Don Hall: The HAWAII Arrays: An Infrared Window on the Universe

Don Hall goes for the Big Picture, helping infrared astronomers progress from laboriously scanning a single semiconductor detector across the sky in the 1980s to today’s sensor chips with 16 million detector pixels, each with sensitivity improved a thousand-fold. This 16 billion-fold increase in observing efficiency has reshaped infrared astronomy from a technology constrained field with very limited scientific potential to a powerful tool for the study of the universe.

The ultra-deep field as imaged by the Hubble Space Telescope using a “HAWAII” array

When cosmic photons travelling across space are focused by a telescope’s mirror onto the infrared detector chip in a camera, they are converted to an electrical charge. Each pixel or “picture element” is a minute bucket of electrons, one for each detected infrared photon, just as visible light in a digital camera. When the exposure is finished the chip is read out to form a digital image.

The ohana of infrared chips developed by the IfA team is known as the HAWAII series of infrared arrays – HAWAII stands for HgCdTe Astronomical Wide Area Infrared Imager. Hg, Cd and Te are the symbols for the chemical elements mercury, cadmium and tellurium, which make up the infrared sensitive crystals used in the sensors. By using infrared sensors in instruments that are mounted on ground-based and space telescopes, astronomers are able to observe the formation of stars and galaxies formed so early in the evolution of the universe that all of their light is red-shifted out into the infrared so that they cannot be seen at all in visible light. A HAWAII array on Hubble Space Telescope has discovered the most distant galaxies known – the light from the oldest of these has taken 13.4 billion years (97% of the 13.8 billion year age of the universe) to reach us. Infrared light penetrates cosmic dust, allowing us to study young stars as they form and the black hole at the center of our galaxy. The spectral signatures of molecules lie in the infrared making it vital for the study of planets around other stars and the search for conditions that could nurture life as we know it.

Along with NASA’s Hubble Space Telescope, the IfA group’s HAWAII arrays have flown on the Deep Impact comet mission and Wide Field Survey Explorer (WISE). Fifteen 4 Megapixel HAWAII 2RG sensors totaling over 60 million pixels will be launched in 2018 in three scientific instruments on the James Webb Space Telescope. The next major NASA astrophysics mission, WFIRST-AFTA, is based on a camera with more than five times as many pixels – 18 HAWAII 4RG arrays for a total of over 300 million pixels! HAWAII arrays are also in use in ground-based observatories around the world, including all eight optical/infrared telescopes on Mauna Kea.

The future is bright. In the IFA’s Hilo facility on Hawaii Island, Hall’s team, under a grant from the National Science Foundation, has recently completed development of the largest infrared array for astronomy yet – a 16-megapixel sensor, the HAWAII 4RG-15. The focus has now shifted to a new HgCdTe infrared detector technology, the linear mode avalanche photo-diode. This offers the detection of individual infrared photons – the “holy grail” of infrared astronomy detector technology.

“These detectors are vital to the long-term success of the James Webb Space Telescope and other upcoming astronomy missions. They also greatly improve the infrared sensitivity of ground-based telescopes such as those on Maunakea today and are critical for the upcoming 30-meter class telescopes.” — Don Hall