[Sheila, Kiwamu]

Looking at the ITMX camera, Sheila found a good alignment of PR3 which centers the beam on ITMX. The below is the slider settings:

PR3 (to get the beam centered on ITMX)

• YAW bias = 758 counts
• PIT bias = -1058.5 counts

It seems that this alignment also kind of centers the beam on ITMY without touching BS. Indeed the biases look similar to what Keita had when he got it centered on ITMY (see alog 8978). However, with this alignment, we were unable to get the PRX fringes even with ITMX steered. To study what is happening, I looked at the PR3 camera which is an analog camera (see alog 8937) attached on the MC spool. Then I realized that the ITMX return beam was hitting the swiss cheese baffle. This was the beam propagating from PR2 to PRM. I tried to steer it with ITMX, but the actuation range was not big enough and also the spot tended to disappear as I steered ITMX. So instead, I steered PR3 to let the beam hit the PRM.

As a result, currently multiple bounces is happening in the PRX cavity. A clear beam spot is visible on the BS baffle of its HR side and the spot is sensitive to both PRM and ITMX, indicating that this is a multiple-bounces beam. Note that I didn't change the ITMX biases so that both pitch and yaw sliders are zero. Apparently the horizontal alignment is not good because I can see the beam is slightly hitting the BS baffle on its right side. I am guessing that we need to re-tweak PR2 horizontally to clear BS.

PR3 (to get multiple bounces in the PRX cavity)

• Y = -107 counts
• P = -312 counts