Keita, Louis, Elenna

Today Louis and I set out to measure the beam spot position on MC2 using A2L. We spent some time doing wrong things to make this measurement, until Keita came along and helped us figure out the correct way to do this.

Here is our correct method:

• set up an excitation on the MC2 M3 Dither for either P or Y
• We chose 8 Hz, starting with a 10 ct excitation and working up to 100 ct depending on the dof and the A2L gain value
• Measure the coupling from MC2 M3 dither to IMC L at 8 Hz, and adjust the MC2 M3 Drivealign A2L gain

We did all of these measurements with IMC ASC engaged, and no offset on MC2 trans QPD, so the beam "should" be centered on MC2.

For both pitch and yaw, we were able to observe a flip in the sign of the MC2 M3 dither/ IMC L transfer function. Because I set up the measurement in that funny way, this actually meant we needed to maximize the transfer function instead of minimize, which is a little confused (I figured this out after having saved several references, sorry). Luckily, we were able to see the sign flip as further proof we were moving through a minimum value.

The results indicate that a P2L gain of -0.85 and a Y2L gain of +0.5 minimized the coupling.

I found this alog I wrote which references several other alogs about measuring the beam position using the A2L gains, 76754. I wrote it about PR2, but MC2 and PR2 are the same suspensions, so the results should be the same.

For a Y2L of 0.5, the alpha factor is 0.0239 (unclear if this should be positive or negative), which means d = 39.28 mm * 0.0239 = 0.94 mm; we are miscentered in yaw by less than 1 mm in some direction.

For a P2L of -0.85, the alpha factor is -0.0405, d = 39.28 mm * -0.0405 = -1.6 mm. Keita and I had a debate about what this could mean. I claim that the P2L gain that matches a centered beam could be different than zero depending on where the suspension wires are attached to the mirror (this is true for the QUADs), but he does not think that should be the case if we are measuring at 8 Hz, above the suspension resonances. Therefore, my claim is that we don't know exactly how miscentered we are in pitch until we figure out how to account for that offset. Keita claims that does not matter. We will reconvene tomorrow to discuss further.

Either way, it it could indicate that MC2 trans QPD is not exactly level with the center of MC2 mirror.

Overall, being less than 1 mm miscentered in yaw is not so bad, and within our tolerances for beam centering while working in chamber.

I attached screenshots of my dtt templates during these injections, labeled by the A2L gains.