I did a quick measurement get an idea of the ETM actuator calibration, relative to the ITMs. This will still hold if/when we revisit the ITM calibration via the Michelson.
For the test masses, I locked the arms individually using MC2 as the actuator, and took transfer functions from the SUS-[OPTIC]_LSC_EXC point to the PO[X/Y]_I_ERR error signals. There were two points with coherence less than 99% that I threw away. I then took the fraction at each point, and am using the standard deviation of those fractions as the reported random error, since the coherence was super high for all points, making the error of each point negligible relative to their spread.
This gives:
**ETMX/ITMX: 2.765 +- 0.046**
**ETMY/ITMY: 2.857 +- 0.029**
With the data from ELOG 8242, this implies:
MC2 data was taken with the arms locked with the ETMs. The results are not so clean, the fractions don't line up; there is some trend with excitation frequency... The ratio is around the same as the ETMs, but I'm not going to quote any sort of precision, since I don't fully understand what's happening. Kind of a bummer, because it struck me that we could get an idea of the arm length mismatch by the difference in IMC frequency / arm FSR. I'll think about this some more... |