Oli, Elenna

Today, we tried to balance the coils on the BBSS M2 and M3 stages. I chose to use AS_C as an optical lever, since we have previously observed some strange cross coupling on the BBSS oplev and I didn't want to get confused.

Coil balancing steps:

• I put IFO in single bounce off ITMY with 10 W input power
• I centered the beam on AS_C by running SR2_ALIGN in the ALIGN_IFO guardian
• For M2, I changed the length euler to osem matrix to drive a pringle mode, so I made the UR and LL matrix values negative, while leaving UL and LR positive. Then, I drove a beamsplitter length line at 8.125 Hz from M2 test
• I observed the 8.125 Hz peak in AS_C pitch and yaw and adjusted the coiloutf filter bank gains to minimize the line height. I ensured that the absolute value of the gains still summed to 4 during my changes
• Then, I reset the M2 matrix, repeated the same process for M3

I was able to get a 20 dB reduction in pitch and yaw on M2, but only a decent pitch reduction on M3. I should revisit the M3 measurement again, since I was just guessing which way to go with the gains, and there is probably a much more procedural way to go about this.

Bs M2 shows the improvement in pitch and yaw and the final gains.

BS M3 shows the improvement in pitch and little improvement in yaw, also final gains.

I SDFed these coil gains (91735)

I had previously measured the L2P and L2Y coupling on M2 and M3 before balancing the coils. I remeasured the transfer functions afterwards, again using single bounce to AS_C. There was only a significant difference in the M3 L2Y coupling- I was unable to measure it at all, despite using the same drive as before. I couldn't bump up the measurement drive since I was close to falling off AS_C.

I have attached screenshots of each measurement. The blue reference traces are from before the coil balancing.

I will fit these measurements and implement them in the drivealign matrices. Then, we can check the decoupling of the L2A during PRMI/DRMI locking.