When we moved the Pcal beam spots on ENDX on Tuesday we were not able to get the outer beam(bottom beam) to exit the vacuum properly. We suspected that the clipping was happening on the receiver side (after reflecting off the ETM). We wanted to confirm this today so I ran a calibration line at 333.9 Hz during an opportunistic commissioning break to check if the displacement predicted by TxPD matched the displacement measured by the CAL-DELTAL_EXT. Following were the displacement reported by different channels:

CAL-DELTAL_EXT : 6.95E-18 m/sqrt(Hz)

CAL-TX_PD           : 6.75E-18 m/sqrt(Hz)

CAL-RX_PD           : 3.42E-18 m/sqrt(Hz)

The ratio between CAL-DELTAL_EXT/CAL-TX_PD is 1.03 and CAL-DELTAL_EXT/CAL-RX_PD is 2.03.  3% difference between CAL-DELTAL_EXT and TxPD could be because CAL-DELTAL_EXT is not accurately calibrated at  few % level but this clearly shows that the clipping is definitely happening on the receiver side (after the beam has reflected off the ETM). This will make the calibration of RxPD useless but we will still be able to use the TxPD calibration.