Cheryl and Ed aligned the ALS beam path using IO_MB_M1 and ALS_M1 (see the first attachment for the mirror names from D0902114). After that, the main beam was rotated such that the beam will not even clear IO_MB_WG1. Second attachment shows this, the beam was hitting the edge of an iris placed in front of IO_MB_WG1 (iris is not on the PSL layout diagram). This is a bit more than an inch of a shift mostly in YAW, and the distance between the iris and IO_MB_M2 is about 32 inches, so this is like ~1/32 rad ~2 degrees (huge).
This seems to qualitatively agree with that the beam position moved towards +Y direction on IO_MB_M1 at some point after new PMC installation (3rd attachment) given that the steering mirror positions cannot be changed due to the PSL mount design (though the detailed numbers depend on how much the shift on IO_MB_M1 was, what was done to EOM path after the beam shift and how good the ALS path alignment is). With the beam position change on IO_MB_M1, we cannot satisfy both the main path and the ALS path at the same time using only IO_MB_M1 and ALS_M1.
There was some suspicion that the EOM motion/rotation could have caused the beam rotation downstream. There could only be a miniscule change due to that compared with O(1deg) we're talking about, and thus I don't see this as an urgent issue. See attached comment.
As such we need to move on to align the main beam path downstream of IO_MB_2 using two irises that Cheryl placed in that path as a fiducial. We first turn IO_MB_1. If that is not enough we iterate using IO_MB_2 and IO_MB_1. We might need to displace EOM using 5-axis mount so the beam cleanly goes through EOM (we need to measure in/out power like Koji did). Then we go back to ALS path and move ALS-M1 and ALS_M2.
About EOM movement. It is a concern but not because it rotates the beam by a huge amount.
1. EOM movement.
This is already reported by others, but I was also able to move the EOM by pushing/pulling using finger pressure. I wasn't able to successfully move the EOM and make it stay there after removing my pressure within the accuracy of my eyeballs, so I don't think that the connection of EOM to the 5-axis mount is loose as of now. That part shouldn't be a disaster.
(Koji's alog mentioned that originally the screws were loose, he should have tightened them. But that means that it can become loose over some time, as Cheryl and Volker tightened them a long time ago before Koji swapped EOM.)
Still, if it becomes loose it's bad, and anyway vibration or sudden movement may still cause phase or amplitude or polarization jitter or glitch. Seems like a long term fix material to me.
2: You cannot cause O(deg) rotation of the beam by rotating wedged material.
Let t1 be the incident angle on the first surface and t3 the exit angle on the second surface, and the wedge angle w (1st attachment).
Deflection is t1+t3-w = t1+asin(n*sin(w-asin(sin(t1)/n)))-w.
This is an even function of t1-asin(n*sin(t/2)) and is always ~(n-1)w for near-normal incident.
EOM has 2.85 deg wedge at both ends, net wedge of 5.7 deg, I don't know the exact index of refraction for RTP but let's say n=1.9. With these parameters, 2nd attachment right panel shows the deflection over a super wide range of t1, and the left panel is just the magnified view centered at the nominal incident angle of ~5.4 deg.
Peter King put the iris in front of IO_MB_WG1 after the EOM replacement and before the PMC swap. It's a new aperture, and given that the other iris placed in the IO path at the same time, in front of the bottom periscope mirror, is not well aligned to the long standing apertures in the IO path, using the iris in front of the wedge is not advised.
T0900475 lists the expected deflection angle as 4.7° for P-pol and 4.2° for S-pol.
Apparently n=1.9 is too big then. From deflection angle in T0900475, it sounds like n=1.82-ish for P and 1.74-ish for S.