Krishna, Jim, Ryan,

This morning, we tried some different blend filter and sensor correction configurations on the ISI at ETMX to see if we could improve the low-frequency performance of the platform.

To give some background, the nominal configuration along X axis, at present, is the 45 mHz blends and the LLO ("0.43 Hz only") sensor correction. The very low blend frequency amplifies the motion of Stage 1, likely through tilt reinjection. We wondered if we could do better by going to a higher blend and relying on a broadband low-frequency sensor correction, which uses the tilt-free super-sensor. We used the CPS sensors as the witness sensors, which should be reliable below 0.1 Hz and ST1 T240 as at higher frequencies.

The first attached plot shows the Stage 1 T240 X sensor ASD for the following configurations:

1. Red - 90 mHz blend and Low-Frequency Sensor Correction (LF SC, also known as Rich's filter or Z sensor correction filter).

2. Blue - 90 mHz blend, LF SC and BRS off (no tilt-subtraction).

3. Green - 45 mHz blend, LF SC with tilt-subtraction.

4. Brown - 45 mHz blend, LF SC with no tilt-subtraction.

5. Magenta - Nominal configuration - 45 mHz blend, LLO SC.

The CPS (before sensor correction) signals for the above configurations are shown on the second plot. These should be used as an estimate of the platform motion at low frequencies. The third plot shows an example of the tilt-subtraction. Wind speeds were between 5-10 mph at EX.

As expected (and seen before 15270), the Green/Brown configurations offer best isolation at the microseism. The rms CPS signal for Case 3 is about the same as Case 5. Turning BRS off shows the clear re-injection of tilt in Case 4.

In Case 1/2, there is still a factor of ~5-10 reduction at the microseism and the usual factor of ~200 at 0.43 Hz. The slight surprise was that in Case 1/2 there was a clear amplification of the primary microseism by factor of 4-5 which limits the rms motion of Stage 1. It is not clear why this is being amplified and is worth investigating further.

The tests were concluded by 9:30 AM and the ISI was returned to the nominal configuration.