Doors to 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.

microshutter array
Each quadrant of the microshutter array is about the size of a postage stamp and contains 62,000 tiny shutters (some are open and some are closed in this photo). Credit: NASA.

Now imagine you could see twice that. Ten times. A hundred times.

Sadly, human beings can’t eye more than one thing at a time. The James Webb Space Telescope’s Near-Infrared Spectrograph (NIRSpec) can, thanks to a piece of technology miniscule enough to be threatened by the motes of dust floating in the air.

NIRSpec is Webb’s primary spectrograph, an instrument that breaks light into its component colors, which creates a spectrum for scientists to analyze. When Webb turns its attention to extremely faint, faraway objects, it will take a long time—at least a day or as long as 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 throughout its life.

On Earth, the solution to this typical telescope problem is simple. Scientists use metal plates drilled with holes to block out light from surrounding objects and focus on the multiple objects they want to analyze. Each time they do a new set of observations, they drill and insert a new set of plates.

Obviously, that isn’t a possibility for a telescope four times farther from Earth than the Moon is. So engineers came up with a tiny, creative solution: the microshutter assembly.

Small but Mighty

How microshutters are used as a mask
Microshutters enable Webb to observe multiple targets at once by allowing the light of targeted objects to pass through while blocking the light from objects astronomers don’t wish to study. Credit: NASA and STScI. GET THE FULL IMAGE IN RESOURCE GALLERY >

The microshutter assembly is a collection of four postage-stamp–sized devices called arrays. Each inch-and-a-half square 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 look at 100 objects at once.

The 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 shutters are just 100 microns long and 200 microns wide. For comparison, a human hair is about 75 microns wide.

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. They then etch the unaltered portions, carving into the wafer. They repeat this process time and again until they have the final result.

It’s a difficult, jigsaw-like process. But when it’s done, there will be a grid of thousands of closed windows, 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

Close-up of Microshutter Doors
The doors of the microshutter array are smaller than your average household dust specks. Credit: NASA.

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. Only the shutter doors aligned with the celestial objects meant to be observed are kept wide open.

The light from stars or distant galaxies pours through the open doors, each door encompassing a different object. The light is split by a grating 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 focused on another cosmic sight.

Last Updated: May 31, 2018

Keywords: Webb Mission

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