Summary: Since I had trouble seeing a noticeable improvement in jitter by looking at the IMC WFS in this alog #89988 where the PSL output power was 2W, I looked at times when we were at 10W PSL output power to see if we were limited by shot noise. The measurements at 10W show that we might have decreased jitter but I checked the QPD sum values for the measurements after JAC was installed vs. before and we are near the edge of the QPDs.

I took reference times, when our input power was 10W to see if this gave a better measurement of jitter

Time 1: 2025/11/17 16:13:17 UTC during initial alignment. Without JAC.

Time 2:2026/03/19 15:31:23 UTC during commissioning when HAM1 was at vacuum.

Image one shows the yaw measurement, there is again a difference in the value at DC of the QPD ASDs. It looks like the peaks seen between 40 and 1000 Hz are slightly better with JAC than without JAC but it is only obvious on WFS A and B QPDs (top left plot comparing green and purple lines for WFS A and red and yellow lines for WFS B).

Image two shows the pitch measurement, there is again a difference in the value at DC of the QPD ASDs. It looks like the peaks between 40 and 1000 Hz are slightly better with JAC than without JAC but it is only obvious on WFS A QPD (top left plot comparing green and purple lines).

I also checked that the QPD sum values changed between my reference times during O4 and after JAC installation. See these four images.

1. 2W reference time during run. A and B QPD SUMs were arpund 0.03 counts.

2. 2W reference time after JAC installation. A and B QPD SUMs were arpund 0.002 counts. Maybe we are now nearer the edge of the diode?

3. 10W reference time during run. A and B QPD SUMs were around 0.16 and 0.13 respectively.

4. 10W reference time after JAC installation. A and B QPD SUMs were around 0.009 and 0.008 respectively. Maybe we are now nearer the edge of the diode?