Figure 5

Equivalent readout noise limit of the SPI sensors for the AEI-SAS platform. The blue line shows the readout noise of the reference interferometer. The red line shows an additional interferometer, partially built using standard optical mounts, with an intentional arm length mismatch of about one meter. The stability is even better above \(50\,\textrm{mHz}\) due to better fringe visibility of the interferometer. At lower frequencies, thermal drift of the mounts is probably the dominant source of noise. These measurement results show that the stability of the ULE base plate is relevant at frequencies below \(50\,\textrm{mHz}\). The more interesting range for gravitational wave detectors is between \(100\,\textrm{mHz}\) and \(1\,\textrm{Hz}\), and here metal mounts are sufficient. The yellow line shows the readout noise limit for the optical lever in pitch and yaw. All measurements were calibrated to show the equivalent sensitivity for the AEI-SAS platform control. The metal test interferometer can be seen in the photograph.