The bracket, which will hold the upper periscope mirror for the ALS system, was installed this afternoon. The attached picture shows the actual bracket installed on the structure with 8-32 screws.



During the installation, one of the threaded holes on the periscope structure (shown as A in the attached picture) was accidentally damaged and hence the screw won't be really tighten any more. The cause of the damage was that --- a lower screw (shown as A in pic) and another lower screw (shown as B) were orthogonally crossing each other in the structure. At first the B screw had been inserted and then the A screw was being tightened. Then I realized that they were interfering with one another, but it was too late. I unscrewed the A screw. However during the removal of the A screw the thread of the hole was mechanically damaged due to the fact that the screw had been bended slightly. The screw can be still in, but it can not be really tight any more. The B screw was removed without serious damage on the threaded hole, though the screw had a clear mark of the A screw. This was then replaced by a new one without a problem. The same situation was found in the upper screws and the issue was avoided by using shorter screws (8-32-3/8"). Currently the A screw is in, but loose. I will remove it tomorrow or later as it doesn't do anything good but it moves around and creates some vibration.

I consulted with Robert about this issue. A main worry was the mechanical stability of the bracket with one screw missing because it may wobble the bracket too much, resulting in unacceptable amount of phase noise in the ALS light (and perhaps even in the PSL main light). The total number of the screws are currently 11 (12 if including the broken one) --- seven of them are on the plane where the B screw is, and the rest of four are on the side where the A screw is. Note that all of the four screws are not shown in the picture. At this point we think that it (hopefully) will not be a matter as we still have so many screws in. We'll see how it goes.