One way of imaging exoplanets around nearby stars is to use a starshade in conjunction with a space telescope. Typically, the starshade, which resembles a sunflower in outline, is ~ 30 meters in diameter. The starshade is flown about 50,000 km in front of the telescope, and when positioned directly in the telescope’s line of sight to the star, blocks the starlight, casting a deep shadow onto the telescope. Exoplanets orbiting the star and having a small angle to the line of sight will be visible because the starlight is extinguished. During the observation period, the position of the telescope needs to be maintained within about 1 m of the center of the shadow for maximum shading of the starlight.
When another star is to be observed, the starshade and the telescope must be reoriented. This will involve one or both of the spacecraft making a transit of typically thousands of kilometers. During this phase (transition), it is necessary to keep track of the position of the starshade as seen from the telescope so the starshade can be quickly repositioned on the new line of sight, without requiring the deep space network.
Optical methods have been devised for measuring the starshade position during both of these phases of a starshade mission, and these rely on two beacons mounted on the starshade. One of the beacons is an infrared laser operating at 1550 nm [the fine guidance sensor (FGS) beacon]. It is used for fine positioning once the starshade is near or on the line of sight. In this case, both the target star and the FGS beacon will be observed through the science telescope and detected by an infrared (IR) camera. The FGS laser is pulsed on and off, thus providing a signal to distinguish it from the continuous light from the star. The long wavelength light (1550 nm) from the beacon does not interfere with science observations, which are made with light of 1000 nm wavelength and shorter.
The FGS sensor has a small field of view (FOV), about 1 arc minute, to enable resolution of the starshade and stellar centroids to less than 1 m. During transition maneuvers, it is necessary to have a wider FOV to keep track of the starshade as its position rapidly becomes uncertain due to variations in spacecraft thrust direction and impulse magnitude. The starshade therefore includes a second beacon (the transition beacon), a blinking LED array that emits visible light. The transition beacon is sensed using the telescope spacecraft’s star tracker camera, which has a very wide field of view (20 degrees), ensuring that the starshade will be detected.