I went down to investigate the issue with the extra noise that I found in the ETMY optical lever yesterday. There were several problems with the optical layout down there - I'm not sure if I remember them all now.

1. Beam reflected from OL QPD not dumped.
2. OL QPD set normal to the steering mirror so that the back reflection goes into the vacuum chamber.
3. HeNe laser mount only dogged with 2 dogs. Needs 3. Looks like some said "Aw, that's not goin' nowhere. Let's just leave that there pard!"
4. First lens downstream of the laser had 2 screws and washers, but neither was even finger tight! They were loose by more than 1 full turn.
5. Second lens was clipping. Beam was so far off center that this lens was being used to steer the beam by a few inches on the QPD.
6. Extra reflections from ingoing beam (I don't know which surfaces) randomly landing on green & red optics.
7. Lenses for the HeNe mode matching are coated for 1064 nm. HeNe is 633 nm, so these lenses must be replaced to reduce the reflections.

The main noise issue, however, appears to be not a layout issue at all. Instead its that the laser intensity noise has gone through the roof. See attached spectra of the quadrants (this is the way to diagnose this issue).

I'll ask Steve to either heal this laser or swap it out tomorrow. After that's resolved we'll need another round of layout fixing. I've done a couple of hours today, but if we want a less useless and noisy servo we'll have to do better.

NOTE: by looking at the OL quadrants, I've found a noisy laser, but this still doesn't explain the excess noise in the ETMX. That was the one that has a noisier error signal, not ETMY. By the coherence in the DTT, you can see that the ETMY OL is correctly subtracting and normalizing out the intensity noise of the laser. Seems like the ETMX electronics might be the culprit down there.