Gravitational wave signals measured by LISA will be superimposed by noise from various sources. Identifying noise sources and suppressing them is a major goal of scientists and technicians working on LISA technology.

A significant potential noise source for the LISA metrology is ‘tilt-to-piston coupling.’ This is the generation of an apparent longitudinal signal resulting from a change in angle between the local and received optical beams, caused by the spacecraft jitter. The primary goal of this activity is to demonstrate that imaging systems are a viable means of supressing tilt-to-piston coupling to 25 µm/rad.

The team aims to demonstrate this by measuring the tilt-to-piston coupling in a representative interferometer. Two imaging systems have been designed based on different modelling approaches – one using conventional ray-tracing methods and one using Gaussian optics – and the theoretical models will be compared to the results of the investigations.

The image shows a CAD model of the experiment comprising an interferometric test bed to study two imaging systems which are designed to suppress tilt-to-piston coupling in LISA. The Minimal Optical Bench has a diameter of 580 mm.

A Telescope Simulator will generate a beam that is representative of a beam from a far LISA spacecraft. This flat-top beam can be tilted to imitate spacecraft jitter and will be directed onto the Minimal Optical Bench to be combined with a local oscillator beam. The Minimal Optical Bench performs the role of the LISA science interferometer, and is interfaced to the Telescope Simulator using a Zerodur tip-tilt adjustable mounting system.

The apparatus will be capable of investigating other aspects of tilt-to-piston coupling such as the difference between using a flat-top compared to a Gaussian beam shape, aperture correction and the effect of quadrant photodiode detector position.