Best to just calibrate the ETM OL in the usual way. I bet the OSEM outputs have a cal uncertainty of ~50% since the input matrix changes as a function of the DC alignment. Still, a 30 urad pitch mis-alignment gives a (30e-6 rad)(40 m) ~ 1 mm beam spot shift. This would be enough to flash other modes, but it would still be easy to lock on a TEM00 like this. I also doubt that the OL calibration is valid outside of some region near zero - can easily check by moving the ETM bias sliders.
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I should've put in the SUSPIT and SUSYAW channels in the previous screenshot. I re-aligned ETMX till I could see IR flashes in the arm, and also was able to lock the green beam on a TEM00 mode with reasonable transmission. As I suspected, this brought the Oplev spot back near the center of it's QPD. But the answer to the question "How much did I move the ETM by" still varies by ~1 order of magnitude, depending on if you believe the OSEM SUSPIT and SUSYAW signals, or the Oplev error signals - I don't know which, if any, of these, are calibrated.
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What we still don't know is if this is due to Johannes/Aaron working at the ETMX rack (bumping some of the flaky coil cables and/or bumping the blue beams which support the stack). Adding or substracting weight from the stack supports will give us an ETM mis alignment. |