Today's shift was devoted to the arm losses measurements. We did it as usual by comparying the power reflected by the arm cavities when these are locked and unlocked, respectively.
Some preparatory work was performed to make sure that no ghost beams were falling on the photodiodes, and that the beams were well centered on each PD. In order to do that, we changed the misaligned position of the ITMs by ~50urad. Any SR misalignment was uneffective. The PR was left in its usual "misaligned" postion, since its misalignments did not have an effect on the power level of the PDs of the central area. Eventually, we made a scan of the TX and TY setpoint of both SPRB and SDB2 in order to make sure about the PD centering.
At the very beginning of the shift, Romain G. zeroed the offsets on all the PDs but B2_PD1 and B5_PD2, where it was not possible to close the shutters.
In order to have more signals available as backup, we looked also at the total power on the quadrants (namely the *QD*Sum* channels) and on the cameras, after making sure to have the right gain and integration time for it not to saturate.
First of all, the coherence between photodiodes was checked in single bounce from each ITM. In particular, it was checked between PDs at the same port and between PDs at different ports. Between the PDs at the same port, the coherence was good, except for B4, probably because of the faint light on it (figure 2). On the other hand, we observed a lack of coherence between PDs at different ports, even at few Hz. (figure 1)
After these checks, we started our "lock/unlock" measurements, one arm at the time. The reflected power was measured in three different conditions:
- cavity locked
- cavity unlocked (but still aligned)
- cavity unlocked and ETM misaligned (only ITM aligned)
You can find all the GPS (commented) in the attached .txt file.
From an online look, it seems that B1p is sensible and at a very first sight not worrying, while the others which seem to be a bit funny, and they deserve a more accurate analysis. (figure 3)
SIDE NOTES:
- when the North arm was misaligned, we could still see some light and flashes on the B5 quadrants (figure 4)
- glitches are visible about every 100 seconds on all the quadrants of the central area (see figures 5, 6 )