Dan, Nutsinee, Evan

Tonight we explored the damping settings for the first harmonic of the violin modes.  Our goal was to reduce the intensity fluctuations on the DCPDs enough that we could engage a second stage of whitening.

We were able to damp nearly all of the peaks that were visible between 1002-1010Hz, these correspond to the ETMs and are well separated in frequency.  The phases required for the damping filters are not amenable to broad bandpass filters (ETMX in particular is pretty random).  In the end I dealt with each mode one at a time, by hand.

After a while I got tired of saving the filters, so I just recorded the gain and phase that led to smooth damping.  These settings can be easily replicated using a 20mHz bandpass filter around the frequency of the line.

We worked on the modes in order of height.  Eventually we ran out of steam, but the first harmonic lines in the spectrum have been reduced by a factor of five compared to the reference.  The RMS of the DCPD signals (about 300 counts) is now dominated by the residual length motion around 3-4Hz.  We should be able to engage more whitening if we want to get some headroom over the ADC noise.

The frequencies in the table below are from Keith Riles' list o' lines in ER7: alog 19190.  Eventually this will get propagated to Nutsinee's new violin mode wiki page.