This page is for writing explanations or plans which we are not confident about and may be subject to change. For instance, a possibe course of action to carry out a particular type of measurement or the plan to follow to identify a noise source.

Noise hunting in SR3 optics' residual motion

Context: klog entries 9702 and 9755.

On Friday afternoon I found high coherence between SR3 L and P degrees of freedom in at least two frequency bands, as measured by the oplev. High coherence is observed in both states ALIGNED and FLOAT.

• Directory: /kagra/Dropbox/Subsystems/VIS/TypeBData/SR3/Noise/Measurements/20190802

• Files: SR3_ALIGNED_TM_OPLEV_ASD.xml, SR3_FLOAT_TM_OPLEV_ASD.xml

Possible reasons for the high coherence are:

• The oplev was not successfully diagonalized in those frequency bands. In this case the high coherence would be a consequence of measurement coupling between L and P and not to a real motion of the optic. In this case we would have to identify which one of the two degrees of freedom exhibits real motion.

• Lack of balance in optic coil-magnet actuators. Although the actuators have not been balanced yet, it's unlikely this is the main cause becasue the high coherence is also observed in FLOAT state, in which the control system is off.

• Pitch motion happens as a consequence of the longitudinal motion in upper stages.

• There is an external environmental excitation. This may be acoustic noise or ground motion leaking through the suspension into the optic. It may be something affecting the oplev optics directly, but it's unikely because the effect is see at low frequencies.

Plan

• Find out the frequencies in which the oplev was diagonalized.
• Balance the coils. Excite the saddle mode and cancel out the motion of the optic.