Sudarshan, Gabriele

To try to reduce as much as possible the beam pointing sensitivity of the ISS photodiodes, we wrote a python script that reads the PD signals and demodulate them with the excitation sent to IM3. In this way we could send a 12 Hz, 1000 cts sinusoid in yaw and a 17 Hz, 1000 counts sinusoid in pitch, and look at time domain traces of the demodulated signals, as shown in the first attached plot. We moved a bit the picomotors, and we could improve quite a lot the situation. As shown in the second plot, right now we almost can't see the sinusoids in the PD signals, since they're below the noise background. Here are therefore upper limtis on the dP/P/dx:

The optimal position we found is not exactly with the QPD centered (both x and y are around 0.3). Maybe we could improve this, but for the moment we guess the situation is good enough.