LIGO Document G1602132-v1

CAD Camera Views from ETM for scatter evaluation

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Photon Calibrator (pcal) periscope motion was found to be responsible for scattered light noise in DARM:

Scattered light analysis of the PCal Periscope, D1200993, is addressed in these design documents:
section 4.9 (pg 54-57) of T1000747, "AOS SLC Arm Cavity Baffle FDR"
item 4 of T1500096, "Disposition of action items from the Photon Calibrator Final Design Review"
T1000747 indicates 4% of "wide angle scatter" from the TM passes through the ACB aperture and hits the PCal Structure or manifold spool or the CMB. The off axis position of the camera mirror mounts is stated to be .09 rad (5 deg).

Photographed views of, and from, ETMX for scatter evaluation:

This DCC entry shows CAD views from ETMy with varying fields of regard. Each view is centered and perpendicular to the ETMy HR face with fields of regard of X degrees (rectangular, i.e. the corners of each image are beyond X degrees from the central line of sight).

Compare this photograph, for ETMx
to the CAD image for a 45 deg field of regard, for ETMy (symmetric reflection).

Remember ETM's are off center in chamber, see

As a result of the commissioning team finding scattered light issues with the PCal Periscope structure, we are looking into:
1) removing the large rectangular periscope mirrors & mounts, used for the camera lines-of-sight
2) passively damping the structure with vibration absorbers, D1002424, and viton pieces between the manifold tube and the PCal structure
3) tessellating the annular region of the PCal structure adjacent to its central aperture, with low BRDF materials (and using some of the rib bolts to attach). Perhaps mAR or DLC on SS (see T1300731).

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