Yesterday afternoon (Monday April 11th) we prepared the SWEB bench for vacuum. This included the following activities:
1/ B8 beam realignment:
In order to fix the problem of clipping observed previously on the mirror SWEB_MMT_M3 (see logbook entry 36637) we recentered horizontally the beam on this optics by acting on the TY position of the mirror MMT_M2. The mirror was moved by turning the knob located near the minitower door (left knob when looking at the front surface of the mirror) in the anticlock wise direction by about one and half turn, performed in small steps. The alignment of the beam on the B8 camera was recovered by adjusting the TY setpoint of the suspended bench to TY=1740 urad (initial value was 5). Due to some horizontal-vertical cross coupling we also had to adjust the TX angular setpoint of the bench to TX=150 urad (initial value was 110). At the end of this alignment the beam was centered on the B8_CAM1 at the position X= -0.15 mm; Y=0.15 mm
Fig.1 shows the images provided by the IP cameras looking inside the minitower when the bench is illuminated, Fig.2 shows the same images when the ITF is locked at dark fringe and before touching the beam alignment. The mirror mount MMT_M3 is visible inside the red circle where a bright spot is visible on the left side of the mount where the beam was clipped. For comparison Fig.3 shows the same images when the ITF is still locked at dark fringe but after the alignment performed today: Clipping on the mount has been significantly reduced.
2/ Installation of beam dumps and bench balancing:
As mentioned in entry 36637, back-scattered light from the quadrants was observed on the bench. Fig.4 shows the back-scattered light from the quadrant B8_QD2 when the ITF was locked at dark fringe. In order to dump this scattered light we have installed two beam dumps in front of the B8_QD2 quadrant (see Fig.5) and another beam dump near B8_QD1 (see Fig.6). The total weight (including support and screws) of a small beam dump is 173g and the weight of the big beam dump is 247 g.
Following this intervention counter-weights were replaced and the bench rebalanced. The overall weight has changed by +7g.
At the end of the shift the minitower doors were closed.
