[Jenne, Keita, Louis]

Following up on the POP path issues mentioned in 91647. Keita suggested measuring the power in the POP beam on ISCT1 to 1.) compare it against what we expect to have and 2.) to compare it with different spot positions on PR2. Today we did the initial measurement. We'll need to come back (hopefully tomorrow) to repeat after moving the PR2 spot position. We did the measurement in single bounce with ITMX with 60 W input power (which put 55.3 W at the PRM). Following Keita's suggestion, Jenne turned on POPX centering and noted a POPX NSUM value of 0.76. After each spot position change on PR2 we will remeasure the power in the POP beam just out of the periscope and note any fluctuations in the POPX NSUM (centered). Having both numbers should help us determine if we are clipping upstream or downstream of the dichroic splitter (M10). There is good reason to suspect that we are clipping on the periscope on ISCT1 (see 91647 and 91481).


We grabbed one of the red Thorlabs power meters from the optics lab and placed it just upstream of the POP beam shutter. At first the power meter read about 71 uW steadily. After a few moments, the power meter readout started to fluctuate from about 69 uW to about 72 uW. We couldn't immediately tell what was causing the swinging. The AS Air camera was stable and not swinging. The REFL camera was saturated (most likely because we had the door open on ISCT1). Trending back I found that POPX NSUM and YAW started being very noisy at 1471643523. This lasted for about 30 minutes. I have not been able to find a source. I checked suspensions and QPDs from POPX back to IM4, + PR3, BS, and TMs and could not identify a culprit for the 'swinging'. There was a power glitch in the afternoon before we went out to ISCT1 but Ibrahim reported the glitch timestamp to be 1471640837, about an hour before the POPX issue. I put a screenshot showing POPX NSUM, YAW_OUT and a few other channels I checked during the same time (pop_isct1_shaking.png. There was some JAC-related ASC testing going on at around the same time but none of the trends I checked showed clear correlation to noise in POPX.


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With 55.3 W at the PRM (taken from IM4 trans, which is assumed to be calibrated to Watts at the PRM), we expect the power coming out of the periscope on ISCT1 to be 

P_isct1 = P_PRM * T_PRM * R_ITMX * 2*T_BS * T_PR2 * R_HAM1_air_bs = 55.4 W * 3.1% * 98.5% * 0.25 * 229e-6 * 90% = 87.2 uW 

With 71 uW measured at at ISCT1, we are missing about 18.5 % of the expected light in the IR POP AIR path.

All optics values were taken from galaxy, with the exception of the BS in HAM1 on the POP Air path. I found this optic on the HAM1 assembly (D1000313) and corresponding bill of materials.