I poked around a bit thinking about what we need for a single AS WFS. 

New things that we would need:

• The WFS head itself: aLIGO ASC RF Photodetector (WFS) - E1400103 (Which will need some modification for 55MHz)
• Interface board for DC power and DC signals: aLIGO WFS Interface Chassis - D1101906
• Quad Demod chassis: Quad IQ Demodulator Chassis -  D0902796 [Variant B (WFS)] (Needs LO, but we have unused outputs in the LSC LO distributor)

Things we have:

• Many Eurocard-style Whitening chassis, such as we use for all of the LSC PDs. 
• Enough ADC (c1ioo has two ADCs, but doesn't even use 32 channels, so we can steal one, and throw it into c1lsc)

We'd have 12 new signals to acquire: 4 quadrants x DC, I, Q. In principle the DC part could go into a slow channel, but we have the ADC space to do it fast, and it'll be easier than mucking around with c1iscaux or whatever.

Open question: What to do about AA? A quick search didn't turn up any eurocard AA chassis like the ones we use for the LSC PDs. However, given the digital AA that happens from 64kHz->16kHz in the IOP, we've talked about disabling/bypassing the analog AA for the LSC signals. Maybe we can do the same for the QPD signals? Or, modify the post-demod audioband amplifer in the demod chassis to have some simple, not too agressive lowpass.