The concept of a Starshade is an Occulter consisting of a space vehicle shaped to occult (block) the light of a star so that a targeted space telescope can examine the star's surroundings directly, e.g., to observe Extra-Solar Planets. Normally, the starlight, which would be 107 to 1010 times brighter than the exoplanet, would saturate the CCDs. There are optical advantages to having a long distance between the occulter and the telescope, which makes the concept of a separate space vehicle is of interest. The New Worlds Mission was a plan to deploy a starshade and Exo-S is a newer plan.
Starshade designs have a sunflower-like shape, specifically so that the starlight refracted around the edges does not converge toward the center of the optical path: ideally, all the starlight misses the telescope or Focal Plane. The size of the starshade is as large as the telescope but small enough to view the space around the star.
A disadvantage is a starshade vehicle requires the capability of moving a considerable distance each time the telescope is aimed at another star system. Another issue in its design and operation is the effect of sunlight on it, refracting, reflecting, and heating it.
New Worlds Mission