[Yokozawa, Aso, Komori]

Abstract:

The point of contact may be located around the IM or TM, rather than the MN, based on the measurement that the DC response of the TM pitch is smaller than that of the MN.

Detail:

We attempted to identify the point of contact in the IX cryopayload.

First, we performed a rough centering of the beam spot on both the MN and TM QPDs by adjusting the moving mass on IX MN.
At that time, the oplev pitch signals in the unit of error function were approximately –0.3 and –0.2 for the MN and TM, respectively.
Since the MN yaw error signal was 0.997 and we do not have a system like the moving mass to adjust the yaw degree of freedom, we could not perform the same thing in yaw unless we relocate the MN QPD inside the tunnel.

Next, we applied several offsets to the MN optic alignment, ranging from –300 to +300 in steps of 100, and measured the resulting oplev values for both the MN and TM, as shown in the attached figure.
The MN response was consistently larger than the TM response, suggesting that the point of contact is around the IM or TM.

It should be noted that the responses were not linear (larger positive offsets resulted in larger DC responses), and the oplev values themselves were not reproducible.
However, the trend—namely, that the MN response was always larger—was reproducible.