Aso, Ikeda, Hirata, Takano, Akutsu; following 29261.
Summary
Confirmed the aligned light beam POP_FORWARD reached two QPDs on the POP table, but mostly clipped at a steering mirror on the POP table. So not yet centered to these QPDs. The work is still on the way.
Details
Yesterday we tweaked IMMT1 and 2 with their oplev setpoint in yaw to their limits. So, we were worried if the actual beam might be off-centered at IMMT2. So, we started with checking this. Before that, we called PROVIDING_STABLE_LIGHT to IO Guadian to make IMC with LSC and ASC. Also, we also set IMC ASC gain to zero (following Ushiba-kun's suggestion) so that we were able to walk across the light beam to IMMT1T without disturbing the aligned IMC. Then, it seemed the spot on IMMT2 was mis-centered somehow. So, we re-considered our plan. At this point, the new plan was to (1) reset the setpoint values of IMMT1 and 2 to the values before yesterday, (2) check the IMMT2 centering, and if not good, adjust with IMMT1 setpoint, (3) align IMMT2 to center the spot on PR2 with setpoint, and (4) adjust PRM centering with its traverser.
When reset IMMT1 setpoint, the beam centering at IMMT2 seemed ok (Fig. 1) , so we simply left the IMMT1 setpoint as reset value. Then, we tweaked IMMT2 to bring the beam spot at the center of PR2 (precisely, its HR target); Fig. 2. With this situation, we checked the beam spot postion at PRM (both at AR (Fig. 3) and HR (Fig. 4) with the relevant targets; these AR and HR spot locations were almost the same), and it was 5-mm off-centered in the minus Y direction.
5-mm would be too large for the PRM traverser to move. The demerit of using the traverser are (1) the small movable limit itself as already mentioned, (2) we need to adjust oplevs otherwise we would lose ALIGNED state of PRM (ALIGNED state would be useful to control a suspension with its setpoint values), (3) even though "ALIGNED" can be obtained, this would not mean the beam reflected at PRM could reach REFL, and (4) at any rate, PRM mid baffles do not follow the move of the main suspension chain of PRM. On the other hand, considering the RoC of PRM is ~460 m (kagra wiki), 5-mm off center might be acceptable or easily to be compansated.
So, the original plan mentioned above was slightly modified: (4) not adjust PRM traverser, but adjust mid baffles for PRM later.
The PRM AR mid baffle seemingly caught two (known) ghost beams from PRM. In this sense, this mid baffle would be also ok. But in fact, looking at the aperture of the PRM AR mid baffle, the aperture edge seemed shining with IR (Fig. 5; taken from a location between IFI and IMM with the Miyakawa-san's IR camera; two ghost beam spots at IMMT1's shield can be also seen, and they may come from IFI...). This shining might be due to the main beam's slight clipping. JGW-T1910659-v2 shows where these ghost beams and main beam would come at PRM AR mid baffle, and this clipping might happen in the current situation: the main beam is shifted with respect to the aperture in the minux Y direction about 5 mm, while this document says the 2.8 sigma edge of this main beam should be nominally 4.8 mm away from the edge. So we will adjust the AR (and HR for balancing?) mid baffle position later. See also Fig. 6; also compare with 20797 and 21654.
Anyway, apart from the slight clipping at this PRM AR mid baffle, the main beam would be well aligned. Then, we detached the duct connecting the POP table and PR2 chamber to see the PR2 transmission IR beam, or POP_FORWARD. Fortunately we confirmed that this beam was somehow reaching relevant two QPDs (Fig. 7). We also confirmed this with QPDs SUM count variation. But we also found that this beam was 90%-ly clipped at a steering mirror just after the periscope (Fig. 8). It seemed no simple way to resolve this clipping...
Note
- We confirmed that the beam spot on PR2 varied depending on the PRM state PAY_FLOAT or ALIGNED. Be careful!
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