《Mitigation of Angle Tracking Errors due to Color Dependent Centroid Shifts in SIM-Lite》.pdf

Mitigation of Angle Tracking Errors due to Color Dependent Centroid Shifts in SIM-Lite * Bijan Nemati , Xin An, Renaud Goullioud, Michael Shao, Tsae-Pyng Shen, Udo J. Wehmeier, Mark A. Weilert, Xu Wang, Thomas A. Werne, Janet P. Wu, Chengxing Zhai Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, CA, U.S.A. 91109 ABSTRACT The SIM-Lite astrometric interferometer will search for Earth-size planets in the habitable zones of nearby stars. In this search the interferometer will monitor the astrometric position of candidate stars relative to nearby reference stars over the course of a 5 year mission. The elemental measurement is the angle between a target star and a reference star. This is a two-step process, in which the interferometer will each time need to use its controllable optics to align the starlight in the two arms with each other and with the metrology beams. The sensor for this alignment is an angle tracking CCD camera. Various constraints in the design of the camera subject it to systematic alignment errors when observing a star of one spectrum compared with a start of a different spectrum. This effect is called a Color Dependent Centroid Shift (CDCS) and has been studied extensively with SIM-Lite ’s SCDU testbed. Here we describe results from the simulation and testing of this error in the SCDU testbed, as well as effective ways that it can be reduced to acceptable levels. Keywords: SIM, Interferometry, Astrometry, Spectral, Angle Tracker, SCDU 1. INTRODUCTION SIM-Lite is a space-borne astrometric interferometer which, over its five years of operation, will rea