This afternoon I went to ISCT1 to begin setting up for the beam size measurements that we hope to use to diagnose the PRC mode matching situation (see LIGO-T1400013).
I took the beam analyzer cart from the optics lab out to ISCT1 and left it set up there. I measured beam powers in two locations just after the REFL periscope: one in reflection of the pick off window directly after the lower periscope mirror, and one after. Since we aim to measure two beams in the REFL path with powers differing by a factor ~4000 it's useful to have low and high attenuation locations. The window is not AR coated so there are two beams visible in reflection. I measured the power of one to be 890uW, and the other to be 800uW, with the low-power head on the VEGA power meter. The beam in transmission of the window was measured to be 123mW. I left the table with the window reflected beam dumped, and the photon inc. beam analyzer behind the dump.The window transmitted beam is still dumped on the shutter.
I just looked for the beam in the same locations with PRM misaligned (sending direct PRM reflection to the parking dump). This should be the beam transmitted through PRM, reflected off ITMX, transmitted through PRM again and sent on to ISCT1. The good news is, the beam was still nice and visible on the IR card after the pick-off window
I tried to measure the power at that location with the same power meter as before, expect with the filter out. Due to the X-arm work going on, the beam was finging quite a bit in the IR, so I couldn't get a very solid reading. It looked like around 50uW on average, with a peak value of ~80uW. Comparing this with the PRM aligned value it seems a little high. The PRM misaligned beam should be roughly T_PRM^2 * T_BS^2 = 2.25e-4 times the PRM aligned beam. This means I would have expected 123mW*2.25e-4 = 28uW. Maybe some of the green light was leaking in, or maybe the X-arm flashes were confusing the power meter... it will be easier to get a good measurement here with ETMX and ITMY misaligned later. I can't take beam profile measurements until then anyway.