Virgo Logbook

The shift started around 6 pm. On first, Romain G. opened the SDB1 shutter (SDB1_rot, channel 1 axis 1, -29500 steps). We measured the optical power of the beam reaching the external SQZ bench from SDB1 to be about 0.1 mW. This beam was clearly misaligned with respect to the SQZ BAB by approximately 1 mrad. Our first task was to overlap the two beams, by centring both of them on two diaphragms with a large optical path difference. So we moved the picomotor of the folding mirror underneath the SDB1 Faraday isolator, in order to center the beam on the diaphragm at the southern end of the SQZ bench, close to the last steering mirror, which was centred on the BAB beam. Differently from the last alignment work (see entry 41016) we could not use the other diaphragm already installed on the BAB path about 3 m upstreams, because of the presence of the new, double stage Faraday isolator. We then installed a new diaphragm just in front of the Faraday isolator, we centred it on the BAB, and we moved the last steering mirror to center the beam from SDB1 on it. The two diaphragms are about 1 m apart, so the alignment precision is a factor 3 worse than previous time. Then we looked for the BAB beam spot on B1p camera. To this purpose we had to remove the ITF beam, which is much brighter than the BAB. We then unlocked the IMC and we tilted the IMC end mirror.On first we could not see any spot from the BAB on B1p camera. We repeated the alignment with the help of the two diaphragms with better precision, by increasing the power of the beam from SDB1 - just by mistuning a bit the polarisation of the SDB1 Faraday isolator (SDB1_rot, channel 1 axis 2, 500 steps); in such condition, the power reaching the SQZ bench was about 0.2 mW. Eventually we could see the BAB spot on the B1p camera, but it was badly mis-centered along the vertical direction. We spent quite some time to try improving the alignment with walk-off of the steering mirrors on the SQZ table, but we didn't manage to have it centred on the camera.

We took the chance to align the beam from SDB1 on the two new cameras (SQZ_B1_CamNear and SQZ_B1_CamFar). The cameras receive the beam after reflection off the second Brewster plate of the first Faraday isolator on the SQZ bench (coming from SDB1). In attachment the images of the spot on the two cameras, in two different conditions of the SDB1 Faraday isolator tuning: in the first two files, the total optical power reaching the SQZ bench was about 0.2 mW; in the last two images, the optical power was minimal, i.e. 0.1 mW.