Requirements and options for a stable inertial reference frame for a 100-micro-arcsecond imaging telescope

The MAXIM Pathfinder (MP) and Stellar Imager (SI) missions are under study to do 100 microarcsecond resolution imaging for a number of different targets using interferometers divided over formation flying spacecrafts. One of the most challenging technical hurdles for these missions is to have an independent directional reference in the sky to use for target acquisition and tracking. This directional reference will guide the placement of separate free flying elements of the interferometers to have ~<30 microarcseconds of alignment with the target. This paper will discuss some of the specific challenges as well as some possible options to explore for achieving this alignment.