|Shown is a close-up view of NASA's next-generation microshutter arrays during fabrication. (Image Credit: NASA/Bill Hrybyk)|
A NASA team led by principal investigator Harvey Moseley, a scientist at NASA's Goddard Space Flight Center (Greenbelt, MD) has demonstrated that electrostatically actuated microshutter arrays are as functional as the current technology's magnetically activated arrays. This advance makes them a highly attractive capability for potential space missions designed to perform multiobject observations.
The team has successfully eliminated all macro-sized moving parts -- in particular, a large magnet -- and dramatically lowered the voltage needed to actuate the microshutter array, says Mary Li, a Goddard engineer. In addition, the team applied advanced electronic circuitry and manufacturing techniques to assure the microshutter arrays' dependable operation in orbit.
Considered among the most innovative technologies to fly on the James Webb Space Telescope, the microshutter assembly is created from micro-electro-mechanical systems (MEMS) technologies and contains thousands of small shutters.
Assembled on four postage-stamp-sized arrays, the 250,000 shutters open or close individually to allow only the light from targeted objects to enter Webb's Near Infrared Spectrograph (NIRSpec). Because Webb will observe faint, far-away objects, it will take as long as a week for NIRSpec to gather enough light to obtain good spectra.
NIRSpec's microshutter array, however, enhances the instrument's observing efficiencies. It will allow scientists to gather spectral data on 100 objects at a time, vastly increasing the observatory's productivity. When NASA launches the Webb telescope in 2018, it will represent a first for multiobject spectroscopy.
Eliminating moving parts
Determined to make the microshutter technology more broadly available, Goddard technologists have spent the past four years experimenting with techniques to advance this capability. One of the first things the team did was eliminate the magnet that sweeps over the shutter arrays to activate them. As with all mechanical parts, the magnet takes up space, adds weight, and is prone to mechanical failure. Perhaps more important, the magnet cannot be easily scaled up in size without creating significant fabrication challenges. As a result, the instrument's field of view is limited in size. This greatly impedes next-generation space observatories that will require larger fields of view.
Magnetic activation also takes longer. With the Webb telescope, the magnet must first sweep over the array to open all the shutters before voltages are selectively applied to open or close specific shutters.
The team replaced the magnet with electrostatic actuation. By applying an alternating-current voltage to electrodes placed on the frontside of the microshutters, the shutters swing open. To latch the desired shutters, a direct-current voltage is applied to electrodes on the backside. Only the needed shutters are opened; the rest remain closed. "This reduction in cycles should allow us to extend the lifetime of the microshutter arrays 100 times or more," Li explains.
And because the magnet no longer dictates the size of the array, its elimination will allow scientists to assemble much larger arrays for instruments whose fields of view are 50 times larger than Webb's NIRSpec, she says.
Just as significant is the voltage needed to actuate the arrays. When the effort first began four years ago, the team only could open and close the shutters with 1000 V. By last year, the team had achieved a major milestone by activating the shutters with just 30 V.
The team used atomic layer deposition to fully insulate the small space between the electrodes to eliminate potential electrical crosstalk that could interfere with the arrays' operation. The team also applied a very thin anti-stiction coating to prevent the shutters from sticking when opened.
Three astrophysicists now are interested in applying the technology to their own mission concepts, which include observing nearby star-forming regions in the ultraviolet, studying the origins of astronomical objects to better understand the cosmic order, and understanding how galaxies, stars, and black holes evolve.
Useful for lidar, too
Although spectroscopy is the obvious beneficiary of the technology's advance, Li says it also is applicable to lidar instruments. A major automotive company also has expressed interested in the technology, she says.
However, before others can use the new and improved microshutter technology, Li says the team must develop an assembly and packaging to house multiple arrays. "If you want to use the microshutter array on a large telescope, we need to make a larger field of view. To make this happen, we need to take multiple arrays and stitch them together," Li says.
Currently, the technology relies on a large computerized switch box -- a heavy device unsuitable for spaceflight missions. The team plans to incorporate an integrated circuit that drives the switching functions. Placed next to the shutters, the circuit would take up only a fraction of the space. The team currently is identifying circuits from different vendors and plans to begin testing shortly.