MEMS: Microshutter array to help space telescope peer back in time

The Hubble Space Telescope currently provides the sharpest views of the cosmos, with imagery hearkening back in time to an era shortly after the birth of stars and galaxies.

Mar 1st, 2007
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The Hubble Space Telescope currently provides the sharpest views of the cosmos, with imagery hearkening back in time to an era shortly after the birth of stars and galaxies. Hubble’s successor, the James Webb Space Telescope (JWST; see, scheduled to launch in 2013, aims to extend the view even farther into the past, all the way to the luminous glow shortly after the Big Bang. Preparing for this capability has required the development of several new technological approaches, including a programmable array of microelectromechanical-system (MEMS) microshutters to optimize detection of the most distant and consequently most faint of stellar targets.

The optical instruments on JWST-which include a near-infrared (IR) camera, a near-IR multi-object spectrograph (NIRSpec), a mid-IR imager and spectrograph, and a tunable-filter imager-will provide access to wavelengths ranging from 0.6 to 27 µm, and will offer a wide field of view and a deep, long observation of the sky including millions of light sources.

Optimal viewing of the faintest and most distant of sources requires masking nearer and brighter ones. Astronomers using ground-based telescopes create masks to keep light that does not come from the area of interest away from the telescope’s detectors. In space, this would either require including a prefabricated mask at launch time-as has been done with previous masks for space telescopes, which only cover large predetermined regions of a single field of view at any one time-or launching a relatively large and bulky mechanical apparatus to fabricate and configure masks as needed in space, according to Murzy Jhabvala, chief engineer of the Instrument Technology and Systems Division at the NASA Goddard Space Flight Center (Greenbelt, MD).

Programmable mask

As a more reasonable solution to this problem, engineers and scientists at the Goddard Space Flight Center developed the microshutter array for aperture control on NIRSpec. The NIRSpec instrument relies on two 4-megapixel mercury cadmium telluride detectors to obtain simultaneous spectra of up to 100 objects in a 9-square-arcminute field of view. It also provides medium-resolution spectroscopy over a wavelength range from 1 to 5 µm and lower-resolution spectroscopy from 0.6 to 5 µm. Aperture control using the microshutter array will provide a programmable mask, blocking out unwanted light from nearby objects and allowing the large-format detector to measure IR spectra optimally for the up to 100 or so distant objects simultaneously.

Jhabvala described the microshutters as an array of trap doors. The 171 × 365 array of shutters fits in an area about 1.5-inch square; each element or trap door measures 100 × 200 µm in surface area. When making observations, only one door per row is open in the shutter; the signal from that open door is dispersed by a grating into a spectrum along the entire row. “It’s a MEMS device created on a silicon wafer, similar to the manner in which computer chips are created,” he said. The array is etched into the wafer, leaving cavities and shutters with flexure hinges (see figure).

Each microshutter has a hinge on one side, which includes a silicon nitride torsion bar providing the spring tension to close the shutter when opening forces are removed. Magnetic cobalt-iron strips coat the upper face of each microshutter element. In use, a magnet is positioned underneath the array to first open all of the doors simultaneously; when the magnet is removed to enable imaging, elements that need to remain open are kept that way by computer-controlled voltage differences that can be turned on and off independently for each element in the grid. (Courtesy of NASA)
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In use, a map of the section of sky to be viewed is used to control the microshutter array, which will only have about 100 shutters open during any single observation. The time needed for setting up an observation is normally about 30 seconds, which is well within the response capabilities of the device, Jhabvala said. The microshutters are designed to operate at 35 K to match the optimal operating temperature for the NIRSpec detectors.

Hassaun A. Jones-Bey

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