With the first detection of gravitational waves (GWs), the most precise (relative) distance measurement ever was made. The first coincidental measurement of GWs with electromagnetic counterparts, GW170817, from a binary neutron star merger has provided a firm basis for the newly founded field of multi-messenger gravitational wave astronomy and an independent confirmation the gravitational wave detector measurements. In the future, low frequency GW detections will give access to even heavier mass black hole inspiral signals. All these monumental measurements would not have been possible without decoupling the test masses of the detectors from the Earth’s ever-present motion. The seismic wall, after the appropriate vibration isolation, is typically limiting below 10 Hz. Many of the world’s most precise commercial inertial sensors are used in LIGO, Virgo and KAGRA. However, inertial sensors cannot distinguish between translational acceleration and tilt. Tilt of the ground or wind on the buildings is limiting. For these angular degrees of freedom, the BRS, ALFRA and PLI were developed. Tilt sensors are also vital in the battle again Newtonian Noise, the attraction of a varying mass distribution around the detector’s mirrors by passing seismic waves.
The Rasnik alignment system was developed initially in 1983 for the monitoring of the alignment of the muon chambers of the L3 Muon Spectrometer at CERN. Since then, the development has continued as new opto-electronic components become available. Rasnik systems are 3-point optical displacement monitors and their precision ranges from below nanometers to several micrometers, depending on the design and application requirements. A result, expressed in the range/precision ratio of 2 × 106, is presented. A modern example application of the Rasnik is some 6000 systems that align the muon chambers of the ATLAS detector at CERN. Recently, we have studied the ultimate performance of Rasnik system. With commercially available standard components (total 300 Euro), a precision of 10 pm/√Hz has been reached. With specially developed CCDs, a resolution below 1 pm/√Hz seems feasible. This makes Rasnik a fierce competitor of interferometers: a straight linear displacement monitor versus a complex system with feed-back regulated locked loop. Rasnik may be well applied in innovated seismic instrumentation for gravitational wave experiments.
