[Ikeda, Hirata, Takahashi, Ushiba]

Abstract:

We moved PR2 about 3.3 mm in + Y direction and IMMT2 beam spot about 6 mm in + Y direction.
Then, we centered IR beam on PR2 and check beam spot on PRM, which is also close to the center.
So, alignment to PR2 seems fine now.

Detail:

1. Move PR2 in +Y direction:

For reducing GRX beam light power to identify GRX beam spot center easily, we requested ALS_FIBX guardian to SAFE state (GRX power becomes about 0.8 mW).
First, we moved PR2 so that GRX beam spot on PR2 HR target became nominal.
Since PR2 target was set as projecting PR2 center along IR injection beam (PRM-PR2 line), GRX should be off centered by 3.3 mm (165.48 mm (distance between PR2 HR surface and target) * 0.02 rad (angle between input and reflected beams of PR2)).
Figure 1 shows the GRX beam at PR2 HR target before moving PR2.
Figure 2 and 3 show the GRX beam spot after moving PR2 by 3.5 mm in +Y direction with the traverser.
Now, beam spot seem shifted in -Y direction more than 3 but less than 3.5 mm according to the fig3, which is almost the nominal value.

2. Move IMMT2 beam spot by moving IMMT1:

After moving PR2, we moved IMMT1 so that beam spot on IMMT2 shifted in + Y direction.
First, we requested PROVIDING_STABLE_LIGHT state to IO guardian and wait until alignment becomes good, and HOLD_ALIGNMENT state to IMC guardian not to kick the suspension when blocking the beam to IMMT1T QPDs.
Then, we set a ruler in front of IMMT2 and check the beam spot on the ruler before and after IMMT1 moves.
Figure 4 and 5 show the beam spot before and after moving IMMT1.
IMMT2 beam spot roughly moved 6 mm in +Y direction.

According to the current situation reported in klog29285, we need to shift PR2 beam spot by 5 mm along +Y direction.
Roughly speaking, 6 mm beam spot shift on IMMT2 moves PRM beam spot by 4.5 mm (6*3/4).
Due to PR2 position changes (3.3 mm along +Y direction), PRM beam spot will change 0.8 mm (3.3/4) in +Y direction when the beam is centered to PR2. 
So, in total, PRM beam spot changes by 5.3 mm, which is the same as we would like to move.

3. Center the beam to PR2 by moving IMMT2:

After moving IMMT1, we centered the IR beam on PR2 by moving IMMT2.
Then, we aligned PRM so that PRM reflection goes to REFL table.
Figure 6 shows the beam spot on PR2 target with aligned PRM, which seems well centered horizontally.
After that, we checked the beam spot on PRM AR target (fig7).
The beam seems hitting almost center.

Note:

This time, we haven't changed beam spot vertically though it is obviously higher than the nominal position on PR2.