nircam coronagraphs

I was invited out to arizona this past weekend to attend the ultimate (read, last planned) NIRCam Team Meeting. NIRCam is the Near Infrared Camera, an instrument aboard JWST that is responsible for many of the awe inspiring images you’ve seen from the observatory. There, I got to see a physical copy of the sapphire chip that hosts the variously sized coronagraphs for the instrument’s coronagraphic imaging mode, the one I’ve been using for the past few years to study giant exoplanets orbiting in distant star systems. We use these tiny patterns to block the light from bright stars and reveal the faint planets near them.

the chip is a sapphire substrate that is transparent at infrared wavelengths of light. then, black squares are created by depositing nickel-chrome layers: we can use these to block a large fraction of the light from a star when we first point the telescope towards it. this allows us to move the star carefully from somewhere in the black square (wherever the star lands in that portion of the image after swinging the telescope around), measure its precise position in the black square, and then carefully move the star right behind one of the coronagraph. multiple layers of other reflective coatings and protectives are added atop the saphire and black squares.

then, a thin layer of aluminum is added to the top of the chip, and a thin layer of chrome atop this aluminum. a lithographic resist chemical was put on the aluminum, and the pattern of the coronagraph that is supposed to be opaque is shot with a beam of electrons that harden the coating. then, the aluminum, chrome, and chemicals are plasma’d off, leaving the hardened dark pattern. the pattern is a series of microdots that approximate a smooth, gray scale design

even though these things are designed to image planets that are far away from their stars, because the resolution of a telescope gets worse at longer wavelengths of light, the nice thing about these coronagraphs is that, because they have a gradual, gray scale pattern, we can still see planets relatively close to the stars that we are blocking. the pattern will block, say, 99.9% of the star’s light, and only 93% of a planet only a few pixels away from the center of the coronagraph. that’s how we got this image of the young, giant exoplanet AF Lep b using NIRCam last year.