Keita Robert Daniel

We misaligned MC2 and made a jitter measurement using the DC readouts of the two wavefront sensors in reflection of MC1. The PSL power was close to 2 W. The top panel of the first attached plot shows the jitter and displacement spectra in units of divergence angle and beam size, respectively. The bottom panel shows the spectra measured by the QPDs of the PSL diagnostics breadboard in arbitrary units. The calibration for these QPDs is not clear at the moment.

The second plot shows coherence spectra between the QPDs of the PSL diagnostics breadboard and the wavefront sensors.

Some observations:

• The jitter below 100 Hz seems to come mostly from the PSL HPO—even after the attenuation provided by the PMC. This rise towards lower frequencies was not present in O1.
• The slope of the uncalibrated PSL QPD spectra differs above 100 Hz from the pointing noise reported by the automated procedure of the diagnostics breadboard. The uncalibrated QPD spectra are a lot flatter in this frequency region. The flatter spectra do a better job explaining the coherence of the 240 and 450 Hz peaks with the wavefront sensor.
• Since we see the coherence of the HPO peaks with the wavefront sensors, its difficult to see that the 1Q spectra (which doesn't show the peaks) can be five times higher than the 2Q spectra.
• The calibration of the PSL QPD spectra can be estimated from the coherence with the wavefront sensor spectrum. For 2Q the calibration factor should be around 60 x 10^–5, and maybe 10 x 10^–5 for 1Q. The attenuation factor of the PMC is around 60.