Vaishali, Kiwamu,

We have implemented the calibration coefficients for the bullseye sensor in the front end today. The PIT, YAW and WID signals are now calibrated in fractional amplitudes. They are defined as

(fractional amplitude) = | HOM amplitude (e.g. E_{10} )| / | E_{00} |

Using this calibration, we virtually propagated pointing jitter (a.k.a PIT and YAW) from the bullseye to the IMC WFSs. We were able to get somewhat quantitative agreement with the measured jitter spectra there for the PIT and YAW degrees of freedom.

Next things to do:

• measure/check the IMC WFS calibration.
• measure the pointing jitter coupling transfer functions (again).

[New calibration settings]

Since Vaishali is writing up a document describing the calibration method, we skip the explanation here and just show the results.

• PIT_GAIN = 1.0
• YAW_GAIN = 1.1
• WID_GAIN = 0.54

Once they were set, we then accepted the SDF, although they are NOT monitored.

[Some noise spectra]

Below, the calibrated spectra are shown in the lower right panel.

The jitter level was about 3x10^-5 Hz^-1/2 for PIT, YAW and WID at around 100 Hz. If these fields go through the PMC, their amplitude should be suppressed by a factor of 63 in amplitude (T0900649-v4). So their level should be something like 5x10^-7 Hz^-1/2 when they arrive at the IMC. By the way, for some reason the intensity fluctuation and beam size jitter are positively correlated: as the laser power becomes larger, the beam size becomes larger at the same time.

Here are plots showing the noise projection for the IMC WFS which qualitatively agree with what Daniel has measured in the past using the DBB QPDs instead (31631) -- IMC WFS noise are superposition of acoustic peaks and HPO jitter.

Finally, if one let these jitter fields propagate through the IMC, they should get an attenuation of about 3x10^-3 in their amplitudes. Therefore the amplitudes of the HOMs after the IMC should be roughly 1.5x10^-9 Hz^-1/2. This number is consistent with what Sheila has estimated for pointing jitter at IM4 (34112). However, a funny thing is that, in order to explain a high coherence of 0.1 for pitch when the interferometer is locked in low noise (see for example, blue curves in the first attachment in 34502), the coupling must currently be about 1x10^-11 m / fractional amplitude, which is more than 10 times larger than what Sheila measured back in February (34112). What is going on?