  A key challenge for starshades is formation flying. To successfully image
exoplanets, the telescope boresight and starshade must be aligned to ~1 m at
separations of tens of thousands of kilometers. This challenge has two parts:
first, the relative position of the starshade with respect to the telescope
must be sensed; and second, sensor measurements must be combined with a control
law to keep the two spacecraft aligned in the presence of gravitational and
other disturbances. In this work, we present an optical sensing approach using
a pupil imaging camera in a 2.4-m telescope that can measure the relative
spacecraft bearing to a few centimeters in 1 s, much faster than any relevant
dynamical disturbances. A companion paper will describe how this sensor can be
combined with a control law to keep the two spacecraft aligned with minimal
interruptions to science observations.
