Astronomy's rosy revolution - infrared detectors - Cover Story

Science News, Nov 16, 1991 by Ron Cowen

The heavens look different in the infrared. Ancient stars, dim and dull in visible light, take on a fiery glow; familiar-looking galaxies dramatically change their shape; dust-shrouded regions blaze brightly, revealing the birthing places of stars.

Astronomers have long known that infrared observations could open a new window onto the cosmos. At some of these wavelengths -- just three to 10 times longer than those of visible light -- dust becomes transparent, allowing researchers to peek at the hidden centers of galaxies. Infrared deep-sky maps reveal galaxies more distant than those seen in visible light. Closer to the Milky Way, only infrared viewing clearly images the longlived population of cool stars, which make up the bulk of a galaxy's mass and ultimately determine its fate.

But for years, scientists lacked the tools they needed to fully explore the infrared frontier -- even at the shorter range of wavelengths clearly visible from Earth. Simply put, the available detectors could not meet the challenge.

Too small and lacking the high sensitivity of devices used to view visible light, the typical infrared detector in the 1970s relied on a single, solid-state sensor to capture streams of incoming photons. That created an astronomer's nightmare: To obtain one full image of a nearby galaxy, researchers had to slowly slew their telescopes back and forth across a patch of sky over dozens of nights, then piece together the thousands of small images generated by a detector attached to the back of the instrument. And because the sensor's field of view exceeded the size of a single star image, the resulting picture often lacked the small-scale detail that the astronomers had labored so carefully to grasp.

"The frustration that has underpinned infrared astronomy ever since it began was that we knew very well what we wanted to do but we didn't have the instruments to do it," says Ian Gatley of the National Optical Astronomy Observatories (NOAO) in Tucson, Ariz. "A lot of the things we aim to do are really very, very simple conceptually. The issue has always been one of technology."

Things started looking rosier in the 1980s. By then, the U.S. military had developed some large, heat-sensitive detectors for viewing infrared-bright objects such as rockets moving in the night sky. Astronomers began adapting these devices to record the fainter infrared emissions of distant stars and galaxies.

"There were a lot of things that for years and years I said, 'Gee, I wish we could do this; wouldn't this be neat?'" Gatley recalls. "Suddenly, along comes this amazing technology that's telling us, 'Okay, fine -- go do it!'"

Since no single material can sense all infrared radiation, designers vary the metal alloys used in detectors, depending on the specific wavelengths astronomers seek to study. So far, researchers have made the most progress in developing devices sensitive to near-infrared wavelengths between 1 and 5 microns -- a region of the electromagnetic spectrum in which dust becomes transparent.

Their efforts were spurred by the push to develop a new infrared detector that NASA plans to install aboard the Hubble Space Telescope in 1997. Scientists at the University of Arizona in Tucson and the Rockwell Science Center in Anaheim, Calif., collaborated on the designs of the instrument, known as NICMOS (near-infrared camera and multi-object spectrograph). Since 1990, research teams at the University of Hawaii in Honolulu and the University of Arizona have used prototypes of the new instrument to explore the heavens.

A sandwich of two semiconductor chips, each serving a separate but critical function, forms the heart of these new, thumbnail-sized devices, known as large-format arrays. The top layer consists of an infrared-sensitive chip divided into individual sensors called picture elements or pixels. A bottom layer of silicon, also divided into pixels, records the electronic signal induced when light strikes the top layer. Tiny metallic "bumps" electronically connect the two layers. The newest arrays contain some 65,000 infrared-sensing pixels; the sensitivity of a single pixel surpasses that of an entire infrared detector built only a decade ago.

The resolution of these detectors still pales alongside state-of-the-art optical devices, which contain some 4 million pixels. And infrared astronomers still must slew their telescopes to capture an image of most sources. But they slew less often, and in just one night they can produce images that would have taken centuries to make with the old, single-element infrared detectors, notes Stephen E. Strom of the University of Massachusetts at Amherst. Best of all, Strom says, these pictures offer unprecedented clarity and detail.

Already, the new detectors have generated a feast of new findings, ranging from surveys of distant galaxies to glimpse of starbirth in our own Milky Way.

Viewed in visible light, the galaxy NGC 309 appears similar to the Milky Way. Like spiral out from a central disk. The luminous arms represent regions where massive, hot stars currently concentrate and where astronomers believe starbirth takes place.