Krishna

The reconfiguring of BRS-X is complete. The new resonance frequency of BRS-X is 5.6 mHz (T ~180 s) and d is zero for all practical purposes. The BRS-X calibration filters have been modified. I have also modified the tilt-subtraction filter banks. The big change is that there is no acceleration correction for the finite d.

The pdf attachment shows the tilt-subtraction achieved under mild wind speeds of ~10-15 mph. The one noteworthy change is that the tilt-subtraction is slightly better in the 5-20 mHz frequency range, likely due to the lower resonance frequency.

I have also modified the C# code, which calculates the Beam angle, to also make an online subtraction of the reference beam angle. Previously, we did not do this since the reference beam angle noise was usually much smaller than the main Beam angle, hence it didn't help much. However, I have recently noticed that during times of excess temperature noise in the VEA, the reference beam angle noise is worse and does contribute to the main Beam angle. In order to do the subtraction (requires extra calculations), I had to reduce the update frequency of the C# code (from 140 Hz to 70 Hz). I have confirmed that this does not negatively affect the tilt-subtraction, though it does increase the high frequency noise floor slightly (by ~sqrt(2)). This code change will also be implemented at BRS-Y.

The pressure in the vacuum can (BRS_X_PumpPressure.png) is currently at ~1e-6 torr and going down steadily. The mean position (driftmon) of BRS-X was reset on Monday (see BRS_X_DriftMon.png). Acceptable range is +/- 16k counts. It seems to be stable now and I will continue monitoring.

Edit: The capacitive damper/actuator on BRS-X now works the same as that on BRS-Y. The Q is set to ~10 when the angle exceeds the high threshold (which is nominally set to 2000 = 2 micorad). Below that, the Q is set to ~100. It is time to say goodbye to the gravitational damper! It was fun while it lasted !