Towards an Advanced LISA Payload Architecture Featuring In-Field Pointing and Spherical Proof Masses

Abstract

The prediction of gravitational waves in the early 20th century promised new means for observational astronomy, since they were expected to allow deeper insight into our universe. Recently, the first direct detection of gravitational waves by the earth based LIGO detectors - a major achievement along gravitational wave observation - confirmed the original prediction. This milestone elevated the interest in the development of enhanced as well as new detectors, which hopefully enable the discovery of new aspects of our universe. One of these new detectors is the planned Laser Interferometer Space Antenna (LISA) space mission, which will represent a giant interferometer for the observation of gravitational waves in space. LISA consists of three satellites, which are arranged in a triangular formation and form three interferometer arms. The detection will be performed via interferometric, inter-satellite laser distance metrology between free-flying proof masses, which act as the end points of the interferometer arms and will change their relative distances with respect to each other when a gravitational wave is passing. However, the strain amplitudes of gravitational waves are small and so will be the distance changes between the proof masses (in the order of a few tens of a picometer), which makes their detection very challenging. LISA will expand the currently available measurement bandwidth covered by the earth based detectors to lower frequencies, hence enabling the detection of new gravitational wave sources. In the baseline LISA payload architecture, changes in the angles of the triangular spacecraft formation, caused by the individual orbital mechanics of the spacecraft, are compensated by pointing of the whole telescope assemblies. This concept is called telescope pointing. During the LISA mission formulation study a new, alternative LISA payload architecture was developed and theoretically investigated. This concept features in-field pointing in order to compensate for these changes in spacecraft formation. The in-field pointing architecture offers potential savings in mass, volume and power consumption as well as potential improvements in measurement performance of the metrology instrument, compared to telescope pointing. Nonetheless, the technical realisation of the IFP concept in compliance with the requirements is highly demanding. This thesis covers two different and potentially limiting aspects concerning the realisation of an advanced LISA payload architecture including in-field pointing. The first one is part of the detailed investigations of the required wide-field telescope, suffering from mirror topography induced (optical) path length changes, so called piston, due to active beam steering and walking of the laser beam over the surfaces of subsequent telescope mirrors. This specific effect and its impact on the LISA measurement performance is investigated in more detail based on experimental measurements as well as a theoretical model. The second aspect investigated in this thesis is a novel concept for a LISA like inertial sensor, which features a spherical proof mass in combination with an all optical read-out. This concept represents an elegant solution for a low-noise, fully drag-free inertial sensor system, which could further increase the measurement performance of the LISA metrology instrument. In particular, the presented investigations include the development of a measurement setup for generating a detailed surface map of spherical proof masses, in order to calibrate its topography with respect to the sphere's centre of mass. First results of one-dimensional topography measurements of a SPM dummy are presented, representing a first step towards the generation of a complete two-dimensional surface map. Both topics are based on picometer-precision interferometric topography measurements, utilising highly sensitive heterodyne techniques. A major advantage of this measurement principle is its non-tactile nature, hence avoiding damage of the examined surfaces, while offering high levels of accuracy and reproducibility.