for the James Webb Space Telescope
Of the thousands of exoplanets that have been detected thus far, it may be surprising to learn that astronomers have actually seen only a handful of them. This is because prominent exoplanet detection techniques rely on residual effects of a planet on its host star, such as the planet's gravitational impact on the star (radial velocity measurements), or the amount of starlight the planet blocks out when passing in front (transit observations). While these techniques have been quite effective in finding exoplanets, they remain limited in the types of planets they can detect, and the amount of information they can glean from them. Directly measuring the light from a planet offers many advantages, such as the ability to directly measure the planet's surface temperature, atmospheric composition, and rotation rates, all important steps in determining a planet’s habitability. Unfortunately, direct imaging also comes with immense challenges. Because stars tend to be much brighter than their companion objects, most planets are very difficult to see. Exoplanets best suited to this technique are typically large, hot, and far away from the host star — not exactly earthlike, to say the least!
Enter the coronagraph, an instrument built directly into a telescope that works to partially shield incoming starlight by inserting a mask in front of a target star, boosting the relative signal of fainter nearby companions. NIRCam is equipped with five coronagraphic masks — three round masks and two bar-shaped masks — that suppress starlight under different conditions of contrast and separation between the star and its companions. Despite the masks' ability to block most of the starlight, however, a halo of scattered light, as seen in figure 1, remains, still making companions difficult to spot. A second step must then be taken to remove remaining starlight using point-spread function (PSF) subtraction, a process in which a second image of a comparable star, or of the same star system taken at a different angle, is subtracted from the original, as seen in figure 2. Here’s how this will work in action:
On the left is a star with two companions. As the coronograph moves in front of the host star, it blocks a significant portion of incoming light. In the middle panel, the brightness of the star appears to decrease, but the companions are still too faint to see. The right panel shows what this process looks like after PSF subtraction: the signal from the star is almost non-existent, and the companion signal becomes enhanced.
NIRCam’s coronagraph represents a significant advancement in direct imaging techniques, and is a big step toward detecting fainter, close-in planets resembling our own.
Beichman et al (2017), "NIRCam Coronagraphic Observations of Disks and Planetary Systems," American Astronomical Society: NASA ADS