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MIF (General)
masayuki.nakano - 0:37 Saturday 14 October 2017 (3508) Print this report
Investigation into PR2 inclination
[Kokeyama, Yokozawa, Miyo, Enomoto, Arai, Nakano]
 
As yesterday's report, we found the reflection beam from PR2 has unexpected inclination. We tried to find the cause of it.
 

In conclusion, the PR2 inclines 5.4 mrad more than design value. Additionally the PR3 also inclines 3.3 mrad more than design value.

1. Visual inspection 

First of all, the PR2 suspension has been checked by our eyes. Check point is 
a. see the mirror from side and check it inclines.
b. check that OSEM is well centered.
We could not find any problem. The picture is attached.
 

2. Inclination measurement by optical lever.

We measured the inclination of PR2 by reflection angle of a optical lever laser injected horizontally. The incident beam was leveled by a laser level (see attached picture). The beam height from incident beam was measured at a oplev breadboard by measuring the distance between the reflected beam and the laser level beam. Afterwards we checked the distance of a measurement point from a AR surface on a drawing. The result is as below
 
Laser height : 14mm at 80(2)cm from AR surface.
 
So the PR2 incline by (14mm/0.8m) /2 = 8.7(0.2) mrad, and it is 5.4 mrad larger than design value (3.3mrad)
 

3. Relative height measurement of mirrors

To check the incident beam angle, we measured the relative height between IMMT2, PR2, PR3 and PRM. All mirrors can be looked into by an auto level from the BS side flange of the PR2 chamber. The picture is the IMMT2 suspension looked by the auto level. We marked of each mirrors height on PR2 chamber (see the attached picture) and estimated relative heights of each mirrors. The result is as below
 
IMMT2-PRM : 11mm
PRM-PR2 : 14mm
PR3-PR2 : 29mm
 
The distance between IMMT2 and PR2 is 19.78m, so the incident beam goes up by 54mm/19.78m = 2.7 mrad under the assumption that the incident beam hit center of IMMT2 and PR2 mirrors.
 

4. Reflection angle measurment of main beam

We measured the reflection angle of the main beam more finely than yesterday. The relative beam height between the incident beam and the reflected beam of the PR2 was measured by a sensor card and it was 15(3) mm. The distance of measurement point from the SR2 HR surface was 1.4m. So the reflection angle is 
15mm/1.4m = 11(2) mrad
 
The inclination angle calculated by 3 and 4 measurement is (11/2) + 2.7 = 8(2) mrad. It is consistent with the result of 2. measurement.
 

5. PR3 inclination measurement by optical lever.

The similar measurement with 2 was done on the PR3 mirror. The result is as below,
 
Laser height : -12mm at 92.5cm from AR surface.
 
That means the PR3 inclines by (12mm/0.925m)/2 = 6.5 mrad. It is 3.2mrad larger than the design value.
Images attached to this report
Comments to this report:
yutaro.enomoto - 18:58 Wednesday 18 October 2017 (3536) Print this report

We found a mistake in calibration of PIT direction sensing of oplevs.
For PIT sensing, arm length of oplev should not be just the distance between the mirror and the QPD. The incident angle to the mirror should be taken into account. See the attached pdf file for a sketch.
Arm length should not be L, but L*cos\alpha.

Izumi-san told me that this effect is summarized in Sec. 4.11 of FINESSE manual document by A. Freise, so see this document for more detail.

Since I heard this effect for the first time (in fact I "found" it by myself), I guess this effect is not so common in KAGRA,
so I would say it is better to check the calibration of PIT sensing of all the oplevs and to make sure not to make the same mistake in the calibration of oplevs that are not yet calbrated or implemented.

Non-image files attached to this comment
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