Space Interferometry Mission: Recent instrument configuration developments

About a year ago, NASA Headquarters requested the SIM project to develop a less costly approach to performing micro-arc-second astrometry. The SIM Team responded by forming a Tiger Team and attacking the problem with vigor. Eliminating two secondary objectives (ing and imaging) opened up several options that led to a new configuration that met the cost target. Another simplification came about when it was decided to launch using the Space Shuttle. The very large payload bay allowed us to eliminate the hinge in the middle of the precision structure supporting the optical elements. In this paper, we discuss the current reference design of the SIM instrument, and illustrate some of the tradeoffs that led to this arrangement.