How Does Webb See the Universe?

Think how much more productive you could be if you could study a book, check your computer, watch the television, and consult a newspaper all at once.

Now imagine you could see twice that. Ten times. A hundred times. That’s what the James Webb Space Telescope’s Near-Infrared Spectrograph (NIRSpec) can do, maximizing the amount of data astronomers can collect.

NIRSpec is Webb’s primary spectrograph, an instrument that breaks light into its component colors, creating a detailed spectrum for scientists to analyze. For extremely faint, faraway objects like distant galaxies it can take at least a day and up to a week for NIRSpec to collect enough light to see a good spectrum. If NIRSpec could only look at one object at a time, the telescope would not be able to observe many objects during its mission.

On Earth, the solution to this typical telescope problem is fairly simple. Astronomers use metal plates drilled with holes or fiber-optics positioned by robots to block out light from surrounding objects and focus on the multiple objects they want to analyze.

Since Webb’s observing location is beyond the Moon, 1 million miles away, inserting new plates isn’t possible. So, engineers came up with a tiny, creative solution: the microshutter assembly.

Small but Mighty

The microshutter assembly is composed of four postage-stamp–sized devices called arrays. Each inch-and-a-half square array contains 62,000 microscopic shutters that open and close to allow only the light from targeted objects to reach NIRSpec’s detector. With the microshutter assembly, NIRSpec can focus on 100 objects at once.

The shutters are just 100 microns long and 200 microns wide. For comparison, a human hair is about 75 microns wide. The shutters’ tiny size is necessary because Webb focuses all the light it collects into a single intense point to create the best possible image. When it’s done, each star or galaxy is just about the right size to fit into one of the shutters.

The arrays are created out of silicon-nitride wafers, the kind typically used to make transistors. The silicon nitride, a combination of silicon and nitrogen gas, is grown atop a layer of silicon and glass. Engineers add layers of metals and other materials to the blank wafer, making it etching-resistant in the areas they want to leave untouched, like protecting a wall with masking tape while painting.

It’s was a difficult, jigsaw-like process. But once completed, Webb was ready, with a grid of thousands of closed shutters, each attached to a strip of wafer only two microns wide—too small to be seen in detail without an electron microscope. These strips are the shutter hinges. The thinness of the hinges is why the array works—they’re so fine that they twist without breaking and snap back into shape.

How It Works

The shutter doors are lined with magnetic strips and sit within a metal box that can be electrically charged. Each shutter can receive its own electric charge. In its resting state, all the doors of the microshutter are closed. A magnet sweeps over the shutters, repelling the magnetic strips on the doors and pushing them all open. The box is charged with electricity, pinning all the doors open. Controllers then change the voltages on the shutters they wish to close, allowing the magnet to pull those doors closed as it sweeps by a second time. The only shutter doors left open are those to be aligned with the celestial objects on the schedule to be observed.

The light from stars or distant galaxies pours through the open shutter doors, each looking at a different object. The light is spread into a spectrum of colors, then directed toward the detector for analysis. Thanks to the microshutters, Webb’s NIRSpec has 100 tiny eyes at its disposal, each working independently, at the same time, to study unique parts of the cosmos