These two designs are functionally equivalent in that they are both capable of meeting all SIM science requirements. The two designs have different benefits and drawbacks. The team has just completed a detailed look at the SIM-Classic design and reported the results to its technical advisory committee. A similar study and report was completed earlier for the SIM-SOS design. The SIM team is currently studying the differences between these two designs, aimed at gathering enough information to make a selection between them by November 1999 which is based upon the best mix of cost, schedule, risk and operability.
TOM is aimed at exploring the response of optical figure to small changes in thermal conditions.
Since the SIM metrology system samples only a small portion of each collecting aperture, sub-nanometer changes to optical figure across the apertures during the course of an observation would result in misleading estimates of the optical path excursions seen by starlight.