Robert, Kiwamu,
This is a summary of the recovery of the IMC alignment after the PSL PZT mirror mount swap today (for which I think Robert will make a separate report).
Synopsis-
IMC seems to have shifted its waist location horizontally by 0.25 mm based on the change seen by the suspension witness sensors. This apparently was large enough to reduce the amount of light at IMC-TRANS by a factor of two presumably due to a worse in-vac clipping in HAM2. Nevertheless, as of now, we seem to be able to lock the interferometer on DC readout without new issues.
Recovery process-
We temporarily placed an iris after the top periscope mirror and before the beam tubing. After the swap, we checked two existing irises that had been on the table and the new one at the top of the periscope to coarsely recover the alignment. Then checking the spot on the PSL wall coming out from the light pipe between the PSL and HAM1, we did a fine alignment. This was good enough to put us back to a position where we see the main light on IMC REFL camera. A final alignment was then done by engaging IMC ASC which automatically servoed the alignment to an optimum. The temporary iris was removed before we left the PSL.
After the successful lock of IMC and successful engagement of IMC ASC, we noticed that the IMC TRANS is smaller by a factor of two. This seems to be due to a slight change in the horizontal direction in the uncontrolled degree of freedom. Here is a table listing several changes in some relevant sensors.
| before | after | diff | |
| MC1 PIT witness | -3 | -2 | +1 urad |
| MC2 PIT witness | 510 | 507 | 0 |
| MC3 PIT witness | -841 | -842 | -1 urad |
| IM4 PIT | -0.33 | -0.34 | almost 0 |
| MC1 YAW witness | -1035 | -1043 | -8 urad |
| MC2 YAW witness | -672 | -672 | 0 |
| MC3 YAW witness | -996 | -987 | +9 urad |
| IM4 YAW | 0.25 | -0.08 | - 0.33 cnts |
As shown above, the only appreciable change is that in DOF2 of IMC YAW (highlighted by red texts). Using Kawazoe's formula (P1000135), one can find that this amounts to a lateral shift of the spot position by 0.25 mm toward HAM3 or away from PSL. [EDIT] Keita pointed out that the direction of the move that I initially reported was wrong. So the correct statement is that the beam shifted by 0.25 mm toward the PSL or away from HAM3.
Things we didn't optimize-
-
MC3 spot position (or IM4 spot position) in yaw
- We are not sure how this is going to change DARM noise.
-
IMC WFS centering
- They are not centered any more as a consequence of today's activity. One can re-center them by picomotors if necessary.
Two addenda.
Firstly, to cope with the fact that IMC TRANS decreased by a factor of two, I have edited the IMC_LOCK guardian and lowered all the FM_TRIG_THRESH_ON and _OFF values by a factor of two. In addition, I manually changed the IMC-MCL_TRIG_THRESH_ON and _OFF values by a factor of two as well. They don't seem to under control of any guardians. IMC locks fine with these new settings. The guardian is then checked into SVN.
Secondly, the spot positions on the PSL wall seem to have shifted by 1-2 mm towards the West. No obvious change was found in the vertical direction. See the attached picture. The new positions are recorded with black 'X' marks as shown in the picture.
Jenne and I did a repeat of what we tried a few weeks ago after different PSL work: (30918) we restored the optics to their old values using the witness sensors, then moved the piezo to maximize build up without turning on the IMC WFS. This brought the spot back to its before position on IM4 trans, although the MC2 trans sum was low, so we think that as expected only the input beam has moved, not the mode cleaner optics or IMs. However, we can't fix the input beam change simply by moving the PZT.
We let the MCWFS run to increase the mode cleaner transmission, and watched the spots on both the ISS QPD and IM4 trans. We walked IM3 and IM2 in yaw to bring both QPDs back to the spot positions before this morning's PSL work, and now Cheryl is doing inital alingment.
To move this degree of freedom, we moved IM2 1.284 urad in yaw for every urad that we moved IM3. Since this is a degree of freedom that our inital alingment and ASC don't control, it may be a good idea to try moving this degree of freedom in full lock to see what impact it has on our noise and recycling gain. For the record, today we moved IM3 -2390 urad, and IM2 -3030 urad.