After lunch SWEB was suspended and under control in vacuum (thank you Alessandro).
The motion of the bench in Tx and Tz was larger than usual (+/- 2 urad).
The control gain was tuned improving a little bit the situation but the motion remained larger than usual.
The bench position was adjusted to allow the IR beam to reach the photodiode.
The west arm cavity was locked.
We then align the green beam using Galvo1 and Galvo2 in order to:
1. Center the green beam on M3.
2. Have the beam retro-reflected on the camera on the Aux bench (= auto-collimation).
In this situation we could see the green beam on WI and some green light on WE but no trace of flashes.
We went to the WE building to check if the auto-collimation was still good.
We looked on the diaphragm placed before the fiber collimator.
Indeed, the autocollimation was still good (no need to tune anything).
We then move the galvos to see if we could move the beam on the WI input.
The motion that we can see on the WI is very small.
Very rapidly the beam disappears because it clips on the SWEB mirrors.
It seems that the galvos are unable to produce large motions of the beam on the WI.
Maybe they can be used only when the alignment is close to the optimal.
We then try to move the bench a little bit (anyway we cannot move it too much if we want to keep the lock).
and adjust the beam on the bench with the galvos.
The result is similar to the one obtained before when we moved the galvos only.
We put back everything close to the optimal position (autocollimation and beam centered on the M3 mirror).
We unlocked the west arm cavity.
We start moving WI and WE in order to align the cavity on the green beam axis.
Depending on the WI and WE mirror positions we could see bright spots on both the WI and WE mirrors.
On the WE mirror the beam has a kind of "comma" (,) shape (also visible on the WI but much less).
It seems that by moving the WI and WE mirrors we can achieve a strange cavity resonance with no flashes.
We asked ourselves if the line-width of the laser that we are using is small enough to see cavity flashes (our cavity has a 50 kHz FSR).
We will have to check this.
Then, thanks to Ilaria and Maria, we had dinner (respecting social distance, of course)
A very useful support.
We then put everything back to the "optimal position" (auto-collimation and M2 mirror centered) and lock again the west arm cavity.
Putting the mirrors back to the positions that they had during the previous lock was particularly painful because the suspension control GUI is not working well (TANGO-SAT issues) and the standard procedure is not straightforward.
From there we started moving the dichroic mirror on SWEB using the pico-motors (we did not move the Aux mirror on SWEB).
Using the dichroic mirror it was possible to move the beam on the WI mirror much more clearly than with the galvos.
We wanted to see if the beam reaches the BS but there is no camera looking to the BS.
We wanted to see if the beam reaches the reflective faces of the PR mirror but there is no camera looking at the reflective face of the PR mirror.
We wanted to see if the beam reaches the diaphragm placed on SIB1 but there is no camera looking at the diaphragm placed on SIB1.
23h30: time to go home.
It is a Koheras BoostiK Y10 from NKT Photonics.
According to the data sheet the line-width is <20 kHz.
We can see the green beam on the WI but the light is scattered by the metal on the back of the mirror (according to Ilaria that is part of the ring heater).
Moving the green beam we can see more or less light.
Moving the green beam on the right of SWEB_M3 seems to move the beam on the WI mirror towards the right.
In this condition some green light is visible also on the edge of the baffle in front of the WI.
See picture.
On the BS we could not find any useful window to look at the mirror (maybe we should have a better look).
On the SIB1 we found a window on the north-west-low side.
From there it is possible to see the baffle placed on the ground in front of SIB1 (but it is very difficult since the angle of view is very large).
Maybe one could try to put a camera there.
On PR we found a very well placed window on the north-west-up side.
We mount a camera there.
We took the camera from the north-east-up side of the detection tower.
This is security camera so we are not sure about its sensitivity.
We placed it so to see the front face of the PR mirror.
We use this camera to look for the green beam on the PR payload while moving the dichroic mirror.
We could not see the green beam on it (while, as usual we saw it on the WI mirror).
In order to look for green flashes, we decided to try the same method used with SWEB in air. Therefore, we started to move Maux1 and Md to change the entry point of the green beam in the West cavity checking the centering on M3 and the beam back-reflected on EWEB. Many different combinations of Maux1 and Md have been tried but we did not succeed in superimposing the axis of the green propagation with the axis of the locked cavity. Finally, we tried to slightly move also the SWEB angular working points, keeping the cavity locked with the IR and compensating the motion using Md, still without success.
The work will continue next Monday, possible different approaches:
- lock the West arm on B4 and induce larger angular movements on SWEB;
- make a narrower snail with Maux1 and Md;
- set the Galvos back on the working points used in air and move Maux1 and Md.