This morning, we made another attempt of measuring the polarization noise of the single bounce beam.

We put DET_MAIN in PAUSE and Francesco put the NI single bounce. 

Then Francesco scanned the angular position of BS_MAR_TX (Fig.1), and we found some ghost beams of the single bounce on the B5 camera after moving BS TX from +63 urad  to -517 urad.

Then Francesco acted on the BS_TY, going from -25 to +75 urad in order to center the main single bounce beam on the camera.

We also misaligned PR_TY further away from its parking position in order to separate better the main single bounce beam from its ghost beam.

Then we adjusted the SDB1 angular position in order to make sure that the B5 photodiodes were properly aligned. Fig.2 shows a scan of the bench in TY and Fig.3 a scan of the bench in TX. We found that with SDB1_LC_TY = 90 and SDB1_LC_TX = 110, we would maximize the power on the two photodiodes.

We took data in these conditions from 08h11 utc to 08h25 utc (Fig.4). The power on B5_PD2 is about 16 mW, which is what we expected with 15W of input power.

Fig.5 shows the noise spectrum of the DC and audio channels of the photodiodes, taken in the first 7 min of the measurement. We notice a noise shoulder that looks like scatter light, with a frequency cut off at ~30 Hz. We took another spectrum from ~08h19 utc (Fig.6), for which the frequency cut-off of the noise shoulder has moved down to 15 Hz.

After this measurement, we restored the standard conditions in the alignment of SDB1, BS, and activated DET_MAIN node.