Nutsinee, Kiwamu, Alastair (remotely)

We have aligned the CO2 Y beam relative to the main interferometer beam. We are ready for various TCS tests again.

We aligned the beam by doing the same exercise as Aidan and Alastair did back in this February (25353). According to what Aidan calculated, the precision of the adjustment is about 20 mm which should be good enough (recall that the CO2 beam size is twice bigger than that of the main laser i.e. 100 mm or so in radius). We hooked up an analog function generator to the modulation input of the AOM driver which resides on the right hand side of the TCSY table. We drove it with an offset of 0.5 V and an amplitude of ~0.2 V at a frequency of 50 Hz. The frequency is chosen to be higher this time because the interferometer had higher noise floor below 40 Hz (dominated by CHARD control noises). We steered pico motor G which corresponds to a mirror on top of the periscope. After an iterative adjustment, we ended up with [x, y] = [-17500, 27000] in the pico motor readout. Note that these values are almost comparable to the previous adjustment by Aidan and Alastair. As a result of the alignment, the CO2 coupling to DARM increased roughly by a factor of 10 -- indicating a better overlap between the CO2 projection and the main interferometer beam.

Side note is that we saw a slow drift (with a small oscillation on a time scale of 10 min.) in the observed CO2 power during the adjustment in which the CO2 power monotonically kept decreasing by 10% or so over an hour or two. After unplugging the function generator, we also saw the power drifting back to the nominal power at a faster rate. A seemingly coherent behavior was visible yaw and pitch of QPDs A and B. No idea why.