Jeff, Sheila
This afternoon during an hour of winds consistently between 20-35 mph, we got a chance to use the three configurations Jeff described in 17729. We hoped to get be able to make a clear statement about whether or not the BRS is helping us on windy days to reduce the motion of the optic, we see a modest improvement with the BRS on, which could also be due to reduced ground motion.
We got at least 15 minutes of data with ALS COMM locked and common tidal runnng to ETMX (ugf set to 0.1 Hz) with three seismic configurations:
- configuration 1 (45 mHz blend, narrow band sensor correction, no BRS) from 23:49:51 April 14th to 0:06:36 Arpril 15th (black traces) (this is the configuration that we have been using as a nominal configuration when the BRS is not working, we know that it is not good when the wind is high)
- configuration 2 (90 mHz blend, narrow band sensor correction, no BRS) from 23:27:30-23:48:51 April 14th UTC (blue traces)
- configuration 3(90 mHz blend, broad band sensor correction, BRS on) from 0:43:43 to 01:08 UTC, a large glitch nearly broke the lock at 0:51:58 (red traces)
The IMC F signal, calibrated in kHz, is a readback of the X arm motion above 0.1 Hz in this configuration, with possible contamination from angular fluctuations. The best performance was measured with the BRS on, where the rms is about 67% of the rms measured with the 90 mHz blends and narrow band sensor correction. From the second screenshot, you can see that the ground motion also dropped when we switched to the BRS sensor correction, the rms of the ground is also about 62% lower for the red traces. The third plot shows X direction GS13s for the three configurations.
The last two attachements are the wind direction ( in degrees from north, this is mostly wind along the Y direction) and speed, and the seismic FOM for today.
What was the status of ITMX? It looks like the rms is limited by the bump at ~50 mHz in figure 1. It is possible that this bump in the ALS signal is coming from the 45 mHz blends at ITMX, though a similar bump in the ETMX GS 13 is also visible, which is a bit odd. With 90 mHz blends, one would expect that the gain peaking would be happenning near 90 mHz.
ITMX was indeed in the nominal configuration, with 45 [mHz] blends and narrow-band, 0.43 [Hz] sensor correction. We'd performed one test once that gave us the superstition that moving blend frequency up on the ITMs made ALS performance worse. I don't think this one test was documented.