Today I went out to End-Y to take a look at the ALS green path on ISCTEY to try to hunt down the source of the bad mode matching to TMSy (and therefore the Y-arm) that we've been seeing here.  Based on the work done back in June (at this alog), the first thing I did was take a detailed look at the beam alignment up to ALS-M7.  To my eye, the beam looks to be off-center horizontally on ALS-L4 by roughly 1 beam diameter in the -X direction (towards ISCTEY's door, away from the chamber).  In addition, the beam looks to be slightly off (1 beam radius or less, also horizontal) on ALS-L5, in the -Y direction (ALS-L5 is oriented 90° w.r.t. ALS-L4).  And in addition to that, I suspect that the beam may be clipping on the entrance aperture to the EOM.  This is very difficult to tell as the laser safety goggles render the green beam invisible, but the beam on the card (which is a horribly inaccurate measure of beam size, but it's what I had to work with) is definitely larger than the EOM entrance aperture (next time I'm at EY I'll try to get a picture, hopefully my old phone can catch it).  A second set of eyes taking a look at this will help confirm or deny whether or not my eyes are accurately seeing this apparent misalignment.  The maintenance window ran out on me so I didn't get a chance to get any beam profile measurements today; I'll head back down with the beam profiler next Tuesday for Round 2 to see if there's a point in the path where the beam becomes elliptical, or if the ellipticity seen in the alog linked above is coming from the laser itself.

I also want to mention that the beam is very off-center horizontally on ALS-HWP5 (after the 2nd Faraday isolator in the green path).  It's so far off-center that my first thought is that it was done intentionally, but I didn't build the table layout so I don't know this for sure (hence my reporting it here).