Virgo Logbook

Between yesterday and today we worked at the alignment of the squeezer bright alignment beam on the interferometer. The original plan was to overlap, on the external squeezer table, the BAB and the ITF beam reflected off the output polarizer of the Faraday isolator on SDB1. Due to the absence of the INJ beam , we tried a blind injection of the BAB towards SDB1, looking for a reflection from NI on the B1p camera. Paolo and Julia kept the payloads along the optical path (SR, BS, NI) in their nominal position, i.e. in the same conditions as last Wednesday. Romain did the same for SDB1.

On first we centered the BAB on the reference diaphragm that was installed on the external squeezer bench after the new Faraday isolator on SDB1 was installed. We then steered the beam to try centering it on the folding mirror beneath the Faraday, looking at SDB1 from the viewport on the north side with a camera (ipcam43). Afterwards we tried to center the beam on the M2 parabolic mirror of the dark fringe telescope, and on the meniscus lens, looking at SDB1 from the viewport on the east side with a camera (ipcamz2).

In a first trial, we could not see any spot on the B1p camera. Ilaria checked the alignment of the Hartmann beam, and no spot was visible on the sensor camera. In order to recover the HWS auto-collimation, Romain rotated SDB1 by about 50 urad in both tx and ty. In such conditions, we tried again to scan the BAB direction and we eventually detected a spot on B1p camera, see attached image.

After some walk-off of the beam we were unable to center the spot on the camera, nor to improve its shape. We cannot exclude that what we see is just a spurious reflection. In any case, in the only configuration we found to propagate the BAB towards the ITF, the beam enters SDB1 with a tilt (pitch) of about 3 mrad with respect to the horizontal. We checked with a bubble level that the external squeezing table is horizontal within better than 0.5 mrad in the north-south direction. The only way to explain the observed pitch is a substantial inclination (3 mrad roll) of SDB1 around tz.

In summary:

- we injected the BAB from external squeezing bench to SDB1
- we observed the beam on the mounts of the SDB1 parabolic telescope and meniscus lens
- we detected a reflected spot on the B1p camera

However:

1) without INJ beam there is some uncertainty on the orientation of benches and payloads, by the way we had to tweak SDB1 for HWS auto-collimation;
2) we need to give a substantial tilt to the BAB with respect to the horizontal plane; this is compatible with an inclination of SDB1 by 3 mrad around tz: does it make sense?

For these reasons we stop for the moment the alignment work, until when the INJ beam will be available again.