We attempted to evaluate how the instrumented baffle could be used to prealign the end mirrors of the arm cavities.

Starting from the locked arms, we took references of the alignment condition of the test masses, of the BS and of the transversal position of the PR (Figs.1,2), we then unlocked the interferometer and misaligned both ITMs by 60 urad on TY to simulate the stage of the recovery when the beam has been centered on the ETMs using the markers by moving the PR and the BS, but the alignment of the test masses themselves is still unknown.

The starting position for the NE is (TX; TY) = (-107.8; 45.4) urad

Once we did that, we started sweeping the NE alignment until the sensors on the NI instrumented baffle detected power. We then refined the alignment of the NE until we maximized the power detected by sensors 0,1 and 2 of the column c17, in the left sector of the baffle. The goal was to use the detected power to attempt to center the beam on the middle sensor, equalizing the power on sensors 0 and 2, assuming that they correspond to the tails of the Gaussian profile of the beam. We achieved this result on the c17 column with intervals of 0.2 - 0.4 urad for the NE, notice that the procedure is cumbersome, because the detected power fluctuates significantly so one needs to be patient and consider an average over a minute or so. 

We then swept the NE right by about 100 - 110 urad until we intersected the symmetrical column c5 and repeated the process. Overall, the power detected were:

• column c17 (East - South - East section of the baffle, fig.3), NE_TX/TY = (-112.1; -7.4 ) urad 
  • sensor r0: 17.81 uW
  • sensor r1: 19.85 uW
  • sensor r2: 17.85 uW
• column c5 (West - North - West section of the baffle, fig.4), NE_TX/TY = (-99.0; 96.5) urad 
  • sensor r0: 17.79 uW
  • sensor r1: 19.84 uW
  • sensor r2: 17.73 uW

The median point identified by these positions is (<TX>;<TY>) = (-105.53; 44.56) urad, that is remarkably close to the initial position.

We repeated the process with a vertical scan:

• column c23 (North - North - East section of the baffle, fig.5), NE_TX/TY = (-57.6; 33.7) urad 
  • sensor r0: 18.37 uW
  • sensor r1: 19.97 uW
  • sensor r2: 18.02 uW
• column c11 (South - South - West section of the baffle, fig.6), NE_TX/TY = (-153.8; 54.2) urad 
  • sensor r0: 17.30 uW
  • sensor r1: 18.82 uW
  • sensor r2: 16.98 uW

The median point identified by these positions is (<TX>;<TY>) = (-105.68; 43.96) urad, which is also close to the initial alignment of the NE.

Our conclusion is that the instrumented baffle is accurate enough to provide a prealignment of the beam that centers it on the NI withing a few urads from the correct alignment, so it seems a promising tool to be used in future recoveries.

We tried to repeat the process for the WI instrumented baffle, but we could not finish the procedure in the remaining time of our shift. Another commissioning windows has been allocated for this tomorrow morning.