KAGRA Logbook

MIF (ASC)

marc.eisenmann - 17:54 Monday 31 October 2022 (22748)

Start investigation of TMSX astigmatism

[Chiaki, Marc, Matteo]

Abstract:
We started investigating TMSX source of astigmatism, the work will continue tomorrow.

Detail:
With the X arm locked and guardian paused we could check the beam propagation on TMSX from the periscope to the installed QPD.
We found some slight miscentering on the 2 lenses and on the first mirror of the periscope.
Also, it seems that the RBS1 is installed in reverse (HR side away from the input beam), see Fig1 with the input beam and Fig2 the reflected beam
We took the beam profiler to Xend and will check tomorrow if any of these items could explain the astigmatism.

Images attached to this report

Comments to this report:

marc.eisenmann - 23:34 Tuesday 01 November 2022 (22762)

[Chiaki, Haoyu, Marc, Matteo]

We rotated RBS1 to have the HR side on the input beam. Then, we realigned the IR beam on the camera and PD in transmission of this optic.

We checked the beam profile far after RBS1 reflection and could still see not perfect round shape.

We installed a pick-off mirror between LNS1 and LNS2 and could see also some non-perfect round shape of the beam (see fig1 far from the optic). We actually suspect some clipping on the right side of the beam (in fig1) more than astigmatism.

We also checked before LNS1 and had some similar conclusion even though we did not had good picture of the beam.

It is a bit tricky to check the beam between the 2 mirrors of the periscope but it seems that even if the beam is a bit low on the top mirror, there is no clipping there. It would mean that the clipping could come from the in-vacuum optics.

A good check could be to lock the GR beam and check from the GR camera if we see similar clipping. If yes, the culprit is on the common path of GR/IR. Otherwise, it might be due to something around the in-vacuum dichroic.

Also, discussing with Ushiba-san, a possible explanation for the clipping could be the cooling of X end that may affect the beam path after the mirror as the mirror is moved vertically during cooling.

Images attached to this comment

tomotada.akutsu - 8:55 Wednesday 02 November 2022 (22763)

The sympton at O3GK was that the GrX did not have strange pitch and yaw coupling, while IR did have. Considering that the 532 nm beam spot can have 1/sqrt(2) times of that of1064nm, a particular situation like "clipping IR while not clipping Gr" might happen on the common path of Gr and IR. I cannot believe that the beam path after separating Gr and IR on BRT-X would have such drastic clipping even after opening the GV in-between EXT and EXC.

Actually, one of my suspects is that the "clipping" might be in the cryoduct shield. Can we slightly change the X arm IR optical path for this??

By the way, note that the currently-installed TMS-VIS does not have sufficient displacement actuators to keep the suspended table at your favorite postion unfortunately; this was from the first of the design review, and confirmed the other day by experiment.