Louis, (Camilla), Vicky

Summary: HAM7 scatter is not from OFI_PD_A, but scatter shelf can be reduced about 100x by mostly 1) raising ZM4 pitch = 750 (from -560), and 2) slightly lowered yaw to zm4y = -50 (from 376). At this "less-scatter" alignment, OFI_PD_A~0.01.

For the scatter measurements, we took the IFO from NLN to NLN_CAL_MEAS at 1346128561, and shuttered all SQZ beams (incl. clf/seed, even green pump/fc), w/OPO unlocked, VOPO "Aligned", before opening the beam diverter.

For most measurments, we drove ZM4_TEST_L_EXC at 0.3 Hz, with 5000 cnts. We started with Camilla's recommended ZMs.

With ZM values for 1) IFO/SQZ alignment, 2) OFI_PD_A "aligned", 3) OFI_PD_A "slightly clipping", no noticeable changes in scatter shelf height nor cutoff freq.

With ZM values for the "scatter model" (at 63880), we saw a ~10x reduction in scatter shelf height! This led us to search around for alignments which could further reduce scatter. To do so, we first "Set ZM IFO" 'd to start from sqz/ifo aligned values.

Then, we scanned ZM4 P/Y to move towards the "scatter model" settings, which meant raising ZM4 pitch, and lowering yaw slightly. This let us first align to OFI_PD_A, then quickly the pitch increased enough that we clipped (pitch high) on OFI_PD_A, so it was back from ~0.04 to < 0.01. We found ZM4 p/y = 750, -50, which brought the scatter shelf down to ~3e-19 m/rtHz, from 1e-16 m/rtHz!

• Clipping values here which slightly increase the scatter shelf from "best" are around zm4y=(-170, 350), zm4p = (650, 850).

To close up, we left IFO back to NLN (successful transition) and closed our beam diverter. We are leaving ZM4 p/y = 750, -50, others are at "Set IFO ZM" values. Would it be interesting to take IR photos at this alignment, to see which spots go away (those might be the scatter sources)? I wonder if the weird IR spots near green-opo-incoupling beams go away in this alignment? Data saved in /ligo/home/victoriaa.xu/Desktop/Screenshots/090122_scatteringTests/090122_scatteringTests_working.