We set up a CCD camera looking at ITMY and the entire baffle and some more, sent a green beam from EY, modulated its angle by using PZTs, and found nothing on the camera. No luck with the baffle diode, either.
It seems like the beam is not getting to the corner station at all.
One (or more) of three things should be true:
- The ETMY is not pointing to the ITMY. The beam from ALS table is very close to retro-reflection by the ETM when TMS and MCL PZTs are in a neutral position.
- Because of a factor of 20 reduction in angle in the telescope, we're not wiggling the angle enough (however I doubt this).
- Mode matching is so bad that, by the time the beam reaches the corner, the beam diameter is 4m or something.
Tomorrow we should use HEPI and/or ISI to move everything by a milli radian or so in YAW.
The reason why I think we're already wiggling enough is because, when we wiggle MCL PZTs, I can see on the CCD camera monitoring ETMY that the motion is so big the beam falls off of the primary. It's not impossible that this is all in translation and not in angle, but we used both PZTs that are placed at different Gouy phase positions and we didn't see anything at IY.
As a side note, 1/20 angle reduction only applies to the tilt upstream of TMS telescope (e.g. PZT on the ALS table).
For the rotatoin of TMS itself, it should be 19/20 or 21/20. However, we (Rana and I) felt as if TMS rotation is not producing enough beam rotation. We should calibrate the TMS angle to know how much we're actually tilting it by measuring the displacement of the retro reflected beam on the ALS table.
Also, we need to keep the optical axis fixed to the TMS even during the alignment, which (among other things) means that the TMS angle and the ALS beam angle on the ALS table should always be moved by the same amount. Elli and Thomas are working on the QPD servo for this.
Some details on the sensitivity of the search:
* For the ETMY camera, we are only a few meters from the TM. The iris is opened up as far as it can go and still stay nearly focused on the mirror (as seen w/ illuminator on).
* For the ITMY camera, we are ~30m away at the spool position. A large maglight flashlight injected from there can easily be seen on the baffle, the chamber, and the optic.
* The light from the flashlight shows up as ~10000 cts on the baffling photodiodes (mounted near the ITM). Wiggling the green beam from the end produces no signal as seen on the camera and if it produces any signal on the baffle PDs, it must be below the noise level of ~50 cts-rms. Given that the flashlight puts out ~500-1000 lumens, how big of a signal should the green beam make if its the right size?
* Assuming that we inject ~10 mW into the ETMY and that it has a transmission of ~1.5% for 532 nm, we should get ~100 uW of light at ITMY. By comparison, when we hunted for the iLIGO beams, we were injecting ~100 mW at the PRM and ~50-100 uW would get through to the ETM before lock (in the single arm config). The silicon CCDs should pick up the green light much better than the IR, so we should be seeing some beam on the chamber walls, etc. We were injecting non-integer frequency sine waves simultaneously into both pitch and yaw for the PZT, so I think its unlikely that we were unluckily sitting in some dark space.
* We need an SEI & SUS expert to inspect the HEPI/ISI & the ETMY angle biases to ensure that they are pointed in the right direction as determined by the IAS.
Video of the swinging beam on the ITM (left) and ETM (right)
Images of the ETMY as seen from the spool cam: illuminator on (left) and off (right)
I calculated roughly what the intensity of light should be on the baffle PDs. Rana's 500 lumen flashlight was emitting around 10W of light. I assumed this was filling an area roughly 1m in diameter - this yields an intensity of 13W/m^2. The corresponding intensity for the green beam at the ITM (assuming 100uW is transmitted through the ETM and the beam radius is ~5cm at the ITM) is about 12mW/m^2. So the response of the PDs to the green beam should be ~1000x less than the response to the flashlight.