Robert, Mitchell, Corey, Tony, Betsy, TJ, and Alena, Eddie and the SLiC team

In O4a, scattering noise at or near the current DARM noise floor was likely produced by 3 sources in the input MC tube (74772). The most important source was the MC baffle by HAM3, followed by the MC baffle by HAM2. I think that the main reason these two baffles produced scattering noise is that they were not angled enough and so they retro-reflected light scattered from certain optics as well as reflecting light between optics. To mitigate this, we increased the angle of these two baffles from 5 to 10 degrees (75637, 75654). We also increased the damping of both baffles in order to reduce the cutoff frequency of scattering shelves.

In addition, the beam tube itself might have been producing scattering noise associated with its axial resonance of 15.2 Hz (Fig. 1).  Because of this possibility, we installed nozzle baffles in all MC-tube nozzles that have blank-off flanges (15 total - 75654 ). Blank-off flanges are generally the most problematic nozzle equipment because they cover the whole nozzle and the circular milling groves retro-reflect some of the incident light from a wide range of angles. We also installed two nozzle baffles in the upper two ports of the +Y door of HAM3 (last page of Fig. 2).

Finally, we installed an unplanned baffle to hide a portion of the MC-tube wall that was likely retro-reflecting light scattered from MC3 (75688). This glint was identified in beam-spot photos, photos that are taken and illuminated  from near the beam spot locations on the optics.

Figure 2 shows beam spot photos taken from the point of view of each of the six optics before and after the mitigations. There were some auto-focus problems with some of the "before" pictures. Some of the photos are annotated with features and possible problems.

Figure 3 shows the damping of the two MC baffles