Open and shut case

NASA has developed microshutters for the James Webb space telescope based on the concept of human squinting.

When you squint, your eyelashes block out light closer to you, which has the effect of improving your focus on a faraway object. It’s a tiny act most of us take for granted but NASA engineers are applying the concept of squinting in a way that will allow them to gather information about the most distant galaxies.

A team from the space agency has developed ‘microshutters,’ which are tiny doorways that could help orbital telescopes bring outlying stars and galaxies into better focus. The technology will go aboard the James Webb space telescope, which will replace the Hubble telescope within the next decade.

The hairlike microshutters — working in a similar way to a person’s eyelashes — will enable scientists to block unwanted light from objects closer to the telescope, thus allowing it to see light from further stars and galaxies.

The devices will allow the telescope to focus on the faint light of stars and galaxies so far away, they were formed just after the Big Bang. This is because light travels at 186,000 miles per second, and it is still travelling through space from the time when the universe started.

The shutters were designed, built and tested at NASA’s Goddard Space Flight Centre in Greenbelt, Maryland. They will work in conjunction with the Webb telescope’s infrared detector camera, called the Near Infrared Spectrograph (NIS), which is being built by the European Space Agency.

The NIS will break up the light from the galaxies into its component colours, which are characteristic of the elements in the stars and gases that make up the galaxies. It will also allow astronomers to measure distances and the movements of stars, nebulae and other objects in space.

Each silicon-based shutter measures 100 x 200 microns and is half a micron thick. The delicate objects are arranged in an array of 171 x 365, creating a waffle-like grid of 62,453 shutters, four of which will be aboard the telescope.

The shutters are equipped with mechanical hinges and electrodes, and each array contains a magnet that is used to open the shutters. The magnet sweeps across the array, flipping back all the shutters, which are then held open with an electric current. Scientists can use specially-designed software to shut off the voltage on the shutters they want to close, which allows them to focus on separate objects in the view field.

Murzy Jhabvala, chief engineer of Goddard’s Instrument Technology and Systems Division, said the concept of microshutters seemed like a pipe dream six years ago.

Normally, he said, NASA does not take a chance on funding the development of conceptual technology for its spacecraft but the agency decided to throw its support behind the microshutters because the technology was deemed to have exceptional contrast ratio, which means the amount of light that can get through the shutters when they are closed is extremely small when compared to the amount of light that gets through when they are opened.

‘It is very important to the science of this mission that no light gets through when the shutters are closed,’ said Jhabvala. ‘There were other technologies but at the time it did not appear that they could meet this contrast ratio.’

The shutters also appeared to be the only technology that could handle the extreme temperatures of space. ‘It’s around -233°C,’ he said. ‘You have to make sure all your materials are compatible so things don’t just fall apart or shatter or twist, and we weren’t sure that other competing technologies could handle the temperature.’

NASA gave Jhabvala and his team three years to research and develop the technology and an additional year for preparation so that it is reliable enough to fly.

There were dozens of incarnations of the array design, he said. ‘We began with a silicon wafer and it took a good three months before we had an array. We started with much smaller ones with just an individual shutter and then we made one with a few hundred shutters, and then a thousand shutters, and all along the way we refined the process of making them.’

The team managed to develop the shutters in the allotted time but there was still not a moment to relax. ‘NASA wanted the technology proven,’ said Jhabvala. ‘You can’t just put a technology up there — if something goes wrong, a billion-dollar satellite just dies.’

So the shutters had to go under rigorous testing and re-testing at Goddard. The engineers opened and closed them, thousands of times, to prove they would last their entire five-year mission on the Webb telescope.

The mission requires that within 248 days over a span of five years, the infrared camera must spot 2,500 objects. ‘That translates to about 40 to 100 objects at any given time,’ said Jhabvala. ‘The microshutters will allow scientists to look at 100 objects in space at once.’

The shutters were also put under vibration testing that simulated the rigorous four-minute take-off of an Ariane launch vehicle. They were also put under acoustics testing. ‘This thing is sitting in a rocket that makes a lot of noise, which transfers to pressure waves,’ said Jhabvala, ‘and we had to make sure that didn’t shatter the array.’

Finally the shutters underwent and passed a high-energy radiation test, simulating conditions in space.

After all this, they were given the technical white-glove test. The arrays’ 171 x 365 grid was studied to see how many shutters broke under testing. ‘Of the 171 rows, only two can have a broken shutter,’ said Jhabvala. ‘those two rows can be entirely broken, or just have one broken shutter — it doesn’t matter.’

Despite the staggering odds, the shutters passed. But the engineers came up with a back-up plan — just in case a shutter broke while in space. So they developed a process where a broken one can be plugged up and permanently closed.

Jhabvala said the scientists don’t care if the shutters are stuck closed, because most of the time when they’re operating the camera, 100 shutters are open and the rest are closed. Scientists are only concerned when a shutter opens when it is not supposed to be.

‘We had to come up with that idea because we knew from the beginning there was no way to make a perfect array,’ said Jhabvala. ‘So we came up with this sort of risk mitigation process.

‘These are the things you learn as you go through,’ he added. ‘You find out your limitations are and then ask “How do we move on to the next step? How are we going to innovate so this isn’t a show-stopper for us?” ‘