Simulating instrumental phase errors for SIM

The Space Interferometer Mission (SIM) has considerable capabilities for imaging of complex targets with the resolution of a diffraction-limited 12 m telescope. The exact performance of SIM in this regard depends-among other factors - critically on its mechanical stability. For example, structural vibrations will lead to errors in the in the delay line position and thus in the derived phase of the incoming wavefront. Depending on the time constants of such vibrations and on whether or not they are random in nature, image reconstruction can be affected in different ways. In order to estimate the impact of such instrumental imperfections, we have improved our simulation code SIMSIM in a number of ways. For example, in order to develop the code further towards a possible analysis tool for real aperture synthesis data, the new version - SIMSIM-II-allows (u,v)-datapoints which are sampled on a non-regular grid. In addition, the user can choose between various models for phase and amplitude errors. Using these capabilities, we have modeled aperture synthesis observations with SIM with varying degrees of error in the visibility data. Simple metrics such as dynamic range or image fidelity are used to quantify the degradation of the reconstructed image. We demonstrate that SIMSIM-II is a suitable analysis tool for deriving hardware specifications and to optimize the stategy for imaging observations with SIM.