J. Kissel, R. Abbott

I'd had a curiousity conversation with the great Rich Abbott at the LVC meeting, for which I just found my notes. The line of inquiry was on the limitations of the wave front sensors (WFS). I was curious what it would take to improve the angular sensitivity -- which is limited by shot noise above a few Hz. (Improving the sensitivity would in turn allow us to increase the angular control bandwidth without polluting the DARM sensitivity).

The numbers:
The WFS are shot-noise limited as long as each quadrant (of which there are four, of course) receives ~3-4 [mA] of photo-current. 
The WFS quadrants' electronics saturate at 15 [mA].

For reference, the LSC PDs are shot noise limited above 1-2 [mA]. It's less because the diode is smaller, so the PD's capacitance is lower.