disconnected the FP-cavity PZT cable from the Laselock (we put a charge of 1kohm) => no change

- then, we switched ON the laser Smaract motors controller and switched OFF the FP-cavity motors controllers => no change

in conclusion, we don't really know where this instabillity comes from.
the amplitude is roughly 1Vpp (when the oscillation is at its maximum) on the laser PZT <=> length oscillation of ~20nm pp

could it come :
- from the air cooling regulation with pressure variation ?
- from vibrations of the hexapod below the table ?
or is it from inside of the laser or FP cavities ?

see these posts for the first measurements on this issue: https://elog.lal.in2p3.fr/FPC/THOMX+commissioning/257