**EDIT by KI on 15/5/2011:**
**The calibration of MICH error signal was wrong by a factor of 2.**
**The open loop transfer functions of the Michelson locking have been measured.**
**The purpose of this excise is to calibrate the coil-magnet actuators on BS and ITMs.**
The estimated actuation coefficients are :
` BS = 3.69e-08 [m/counts]`
ITMX = 8.89e-09 [m/counts]
ITMY = 9.22e-09 [m/counts]
I guess the accuracy is something like **5 %** because the calibration of the MI optical gain relies on a peak-to-peak measurement.
A next step is to calibrate the PRM actuator and the PRC optical gain.
**(measurement)**
The Michelson was locked with different actuators in every measurement. I locked the Michelson to the dark fringe with BS, ITMX and ITMY in each time.
The measured peak-to-peak value in the error signal was 20.2 counts, corresponding to a sensor gain of 5.96e+07 [counts/m]. Note that I used AS55_Q for the locking.
After locking the MI I took the open loop transfer function by injecting broadband noise from DTT.
Then the data were fitted coarsely. In the fitting I used the resonant frequencies that Leo reported recently (http://blue.ligo-wa.caltech.edu:8000/40m/Mechanical_Resonances).
The Q-values are assumed to be 5 because of the local dampings. As a result the fitting gives us the actuator coefficients.
Here is a plot showing the measured open loop transfer functions. The solid lines represent the fittings.
**(by the way)**
- The delay time including ADC/DAC and RFM looks quite big. According to the fitting the delay is something like **600 usec.**
This is about two times larger than the one reported before (see this entry). I will re-measure it with empty filters. |