Summary: We think that the sqz beam is seeing similar loss to the IFO beam in th alignment that we were running in until Friday, but double passed. The sqz beam transmission can be restored by moving the beam, and there should be enough power in this beam to be a useful diagnostic in chamber to distinguish between OFI and SRM damage.
Camilla and Noaki have measured 1.3mW coming from HAM7 towards HAM5 with the OPO locked on the seed. SRM transmission is 32.34%, ( 72680 ) so if nothing was damaged we should have something like 0.41mW of the OPO seed beam in transmission of SRM, which we should be able to measure with a power meter.
The table in 79101 indicates that the single bounce transmission to AS_C has dropped to 64% of what it was last Thursday. Here's a table of power predictions for the sqz beam arriving in HAM6:
| no damage | sqz seed beam we expect at AS_C |
| no damage | 1.3mW*(1-0.3234) * 0.99^2 = 0.86mW |
| single pass of damaged optic with 0.64 transmission | 1.3*(1-0.3234)*0.99*0.99 * 0.64 = 0.55mW |
| double pass of damaged optic with 0.64 tramission | 1.3*(1-0.3234)*0.99*0.99 * 0.64 *0.64 = 0.35mW |
With the alignment used for observing in recent weeks Naoki and Camilla measured 0.2 mW on AS_C QPD (not well centered on AS_C, but on the diode), which is calibrated into Watts arriving in HAM6. They moved ZM4 +5 to improve the transmission of the sqz beam to 0.9mW when centered on AS_C. These numbers don't agree super well with the predictions above, but they seem to suggest that the SQZ beam sees similar damage to the main IFO beam, but double passed.