67 Realization and Testing of the In Field Pointing Mechanism for the Evolved Laser  Interferometer Space Antenna  

Gert Witvoet* and Jet Human*  

Abstract    An active tilt mirror mechanism,  meant for correction of the constellation breathing of the evolved Laser  Interferometer  Space Antenna, has been designed and built. Its open- loop performance has been  characterized in both the time and frequency domain. Based on this, a feedback controller has been  designed and the resulting  closed- loop performance has been assessed. Up to what  is measurable in a  normal lab environment, these experiments  demonstrate compliance with the extreme pointing jitter  requirement  even when using the internal encoder  as feedback sensor.  

Introduction  

The evolved Laser Interferometer Space Antenna (eLISA) mission, which is currently under development,   is an ambitious project meant to accurately detect gravitational  waves [1]. It will consist of three  spacecraft  flying in a triangular formation mutually 10 9 m apart  in an Earth- like orbit around the sun, each  carrying a free-flying proof mass. Detection of a passing gravitational  wave requires measuring the  distances between the  proof masses with an accuracy of 10-11 m, which makes  eLISA an extremely high  sensitive instrument.   The eLISA mission still faces scientific and technological  challenges to be solved before its tentative  launch in 2034. One of which is constellation breathing, i.e. , how to deal with slight variations in the angle  between the interferometer  arms over a period of a year due to orbital  dynam ics. In Telescope Pointing  this is accounted for by rotating each of the six complete optical assemblies  (two on each spacecraft). As  a promising alternative, Airbus  Defence & Space has developed the In- Field Pointing  (IFP) concept  [2], in  which only a small tilt mirror,  located in an intermediate pupil plane of the telescope,  provides the means  to steer the beam. IFP comes with a number of advantages over Telescope Pointing, such as actuation of  a much smaller mass, possible smaller payload sizes , and possible simpler payload architectures  [2, 3].  The feasibility of the IFP concept will be demonstrated in an experiment that Airbus DS is currently  developing [3]. One of the critical components in this experiment is  the In- Field Pointing Mechanism  (IFPM), responsible for  actuating the tilt mirror, on which there are high stability  requirements. Built upon  heritage with the Point -Ahead  Angle Mechanism  [4], which has similar stability  requirements, TNO  (Netherlands Organisation for  Applied Scientific Research) has successfully carried out  breadboard tests  and made a design for this IFPM [5]. In this paper we will present the realized IFPM hardware,  together  with some important test results (in a normal  laboratory environment). These tests include open- loop  characterizations as well as closed- loop performance validations.  Up to what is reasonably measurable in  such a lab environment, the results show that the IFPM is indeed compliant with the requirements.  

Hardware Design and R ealization  

The heart of the IFPM is a 50- mm flat tilt mirror, which  needs to rotate ± 2.5° in a whole year (equivalent to  ±5° beam steering) to accommodate constellation breathing.  In science mode it should do so with a  maximum  pointing jitter of just 5 nrad/ √Hz over a large frequency  range. This combination of large stroke                                                    * TNO Technical Sciences, Delft, T he Netherlands  

Proceedings of the 43rd Aerospace Mechanisms Symposium, NASA Ames Research Center, May 4-6, 2016