Josh, Jeff B., Guillermo

Scattering arches in DARM between 40 Hz – 100 Hz are coincident with scattering arches in the ASC TR A and B photodiodes at frequencies between 3 Hz – 10 Hz.

Summary:

• We found loud scattering arches in DARM for some of the Hveto winners on Dec 10, Dec 15, and Dec 16.
• While looking at the omega scans, we found that these arches are showing up in ASC Y transmitted light too.
• At L1, we were seeing scattering in the ASC Y (alog 41960), and more recently in the ASC X transmitted light (alog 42402).
• We did two things:

1. We searched, using the HHT tool, for surfaces that could be generating the scattering.
2. We generated Q-scans to check where does the scattering arches manifest with the highest amplitude.

Results:

1. As expected, the HHT tool points the Y end as the culprit. The tool points the ETMY M0 and ETMY R0 as the surfaces generating the scattering. The first 5 scattering harmonics of the scattering predictor do not reach the frequency band 40 Hz – 100 Hz but they reach the 3 Hz – 10 Hz frequency band. See figures Scattering_LHO_1.png, Scattering_LHO_2.png, and Scattering_LHO_3.png. In the plots, F1, F3, and F5 are the first, third and fifth scattering harmonics. We intentionally omitted F2 and F4. 
2. In the Q-scans, of all the scattering instances we found, we see that the arches are shown with more energy in H1:ASC-Y_TR_B_NSUM_OUT_DQ and H1:ASC-Y_TR_A_NSUM_OUT_DQ, see the top row of Figure_1, Figure_2, Figure_3, and Figure_4. The scattering features show with less energy in the ASC_X_TR.

Note: scattering predictor in Hz is defined as F_fringe = 2*abs(v_surface)/lambda and the scattering harmonics are the fringes or arches generated after the scattered light hit more than once the perturbed surfaced; see Noise from scattered light in Virgo's second science run data.