Dan, Hang

Using the dithering lines we set up for the past few days for sensing matrix measurement, and the suspension calibration provided in DCC:T1100378, we calibrated the spectra of the ASC error points into physical rad/rtHz. The results were attached.

The best sensor we have is AS45 for DHARD, and the sensing noise is ~ 1e-14 rad/rtHz. While a single QPD has a sensing noise similar to AS45, when we use a QPD combo for CHARD to decouple CSOFT, the sensing noise get slightly worse at ~ 3e-14 rad/rtHz level. The soft loops' sensing noise is about a factor of 10 higher than the hard loops at a few x 1e-13 rad/rtHz level.

On the other hand, the REFL sensors are significantly noisier than AS45/TR QPDs. Using the REFL sensors the CHARD sensitivity is only 1e-12 rad/rtHz at 10 Hz and the slope suggests that it is not shot noise limited. This indicates the necessity of doing CHARD blend from the noise point of view.