Summary:
ETMX M0 stage was pushed towards the reaction chain so the F1 flag comes out of the BOSEM. It got slightly better, BOSEM reading changed from 7300 to 8300.
Reaction chain was pushed away from the test mass chain so that L1 and L2 stages don't get too close to the test mass chain.
We don't know yet if this made any change to the damping etc.
Details:
Due to a huge pit offset we need to align ETMX (it was large before the vent, but it got worse after), F1 flag is too much into the coil, and supposedly that is one of the reasons why the PIT damping is not that good. In the first attachment, you can see the BOSEM reading of F1 to be about 7300 (ideally this is 16000 or so).
I gave the top stage a length push such that the test mass chain is pushed away from the top face BOSEMs, and gained about 1000 counts in F1 BOSEM. With this offset the coil outputs should still have some headroom even if you misalign the ETMX to point to baffle PDs.
One drawback of this is that the test mass chain is pushed towards the reaction chain, which is fine under a normal circumstances, except that Sheila tells that misaligning the reaction chain of ETMX affected the actuation behavior of L1 and/or L2. I'm worried that something is close to touching, and pushing flags further into OSEMs doesn't sound like a good idea.
So I gave a length offset to the reaction chain, pulling it back from the test mass chain.
In the second screen shot you can see that I gave offsets to L2L, L2Y and L2P drivealign matrix rather than M0_LOCK_L because of a very small DC gain of the latter, and because it's somewhat more convenient than to give length offset to TEST_L and compensate the misalignment caused by that by using alignment sliders.
If this makes no negative impact on locking, the offset could be moved to TEST_L and sliders.
Reaction chain offset H1:SUS-ETMX_R0_TEST_L_OFFSET was set to -100k. I can go further easily, but I left some headroom so the reaction mass could be misaligned when fringe wrapping between the test mass and the reaction mass is observed.
After these, M0_OSEMINF_F1_INMON changed from 7300 to 8250, L1 LL from 6700 to 7000, L2 UL from 7100 to 7500.
I took a very very quick top P2P TF of the test mass chain for before(blue)/after(red) comparison and I see no ill effect (but no difference means no change in the PIT sensing). This comparison was done without oplev damping.
Just a reminder for some history on the problematic pitch damping of ETMX: I investigated the effect of the alignment offsets on pitch damping last month. See 16107. I plot some pitch TF measurements from the morning of Jan 16 that suggest the pitch offset influences the OSEM sensitivity around the first two modes. However, a repeat measurement that afternoon was completely normal, i.e. no influence of the alignment offset at all.
Don't know what this means. It's possible the damping problem is not due to the alignment and what I saw was something else mysterious. Or maybe it comes and goes, which sounds worse to me.
When I compare today's measurement with Brett's (alog 16107), the TFs today are somewhere between his good data (cyan) and bad one (green), somewhat closer to the good than the bad.
Anyway I'll leave the reaction chain offsets on because of the claim about reaction chain misalignment VS the actuation of L1 and L2, and the test mass chain offsets on because of the same possibility as the L1 and L2 (that the flags are very close to hitting something, making them hit when driving hard).
