[Tanaka, Ushiba]

Abstract:
Tanaka-kun centered QPDs of MCI, MCE, and MCO.
Then, we set HWP just after laser shutter of main IR beam to change the s-pol to p-pol.
The IMC REFL DC becomes less than 0.02 W, which is over 100 times smaller before putting HWP.
Now, IMC cannot be locked due to the lack of REFL power, so we stop IO guardian at PSL_LOCKED state and keep them as they are.

Detail:
First, Tanaka-kun centered the QPDs of OpLev of MC suspensions at around 9:30 JST (fig1).
Due to the drift, MCI OpLev iwas slightly off from the center but seemed no problem.

Then, we entered PSL room and changed the IR beam polarity.
First, we set one of HWPs found in the decicator in the pre PSL room (fig2); however, we found that this HWP is not the one for changing s-pol to p-pol.
So, we replaced it to correct HWP (fig3, fig4, fig5), and the initial angle of HWP was 171 degees (fig6).
Since we are not sure HWP angle is good because the screw for fixing the HWP angle was loose, we confirmed the good HWP angle with the same way in klog15830.
The final HWP angle is 156 degrees (fig7) and tighten the screws to fix the HWP angle.

The REFL DC PD value is less than 0.02 (fig8), which should be calibrated to Watt, while the value before putting HWP was 2.4 - 2.5 W (fig9).
So, power reduction is more than 100, which is enough for venting hopefully.
Due to this power reduction at IMC REFL table, IMC cannot be locked now.

After this work, Kimura-san started venting.

Note:
Since the apearture of HWP is very small, a part of IR beam can be clipped by HWP.
So, it is nice to prepare larger HWP for this purpose in future.