We need to lock our ISS to the PD output in real time without worrying about DC fluctuations in the PD response.

I made a dual voltage follower using an OP275 and some capacitors. 

The schematic is on the box in picture 2.

I checked both voltage follower input/output pairs with an SR785.  We see a nice low pass with cutoff frequency ~0.7 Hz.

I'll install this tomorrow.