Naoatsu, Kiwamu and Ayaka (over the phone)

We mechanically locked the PR3 mirror to its recoil mass because we had another incident where the mirror angle slipped today.

We are hoping that this will reduce the chance of the PR3 slippage.

[Slippage of the day]

As reported by Yamamoto-kun (3777), the PR3 control machine had a trouble in its digital system. The obvious symptom was that the IOP outputs were unable to receive the digital counts from the user front end code. In fact, they digitally kept zero values at their outputs.

As soon as we recovered from the issue, we noticed that the PR3 oplev had lost its beam on the QPD. See the attached picture in which you can see the oplev beam hitting a place well outside the QPD active area. This roughly corresponds to misalignment of 10 mrad which is quite large. The direction of the misalignment is such that the HR side pitched up or equivalently the pitch went to the negative side.

Remember that we had similar incidents on the same mirror in the past months (for example 3634).

[Locking the mirror to the recoil mass]

Consulting with Shoda san and Saito san over the phone, we decided to lock the mirror to its recoil mass. One plausible hypothesis for this type of slippage is relocation of the wires that go underneath the optic. Since the bottom part of these wires may not be mechanically constrained well against the mirror, they can change the position. However, one strange thing is that rebooting a digital system should not give a jolt on the suspension as they start from digital zero values at the outputs. So we are not quite sure how this could happen.

Anyhow, we locked the mirror by doing the following procedure:

1. The IM stage was locked by securing the EQ stops.
2. The recoil mass was locked to the cage by tightening the EQ stops.
3. The mirror was locked to the recoil mass by tightening the EQ stops.
4. We then released the recoil mass from the cage by loosening the EQ stops.
5. Finally, the IM stage was released by loosening the EQ stops.

After this process, we then adjusted the OSEM positions to make them at around the half-light values as usual. Also we visually checked the OSEM flags to make sure that they are not rubbing.

[Finally we are back to where we were yesterday]

After locking the PR3 mirror, we steered the IM stage in both pitch and yaw roughly by 100 urads to get the beam at the EX PD. So we are finally back to business.

Sad but true....