[Paco, Ian]

We aligned the X-arm IR laser

• First fix pointing on the ITMX by moving the BS (mostly pitch)
• then open etmx chamber and fine tune the pointing using the BS until it is centered on ETMX
• using beam card we quickly see the third reflection back misaligned on pitch so we move ITMX pitch to center it on ETMX
• fine tune the ITMX alignment on ETMX and check higher order reflaction overlaping with input

at this point we checked C1:LSC-TRX_DQ using ndscope and luckly we see a tiny bit of flashing (about 0.04 in normalized high gain PD counts). So we closed ETM chamber and ITM chamber and go to control room to optimize this signal. The optimization was done in the following way:

• First, just reiterate on the last steps from above, by maximizing the peak transmission using ITMX/ETMX pair.
• Then, slide BS alignment, mostly in PIT, and return to ITMX/ETMX pair.
  • At some point, turning the BS PIT made a huge improvement, so I turned the control room light off and looked at the camera on the quad monitors.
• Based on the location of the flashes (now brighter) on the ITMX/ETMX, the beam seemed to be off in PIT more than YAW, so we focused on correcting the pointing (moving BS) and then correcting with ITMX/ETMX, until the flashes got centered around ETMX.

Final peak transmission (C1:LSC-TRX_DQ) was ~ 1.3 in normalized high gain PD counts. Power budgeting tells us the peak should be ~ 2.0. The flashing on this arm is much better than YARM, so we will press on by installing POX11 RFPD and attempt locking tomorrow. This also means that YARM can be improved by a combination of alignment and/or SUS damping.

In the end we turned on the ETMX oplev and centered it to "save" our flashing position using this reference.