Ushiba, Tanaka
## Abstract
In response to klog31962, we investigated the LSC and ASC beam path on the REFL table. we found roughly 3 issues; clipping on the first iris, clipping on BS3 on the ASC path, and the dirty (or scratched?) HWP1, and we found the 2 concerns; the reflection from the beam dump illuminates PDA1 and PDA2, and the inopitmal mirrors are used for the WFS f3 QPDs.
We solved two clipping issues but other issues remain. We need to consider how to deal with them.
Also, we put the beam dump to receive the beams that hit the wall as reported in klog31962.
## What we did
First, we aligned PRM and misaligned two ITMs to use the single bounced beam from PRM. We started the beam tracing from the upstream on the REFL table IR camera. We found roughly 3 issues as below.
### clipping on the first iris
As already reported in klog31962, the beam clipped on the first iris. Fig.1 shows the beam spot on the first iris when we closed the iris. We adjusted the iris position so that the beam pass through the center of the iris. Fig.2 shows the beam position on the iris after the adjustment. The beam passes through the center of the iris.
Then, we found that the beam position on the second iris in front of HWP1 on the LSC path also shifted slightly from the center (Fig.3). So we adjusted this iris (Fig.4).
### dirty (or scratched?) HWP1
Next, we investigated the beam position on HWP1 and the aperture of the plate because the ring had previously burned due to the wrong beam position (klog13794). Fig. 5 shows the beam position on HWP1. (In this time, we removed the second iris.) The beam position seemed to shift in the vertical direction. On the other hand, we found there were something like white stains on the plate. We picked up the plate and looked into it. There appeared to be some dust on the HWP. We tried blowing them away with a blower, and it appeared that the dust had disappeared with our eyeballs.
While as it, we measured the HWP1 aperture with a rular (Fig.6). The inner diameter of the rotator seems to be ~25.4 mm, which is ~ 1 inch. the width of the rind inside the rotator seems to be ~2 mm. So the aperture of HWP1 seems to be ~20 mm.
Then, we reinstalled HWP1 and adjusted the height so that the beam passed at the center of the plate. Fig.7 shows the beam position. The beam position seems to be fine. However, the white steins remain. When we took the HWP off again and looked at it carefully, we found that there were scratches or stains spread across the top of the HWP, and there were also a few of them in the center (Fig.8). The cause of these scratches is currently unknown (damage caused by high-power?). As there is no countermeasure for today, we put it back as it is for now.
### clipping BS3 on the ASC path
We found that the beam clipped on BS3 in the ASC path. We adjusted the position of this BS, and then we aligned the downstream path.
The beam was either centered on the other optics or slightly off-center. For optics with shifted beam positions, we adjusted the alignment of the optical system and the position of the optics so that the beam would be centered on them.
After the work, we performed centering on all PDs and QPDs. Also, we put the beam dump to receive the beams that hit the wall as reported in klog31962 (Fig.9). Furthermore, we opened two irises completely.
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## Note
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The reflected light from the beam dump, which is receiving reflected light from TFP2, is quite strong (Fig.10). This light is shining on PDA1 for the CARM control and PDA2 for the 3f controls (fig.11). To prevent this, it may be better to put a screen between them.
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We noticed that the pico3 and pico4 mirrors in the WFSf3 path were not polished on their back surfaces. In this case, the light transmitted through the mirror is scattered on the back surface, so it might be better to replace them with back-surface-polished mirrors.
