After an initial assessment that stated "not too bad, not too good" (https://alog.ligo-wa.caltech.edu/aLOG/index.php?callRep=8919), which was based on just two data points (beam radius on WFSs), I added the third point further downstream of the WFS2 and it turns out that actually it's excellent.
In the attached plot, blue circles represent the measured beam width, blue crosses on both sides of blue circle represent the potential position error (Jamie claims +-1cm though probably it's too generous), red line is a fit over the three points, green is the curve generated by the design parameters.
As you can see, in both horizontal and vertical direction, the waist is very close to the middle of the WFSs, and the waist radius is very close to the designed 250um. Gouy separation for X (horizontal) and Y (vertical) are 82.6 and 87.1 degrees, respectively.
This was obtained from just two iterations of measurming at WFS1 and WFS2 and pushing RM2 toward RM1 based on the measurement. Downstream was measured just once after we were satisfied with WFS1 and WFS2.
| relative position (mm) | position error (mm) | X diameter mean (um) | X diameter std | X goodness of fit | X goodness of fit std | Y diameter mean | Y diameter std | Y goodness of fit mean | Y goodness of fit std | |
| WFS1 | 0 | +-10 | 746.79 | 4.6 | 0.01 | 0.000 | 688.9 | 4.2 | 0.00 | 0.000 |
| WFS2 | 369 | +-10 | 746.25 | 10.1 | 0.00 | 0.000 | 758.18 | 4.4 | 0.00 | 0.000 |
| downstream | 763.2 | +-10 | 1574.92 | 4.95 | 0.02 | 0.000 | 1666.38 | 1.79 | 0.01 | 0.000 |
Distance from RM3 to the first lens on the sled is, according to Jamie, between 48.0 and 48.25 inches.
Also, as noted earlier, the above data was obtained after having moved RM2 toward RM1 by 22.5mm. Everything else is the same as what Sheila reported much earlier.
We also measured at one point between RM3 and the sled (14.5" downstream of RM3). Together with upstream number measured yesterday (https://alog.ligo-wa.caltech.edu/aLOG/index.php?callRep=8893), these are:
| position | position error | X diameter (um) | X std | X goodness of fit mean | X goodness of fit std | Y diameter | Y std | Y goodness of fit | Y goodness of fit std | |
| After RM3 | 14.5" downstream of RM3 | +-10mm | 3750.18 | 4.885 | 0.01 | 0.000 | 3923.63 | 2.137 | 0.00 | 0.000 |
| Upstream of telescope | 44.7" downstream of 50:50BS right after the HWP | +-1" | 3851.01 | 1.982 | 0.01 | 0.000 | 3959.23 | 1.232 | 0.01 | 0.000 |
| Upstream of telescope, head rotated 45 degrees | same as above | same as above | 4117.74 | 1.475 | 0.02 | 0.000 | 3845.62 | 0.763 | 0.00 | 0.000 |
In all of the above measurements, "Profile averages" was 10, "Rolling profile Averages" was 3, the actual number of measuremets (i.e. the number of scans performed before I stopped the measurement) were larger than 10 but I don't know if the software was taking more than 10 points into the statistics or not.
Several things to note.
We decided that we did NOT want to propagate the upstream measurements to downstream, because it was difficult to obtain more than one data points. (ModeMaster is supposed to overcome this, but in reality there are many caveats and you should know how to tell when which caveat applies.)
With NanoScan and limiting ourselves to the measurements with small beam, it was still difficult to obtain good data because of some kind of glitches. It's not clear if it was due to NanoScan or the beam, the beam was well damped and was not moving on the viewer card, there was no noticable intensity glitch either. But the symptom was that the statistics window shows nice steady data for anywhere from one second to 30 seconds, then there's some kind of glitch and the scan/fit image looked noticably different (not necessarily ugly), the diameter mean becomes larger and the stddev jumps to a big number (like 10% or more of the mean, VS up to a couple % when it's behaving nice), and the goodness of fit also becomes large. Somehow no glitch made the beam diameter number smaller. I just kept waiting for a good period and cherry-picked.
When the beam was moving it was impossible to obtain good data.
Another kind of glitch was "saturating" glitch where the software says there is a saturation. We disabled AGC of NanoScan and lowered the gain by 3dB in an attempt to eliminate saturation, it seemed to help but we couldn't kill that error completely.
We (I and Jamie) will go in HAM1 tomorrow to set the eddy current damper spacing (now that Bram wrote a procedure to do that, plus it turned out that Jamie didn't check the ECD spacing on the back plates).
Measurement apparatus. We flipped one steering mirror on the sled to direct the beam to Nanoscan that is placed at the same distance from the steering mirror as the WFS. Made measurement, flipped the mirror back, and moved to the next one.
Very nice! This will become version 12 in D1000313. It would be good to add the corresponding alamode file with the final distances here: https://dcc.ligo.org/T1300960-v1, I added a note to remind us that this is the relevant log entry.
A update on 2014.Jan.12:
Alexa, Koji and I changed the position of both WFSs to dump the reflected light off of the diode (see alog 9226).
- The position of WFS_A (or equivalently WFS_1) was shifted by 9.4 mm toward the west.
- WFS_B (or equivallently WFS_B) was shifted by 10 mm toward the west.