The NI and WI etalon control requires a measurement of the NI and WI mirror temperatures. The most accurate temperature measurement is found from the temperature dependent payload drum mode frequencies  which are computed from SDB2_B1_PD2_Audio_100kHz (and are visible on the sensitivity). In the event that the interferometer goes down, these signals also go down. In order to keep the control active, the signal for the control is then switched to the thermometer on the ring heater. In order to make this transition smooth, the frequencies are calibrated as a temperature by using the ring heater thermometer measurement.

We applied a voltage on the heating belts which are mounted around the NI and WI towers and looked at the signals during the transient. In figure 2 we have (in a row): the voltage applied to the belts, the temperature as read by the thermometer attached to the tower, the temperature of the RHs, the frequency of the mirrors drum modes lines, the signal monitoring the interferometer powers at different pots and, finally the Optical gain computed from the low frequency lines, which is below the arm cavity pole, where the Finesse asymmetry is more effective(HrecTest_OGcaviies and HrecTest_OgPR) and high frequency lines.

A linear fit was calculated for drum frequencies to the ring heater temperature. The fit is plotted against the data in Figure 1. The calibration parameters for the two mirrors are:

• NI PAY_DRUM1_FREQ to INF_TCS_NI_RH_TE:
  • Slope 0.973775 [K/Hz]
  • Intercept 19.3405 [K]
• WI PAY_DRUM3_FREQ to INF_TCS_WI_RH_TE:
  • Slope 1.07607 [K/Hz]
  • Intercept 18.2165 [K]

The values for NI and WI are about 17% and 7% different that those calculated about 1 year ago in logbook entry 42370, respectively. At present it is unclear as to why these values have changed. The current values were calculated using 20 hours of data from GPS 1252652078 for NI and GPS 1252649760 for WI. This time period did not included points where the band heaters were turned on in order to measure the thermal response time of the mirror.

The next steps for the implementation for the etalon control is to measure the thermal response time of the mirror and calibration factor of the band heater in V/K. These data will be reviewed in the next few days in order to determine these parameters. The algo.py script for Py_Etalon has been prepared and debugged thanks to the help of Franco, and will be tested when these parameters are measured.