Mudge, D.Veitch, P.Munch, J.2008-02-282008-02-28200516th National Congress of the Australian Institute of Physics [electronic resource] : physics for the nation : congress proceedings / [Max Colla (ed.)]0959806482http://hdl.handle.net/2440/40888The performance of a gravitational wave interferometer (GWI) is degraded by distortion of the optics due to absorption of optical power. To maintain performance, detection and active compensation of the distortion are necessary[1]. We are developing an off-axis Hartmann wavefront sensor that can measure the distortion without interfering with the circulating optical power in the interferometer. Our measurement analysis yields the axial and radial distribution of the optical distortion. This allows us to resolve different thermo-refractive and thermo-elastic effects from a single measurement. We shall discuss the development of the sensor, analysis of off-axis measurements and the latest results. [1] R. Lawrence, M. Zucker, P. Fritschel, P. Marfuta, and D. Shoemaker, Classical and Quantum Gravity. 19(7), 1803, 2002.enConference paper00200751722008022814175545904Veitch, P. [0000-0002-2597-435X]