added a simple DC-coupled ISS using the transmitted beam of the PMC compared to one of our voltage calibrators and using a SR560 as the servo (gain 1e6, LP 100Hz) and our Thorlabs PZT driver as an amplifier (offset 30V). Don't know what the long-term stability of that setup will be but hopefully better than a few percent over hours. Let's wait for the results. 3mm diode is scanned with 25um resolution - should take about 4h.