Dave, Elli, Alexa, Evan
We are trying to decrease the observed loss in the Y arm by changing the position where the cavity mode falls on the test masses.
We have done the following things:
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We locked the Y arm in IR.
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We set up ASC loops which take REFL WFSs signals and actuate on IM4 and PR2. This keeps the input pointing to the arm maximized.
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We move the alignments of ITMY and ETMY and monitor the dc signals on the TRY and ASAIR_A diodes.
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The unlocked reflected power as seen by ASAIR is 1180. While locked, Dave has been able to increase the ASAIR signal by hand from ~960 ct to ~1020 ct by adjusting the ETMY and ITMY pointings.
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I’ve written a python script that will adjust the test mass alignments in a spiral pattern in order to map out the cavity loss. It is currently running.
Dave, Evan
We found a sign error in the script, but it doesn’t matter because the Y arm broke lock soon after we started the script so there is no data.
We have tried to relock the Y arm, but have not been able to. Maxium buildup on the TRY PD is about 3 cts (I expect we should se flashes of 6 cts or so). We've left the Y arm test masses "aligned" as determined by their saved guardian values.
It seems there is no green light anywhere in the arm. The ALS Y camera is completely dark, the REFL PD is completely dark, and I cannot move the TMS (either by hand or with the baffle script) to put light on the ITMY baffle PDs. Perhaps there is an issue with how the green light is being launched into the TMS, but I cannot diagnose it.
Ryan called and walked me through unsticking the green QPD loops at the Y end, which had become railed.
We're leaving the rest of the work for the morning.
I and Alexa looked at the video image of ETMY and ITMY.
We confirmed that the green and the IR are on top of each other on ITMY when the alignment was good-ish for green and IR. There's no particularly bright spot.
We couldn't tell if the green and the IR are on top of each other on ETMY. But there are two bright spots on the ETMY camera that only appear when the arm is locked. See attached images (green was misaligned for these images so you only see IR).
The ETMY camera is looking at the ETMY from HR side. These spots could be something on the HR coating itself, or could be an image of something else (e.g. some scattering and ACB or some unknown ghost beam hitting near by structure?) reflected by the HR coating. This cannot be behind the HR coating, but it's still not impossible that these are some ghost beams from TMS structure taking strange path and coming into the camera at a large angle.
While Dave was walking the beam in PIT, the high spot became brighter or weaker but the lower spot didn't change much (the first three images).
When the beam was moved in YAW the lower spots got brighter (the 4th image).
Update:
This morning I locked green to Y arm without IR, and I can see bright spots on the same position (5th picture), so now it seems more likely that they are on the HR of the mirror.
The sixth picture is taken with green and IR both locked.
If these are on the mirror, the low spot is somewhat low and somewhat to the right
leftto the center of the HR.Update 2: Is the loss by shmutz big?
To see if the loss caused by the shmutz is significant, Alexa and I moved the beam about +-3cm in YAW on the ETMY without moving ITMY position: (ITMY, ETMY)= (-7.5, +5.88) urad in YAW moves the beam on ETMX to the right, flip the sign to move to the left.
We saw an improvement in both of the cases but it was better when we moved to the left. This is probably because the shmutz is to the right to the center of the mirror.
Alexa will post the StripTools plot, Dave is working on the loss number.
In the attached picure, the first one is when the beam was moved to the right, the next one is when it was moved to the left. Ignore the bright thing at the center of the shmutz as that's just green, look at the halo, and you can see that the IR is indeed dimmer than the last picture from my previous entry.
Keita and I had intentionally misaligned the green PZTs to examine the ETMY and ITMY camera with only the IR beam, and we never realigned it. This is why Evan found the EY green QPDs railed.
Also I found that one of the two IR input pointing WFS had not been cleared, which also explains why the IR beam was not flashing in the cavity.
The above tables lists the positions and the counts on ASAIR. When the IR is not resonanting in the arm, ASAIR reads about 1360 cnts.
In the StripTool I have shown ASAIR_LF_OUTPUT which we are using to measure the power loss, and LSC-Y_TR_A_LF_OUT which measures the IR transmitted power.
At -75 min the IR is flashing in the arm cavity, and ASAIR is about 1360 cnts. Shortly after the IR locks, and then the input pointing WFS engage, as seen by the power build up in IR.
At -70 min we have full IR buildup (TR is about 11 counts), meanwhile ASAIR has dropped to about 1068 cnts.
At -45 min I begin to move the beam to the right; this is completed at about -40 min. At this point the transmitted power has remained the same and ASAIR is now up to around 1117 cnts
Shortly after we lose lock, and I recover the lock.
At -30 min I begin to move to the left; this is completed at about -20min. Again the tranmistted power has stayed the same, and now ASAIR is at 1129 cnts. Eventually we lose lock again.
Mirror cage outline and guesstimated mirror center line.
Note that we are looking the mirror from the left at a big-ish angle.
Blue lines are inner edges of the cage.
Using the ASAIR data in LHO#15467, the loss 850 ppm for original, 690 ppm for left 3 cm, and 655 ppm for right 3 cm, again ignoring mode-matching issues.
![[ 1 - L_E = left[frac{A - r_I}{A r_I - 1}
ight]^2,]](https://chart.googleapis.com/chart?cht=tx&chl=%5C%5B%201%20-%20L_E%20%3D%20%5Cleft%5B%5Cfrac%7BA%20-%20r_I%7D%7BA%20r_I%20-%201%7D%20%5Cright%5D%5E2%2C%5C%5D)
, and 
.
The formula I’m using is as follows:
where
Rick sent us this picture of Pcal camera. We can clearly see bright spots, which I assume is caused by the IR.
Note that pcal camera is looking at the optic from the right.