Anamaria, Robert
We photographed, from a second angle, the beamspot of a stray beam that I noticed last visit (76969 - Fig. 3 ). The photos (Figure 1) confirm that there is indeed a bright stray beam hitting, at a grazing angle, the bellows and other parts of the spool piece between HAM3 and the IMC tube.
I also wanted to measure the power of the stray beam that had been producing a large 48 Hz peak in DARM during early O3, in order to improve my scattering coupling calculations. This is the beam that reflects off of the PR2 scraper baffle and hits the illuminator viewport on HAM3, which we mitigated by inserting a black-glass viewport cover (52184). We were surprised by how bright the beam is, reaching 20 mW on the power meter (see Figure 2), even though we could not fit the whole beam on the meter. We estimate that the beam may reach 60 mW.
Finally, we made movies as we swept the ITMY compensation plate, hoping to see some indication of where the ghost beams are hitting, but did not. We will have to wait until Alena gives us the likely position of the CP ghost beams based on the angles that Anamaria showed us how to measure.
Minhyo, Anamaria,
I've calculated the point at which the tail of the Gaussian laser beam should touch the PR2 scraper baffle to reflect about 60 mW, following Anamaria's idea.
Assuming that the estimated beam size at the PR2 scraper baffle is 7.47 mm, and the total beam power is 3 kW, the beam touches the side of the PR2 scraper baffle's hole at a position 15 mm away (4 sigma) from the center of the beam. I'll include the calculation later on how the beam is actually positioned within the 70 mm aperture of the PR2 scraper baffle.
The original alog referenced above (52184) has photos that show that the aperture of the baffle is visible at the same location as the beam when the interferometer is unlocked, and argues that this means that the beam is hitting the edge of the baffle aperture.
Minhyo
1) Made typo in above comment from mine. The estimated beam radius 7.67 mm -> 7.47 mm (edited on the original comment as well)
2) I'll elaborate the result of calculation in above (15 mm from the center) in below:
Assuming the total power is 3 kW, the percentage of 60 mW power is 0.002%. Since the percentage of normal distribution for 4 sigma is 99.9937%, the partial integration in the upper limit is around 0.0032%.
Using this approximate number, I searched for the exact number to produce 0.002% with a 2D Gaussian beam model with a 3 kW power and 7.47 mm radius. From the beam model, it showed that integration from 4.11 sigma produces a power of around 0.0604 mW, suggesting that the beam is clipped at a point around 14.94 mm (4.11 sigma) from the center of the beam. I have attached a summary figure of my calculation.
Rechecked situation at LLO: https://alog.ligo-la.caltech.edu/aLOG/index.php?callRep=70996
TLDR: we see the same, but seems much lower power. Will measure when we get the chance.