During ER7, the L degree of freedom as measured by SUS-OMC_M1-DAMP_L_IN1_DQ was seeing position changes of ~microns (peaking at 0.8Hz and 3Hz), about a hundred times the equivalent channel at L1. This motion predicted well loads of scattering events reaching from 25Hz up to 50 Hz in DARM during ER7 (the interested reader can find plots of all scattering events in ER7 <here>). Some of these showed up in the burst search background. 

 

To improve this, there was recently work to decouple the angle-to-length for alignment feedback being sent to the OMC suspension <19691> and reduce the overall motion of the OMC suspension. 

 

To see how this affected scatter in DARM, we calculated the predicted fringe frequency for the ~15 hours of analysis ready data on July 25th and 26th, after the angle-to-length decoupling, and found it to be better - zero times when f_scatter reached 25Hz.

 

We'll continue to monitor. We’ve heard that it may also be possible to squash the 0.8Hz peak in the alignment feedback to OMC SUS - that would be great. 

 

Plots are: 

1. A scattering event from ER7 2015-06-04 with predicted fringe frequency from f2=4v/lambda
2. Another one from 2015-06-11
3. Spectra of SUS-OMC_M1-DAMP_L_IN1_DQ for H1 in ER7, H1 nowish, and L1 in ER7
4. scatMon.py a draft python script (uses nds and gwpy) to predict scattering from a position channel and overlay it on a spectrogram