Related: https://alog.ligo-wa.caltech.edu/aLOG/index.php?callRep=14567
Summary:
We're still close to the East edge of the PR2 baffle, it may be better to go further to the left for safety but we won't gain much as far as the power recycling gain goes.
The centering on PR2 is off by a centimeter-ish (or the baffle is offset from PR2 or the baffle is smaller than it should be).
The beam seems to be somewhat larger than it should be.
There is some clipping downstream of PR2 so we need more scans to find where that is.
In the right plot, blue circle is the PR2 baffle looked down from the PR2, green circles are the IFO beam ideally located, red cross is the PRM-PR2 beam position calculated from the slider, and the red circle is the beam position calculated from the fit.
Note that red cross and red circle have +-14%-ish error bar (both position and the size), but it seems like the beam is bigger and the beam closer to the East edge than it should be.
The left plot is the same as the one in the above mentioned alog, but replotted such that the X axis was rescaled as the beam distance from the East edge of the baffle using the new calibration of the IM4 slider (explained later). This X axis itself has +-14%-ish error bar.
The curve is the fit of POP_A and POP_B combined for the left half of the plot, assuming that the baffle transmission is approximated by a beam clipped by a vertical edge. Beam radius, edge position and the overall scale were the three fitting parameters (the overall scale is necessary because we don't know if the maximum transmission corresponds to zero loss).
Green vertical line is where we were at this morning, blue is where we used to be last Tuesday.
| Distance from the edge | Radius of PRM-PR2 beam on the PR2 baffle | Loss from the clipping | Centering on PR2 | |
| Ideally | 26mm | 6.1mm | Totally negligible. | centered. |
| According to the IM4 slider | 12(1+-0.14)mm | NA |
42ppm when the radius is 6.1mm, 0.62% when the radius is 9.6mm |
14mm off |
|
According to the fit (which depends on the slider) |
16(1+-0.14)mm ??? | 9.6(1+-0.14)mm ??? | 430ppm | 10mm off |
It would appear as if we should go further to the right on the plot, but clearly there is some kind of clipping on the sled path in that direction. As far as there's no clipping in the POP_AIR path that is OK, that's yet to be seen.
IM4 slider recalibration:
IM4 YAW rotation, though the slider has some calibration in the filter, is actually smaller by a factor of 0.175 than the slider number.
Looked at PR2 camera, and moved IM4 in YAW until the beam is blocked by the baffle.
| IM4 YAW slider value | |
| Left edge | 1376.7 +- 1000 |
| Right edge | -9623.3 +- 1000 |
| Where we were this morning | -7623.3 |
The baffle looks like a 65.4mm diameter circle from when viewed from PRM.
Since IM4-baffle distance is about 17m, we can recalibrate the slider:
IM4 rotation = 65.4mm/17m /2 = 1.92 mrad.
Slider calibration = 1.92 mrad/(1376.6 + 9623.3 urad) = 0.175 [rad/rad]
Fit:
That's a simple fit using the fit function
transmission = @(coeffs_, x_) coeffs_(3)*(1/2+ erf(sqrt(2)*(x_-coeffs_(2))/coeffs_(1))/2);
The script is this:
/ligo/home/keita.kawabe/Clipping/clipping1.m