Yuzurihara, K. Tanaka

Continued to 17903

We continued to investigate the cause of the strange beam shape. However, we couldn't find anything in particular that seemed to be clipped. In other words, the shape of the beam seems to be distorted by the lens L2 effect. I'll do some analytical calculations tomorrow.

### What we did

• We checked to see if the beam was clipped before or after the periscope at the far end. Figures 1 and 2 show where the beam was hitting the upper mirror of the periscope. Figures 3 shows where the beam hits the lower mirror of the periscope. The beam is not in the middle of the mirror in these pictures, but it does not seem to be clipped. Figures 4 and 5 show the shape of the beam as seen with the beam profiler placed just before and just after the periscope. The beam looks something like a dekopon (デコポン) (main and ghost beams overlapping?). This shape does not change before and after the periscope.
• We looked at the beam profile on the path towards the DCPD, with the point where the distances from l1 to l2 are equally being the zero point of the measurement. Figure 6 shows a superimposition of the beam profile measured in the DCPD pass and the QPD pass yesterday. You can see that the transition is basically almost an ellipse.
• On a different note, it is also an ellipse in front of the gigE camera (Fig. 7). On the other hand, it looks like a circle in the camera image.