We've had two measurements which suggest ITMY actuation is weaker than it should be, (Georgia's measurement of the ITM ESD actuation strengths 42315 and 42302 and Gabriele's measurement of the ASC actuators 43797), so I have double checked the optical lever calibration.
I found a time in the past when (August 13th at 21:22 to 21:25 UTC) we ran the script that adjusts the alignment of the ITM by pointing the green beam onto the EMT baffle PD's.
The optical lever indicates a 16.5 urad change in ITM pitch between the time when the beam hits PD1 and PD4, and a 14.1 urad change in yaw.
According to the drawing that Keita and Gerardo put together, 9087, the ITM pitch difference between pointing to the two diodes should be 11.76''/4km/2 = 37.2 urad pitch and 37.4 urad for yaw.
This means that our current calibrations (pit gain = 23.9 yaw gain = 24.0) are too small by 2.25 for pit and 2.65 for yaw.
I have not attempted to check the slider calibration since people are locking the interferometer right now.
The calibration has not changed since October 5th 2014, and this is the closest alog I can find: 14312
Summary:
Now OL PIT and YAW agree with M0 PIT and YAW sliders within 10%-ish, so everything is sane now.
I confirmed that the whitening was OK.
I see that OL beam size on the OL diode for IY is much bigger than IX though it's not clear how long this has been the case. This explains why IY OL has a smaller sensitivity and thus needs larger digital gain.
It seems like either IY OL calibration was off from the start or the OL beam size changed at some point.
What I did:
1. Moved M0 PIT and YAW sliders and confirmed that OL reasonably agrees with the sliders (1st attachment).
2. Misaligned ITMY so the output of OL SEG1 is maximized, then SEG4, SEG2, SEG3, each time looking at the spectrum.
Maximized segments look all similar (except f>1kHz), which it should be as this is dominated by the intensity noise. This means that whitening is OK. (2nd attachment).
3. Also looked at DC and saw that the beam is larger than the segment.
When a segment output is maximized, the output is ~16.3k regardless of which one is maximized (attachment 3, SEG1 was maximized here).
When OL is centered, SUM is about 30.4k, so each segment can accommodate only slightly more than 50% of the entire beam at best.
Dark offsets are negligible (~>10 counts).
4: ITMX OL beam is smaller than ITMY.
When SEG1 of ITMX oplev (not ITMY) was maximized it was ~24.4k (4th attachment) while, when OL is centered, SUM is about 27.6k, so this is about 90% of the entire beam contained in one segment.
Apparently OL beam is smaller on ITMX OL diode than on ITMY, which explains why the OL sensitivity is smaller for ITMY now.