how the cavity beam axis is moving during a lock when the cavity is hot ?
could it explain a part of the Transmission / Coupling signal decay ?

we placed 2 Basler camera, one at (30+Z) cm and the other at (85+Z) cm (Z is about 15cm) from the M3 mirror, we recorded the video during a lock and we analyzed the centroid X and Y displacement at 2A and 3A.
frames acquisition speed is a quite slow ~ 100ms => we need to acquire the frames faster !

with these data, the displacement is no more than some pixels, which means << 100µm ... it should be completely negligeable for photdiode thorlabs DET100 with ~10mm of diameter.

the last picture displays typical locking curves (before and after lock) :
- transmission : yellow
- coupling : orange
- PZT correction : blue