ULBCam: The World's Largest Infrared Camera

by Don Hall

ULBCam mounted on the UH 2.2-meter telescope. Credit: F. Hee.

Thanks to a new camera incorporating a huge mosaic infrared array detector, the relatively small, 30-year-old UH 2.2-meter (88-inch) telescope is, for the moment, the most powerful in the world for wide-field near-infrared imaging. The Ultra Low Background Camera (ULBCam), which had its first light last September, is a result of a detector development project funded by NASA and run out of IfA's Hilo facility.

The detectors in ULBCam utilize new technology developed to meet the demanding requirements of NASA's James Webb Space Telescope (JWST). This 6-meter-class telescope, planned for launch at the end of the decade, will be the successor to the Hubble Space Telescope (HST). It will have six times the collecting area of HST and will be launched into an orbit far beyond the Moon at the L2 Lagrangian point. There, the segmented mirror will unfold, and the telescope will cool to temperatures near -400 degrees F, allowing extremely sensitive infrared observations that will probe far deeper than those of HST or existing large ground-based telescopes.

To take full advantage of JWST's incredible near-infrared potential, its instruments will need near-infrared arrays of unprecedented size and sensitivity. To demonstrate that our new approach to building infrared arrays would work, we built the ULBCam as a prototype. The heart of the ULBCam is a 4096x4096-pixel detector array configured as a mosaic of four 2048x2048-pixel detectors. The new camera provides a sixteenfold increase in sky coverage together with much higher sensitivity than the 1-Megapixel (1024x1024 pixels) detector cameras in widespread use for the last decade.

Four chips "tiled" in the ULBCam. Credit: F. Hee.

Until now, nearly all space and ground-based instruments have used indium antimonide (InSb) detectors for near-infrared observations in the 0.6- to 5-micron range. However, another detector, mercury cadmium telluride (HgCdTe), proposed by a partnership of the IfA and Rockwell Scientific Company (RSC), showed such promise that NASA initiated a multiyear program to develop and compare the competing detector technologies. Funded by a nearly $7 million award from NASA Ames Research Center, a team at IfA Hilo worked with RSC to develop 4-Megapixel chips. The chips use the new infrared detector materials along with state-of-the-art, 2-inch-by-2-inch silicon chips that are among the largest ever produced. In partnership with GL Scientific, a small Honolulu firm, the team has created an innovative approach to mounting the individual 4-Megapixel chips so that four of them could be "tiled" into a 16-Megapixel camera. This approach allows for even larger mosaic cameras in the future.

The project team provided technical direction of both the development effort at Rockwell Scientific and the silicon chip fabrication at the UMC foundry in Taiwan. We also established a state-of-the-art Ultra Low Background (ULB) test facility is modular, allowing it to be configured for either laboratory testing or use at the 2.2-meter telescope.

In mid-2003, NASA conducted a comprehensive review of the two competing near-infrared detector technologies. On the basis of this review, the technology developed by the UH/RSC team was selected for the JWST's near-infrared camera, NIRCam. It will probably also be used in its near-infrared spectrometer (NIRSpec) and its guider/tunable filter.

In parallel with the JWST, large ground-based telescopes are already racing to take advantage of this new technology. On Mauna Kea, both the Canada-France-Hawaii Telescope and the Gemini Telescope Project are forging ahead with 16-Megapixel infrared cameras.

IfA Director Rolf Kudritzki recently cited the ULBCam project as "an excellent example of IfA's nurturing of extremely high-tech projects in its Hilo facility." He stated, "It is particularly gratifying that a number of the key personnel on this project grew up in Hilo and were recruited back from the mainland, and that several others were new graduates recruited directly out of UH Hilo. The project also provided important training for undergraduate assistants from UH Hilo, many of whom have gone on to positions in related fields."

A number of IfA faculty and graduate students have already used ULBCam for a wide range of observing projects. Richard Wainscoat has been using it to study galaxies similar to our own galaxy. The wide field of the new camera allows him to image galaxies in a single frame, rather than mosaicking together many separate images (which takes much more observing time, and is less accurate). By studying these external galaxies, he hopes to learn more about the structure of our own Milky Way galaxy.

Peter Capak has been using ULBCam to study galaxies when they were first born, about 13.5 billion years ago, and emitted most of their light in the ultraviolet region of the spectrum. In the last 13.5 billion years, the Universe has expanded, which has stretched the light emitted by the baby galaxies into the infrared, making them invisible to optical cameras but visible to ULBCam. Another period previously unseen is their adolescence, between 9 and 12 billion years ago. Finally, as adults galaxies can merge and produce stars, but often during this process they hide behind a veil of dust. These veiled galaxies can also be detected and studied with ULBCam.

Don Hall is the head of the team at IfA Hilo that developed the 4-Megapixel chips and ULBCam. Both he and IfA astronomer Klaus Hodapp are members of the JWST's Near-Infrared Camera Science Team.