[Hang, Gabriele, Jenne]

Since the Xarm green ITM camera was showing us that the spot on ITMX was more than a spot size different in pitch than we had been using the last few weeks, we wanted to see if we could find the geometric centers of the test masses, and use that as a new alignment reference. (Spoiler alert: for locking today, we're using the alignments from yesterday, but those are pretty close to the alignment with the spots at the centers of the Xarm test masses).

I had tried this in the past, but hadn't tried increasing the PD whitening gain, which turned out to be the key to being able to see the dither lines.

The guardian set us up for locking the Xarm in IR on POP45 (once upon a time we used to use ASAIR45, but we switched to using POP45 when we pulled out the ASAIR PD, and haven't changed things back to use AS_A_SUM_45 once the model changes were in place to use it - probably we should make that change, since AS will get much more light than POP).  Anyhow, before Sheila pointed that out to me, we stuck with POP and increased it's whitening from one stage to 2, and increased the analog gain from 30dB to 42dB (I also changed the I and Q digital gains from 1 to 0.1 to compensate for the gain change). 

We used the ADS system, oscillators 8 and 9, with 300,000 counts amplitude at 19.1 and 23.1 Hz on ETMX and ITMX pitch.  We utilized the DARM filter banks with gains of 1 and all filter modules off (DARM triggering forced to prevent any signal from going to any suspensions) to send actually the Xarm error signal to the input matrix of the ADS demodulators.  Gabriele made new band passes for the demods, and set the phases of the demods.  I set the P2L gains of both ITMX and ETMX to zero.  The ALIGN_IFO guardian was in INPUT_ALIGN, so the INP1 and INP2 loops were making IM4 and PR2 move such that the beam was following the Xarm cavity axis. I also slowly increased the PSL power using the guardian 1W at a time to 10W input power. Hang moved ITMX and ETMX to minimize the dither lines' appearance in the Xarm error signal.  We then repeated this procedure for yaw. 

Hang has screenshots of our final slider positions in this state.  They weren't all that far from the slider positions from Sunday's locks, which is good.  It means that the full IFO locking over the weekend was using something closer to the centers of the optics, and the alignment we'd been using the last few weeks just wasn't so great. 

Since Team Alignment ran out of time on our morning shift, we just put everything back to how it was on Sunday, so that Team Length locking could work on the IFO.  But we may think about this alignment in the future. 

Moral of the story is, we can measure test mass spot positions with single arm locks.  But, maybe we'll just measure the spots with the full IFO, now that we're getting close to lock, and we know that the full lock alignment is pretty close.