starshade

A starshade is a concept for an occulter that would consist of a space vehicle shaped to occult (block) the light of a star from reaching an associated space telescope, such that the latter can examine the star's immediate surroundings directly, e.g., to observe extra-solar planets. It can be considered a variant on the coronagraph concept, but with the occulter far in front of the telescope. 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 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 less starlight refracted around the edges converges toward the center of the optical path: ideally, all such starlight misses the telescope or at minimum, its 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: any of these may reach the telescope, constituting a kind of stray light.