SOFIA Upgrading One-of-a-Kind Camera

After making numerous discoveries about how magnetic fields shape our universe, an instrument flying on board the Stratospheric Observatory for Infrared Astronomy (SOFIA, Cold Facts Volume 35, Number 5), is about to get even faster at gathering data. Announced April 23, SOFIA is upgrading the High-resolution Airborne Wideband Camera-Plus, or HAWC+, with four new detectors that will allow it to study magnetic fields in distant galaxies four times faster than its current rate.

The HAWC+ upgrade is expected to be completed by 2023 and is the first step in the proposed outline for future instrumentation of SOFIA, a joint project of NASA and the German Aerospace Center – DLR. Based on feedback from a scientifically diverse group of astronomers, two additional instruments are envisioned that will enhance SOFIA’s ability to make new discoveries. “We want to speed up the pace of scientific discovery, and we can do that by making HAWC+ even better,” said Dr. Margaret Meixner, director of science mission operations at Universities Space Research Association. “This upgrade is part of a number of initiatives we’re implementing to take SOFIA into the future.”

HAWC+ is currently the only operating instrument in the world in an observatory that uses both far-infrared light and has a polarimeter, a device that measures polarized light from celestial dust grains, to infer the shape and direction of magnetic fields. Scientists are eager to learn more about the role magnetic fields play in shaping galaxies and the formation of stars, and observations like those SOFIA provides, using far infrared light, are critical to getting a clearer picture.

Flying at 40,000 feet and above the interfering layers of the atmosphere, SOFIA offers a one-of-a-kind platform for observing the infrared universe. Because it returns to the ground after each flight, its instruments can easily be exchanged, serviced or upgraded to harness new technologies that may one day be optimized for flight in space.

As Dejan Stevanovic, lead systems engineer for SOFIA, told Cold Facts in 2019, infrared radiation is effectively heat. The colder the detector—and the surrounding optics—the more sensitive that detector is to the faint heat signal from these distance cosmic sources. Most instruments on SOFIA operate at or around 4 K, with detectors at less than 1 K, by using a combination of nitrogen and helium cryogens, closed-cycle cryocoolers (mainly pulse tube coolers), helium sorption refrigerators and adiabatic demagnetization refrigerators (ADRs). SOFIA’s HIRMES1 (High-Resolution Mid Infrared Spectrometer) utilizes two PTC coolers that provide a stable 4 K environment for the optical elements. It includes an 4He sorption cooler with the sole task of cooling a detector baffle and an 3He/4He two-stage sorption refrigerator coupled with an ADR to achieve a stable detector temperature of 70mK for at least 12 hours.

While all infrared astronomical instrumentation, ground-based or airborne, requires cryogenic cooling to some extent, most of the ground-based telescopes are focused on near-IR (0.8-1.2µm) observations, so detectors generally do not need temperatures lower than 60 K to achieve high sensitivity and low noise. SOFIA is in a unique position with its ability to capture mid-IR and far-IR radiation by flying above 99% of atmospheric water vapor.

However, the ability to capture far-IR radiation comes with its own set of challenges and a set of very specific detector technologies that require 0.1 K or lower operational temperatures. These detectors and their cooling systems are so sensitive that any vibration, even the smallest one, can be detected as heat and adversely affect the science.

Vibration sources are very diverse, coming from usual aspects of aircraft operation (engines, airborne turbulence, etc.) to some that are very specific to SOFIA, like wind buffeting and turbulence around and inside the telescope cavity. The vibrational environment on-board SOFIA B-747SP drives a set of complex design limitations and requirements that are imposed on instrument cooling and vibration isolation systems, requirements that are rarely imposed on ground-based or even spaceborne instrumentation.

According to the roadmap published earlier, two new instruments envisioned for SOFIA include a spectrometer and a terahertz mapper. The highly sensitive spectrometer improves SOFIA’s ability to measure faint signals by a factor of 10. With this spectrometer, SOFIA could for the first time measure the mass of gas, water vapor and water ice in the earliest phases of planet formation, enabling astronomers to learn how planetary systems form. The new terahertz mapper would build on the success of another of SOFIA’s flagship instruments, the German Receiver at Terahertz Frequencies, or GREAT, by using similar technology with 100 pixels, an increase from GREAT’s 14 pixels. This will allow the new instrument to make observations 14 times faster.

SOFIA is a joint project of NASA and the German Aerospace Center. NASA’s Ames Research Center in California’s Silicon Valley manages the SOFIA program, science, and mission operations in cooperation with the Universities Space Research Association, headquartered in Columbia MD, and the German SOFIA Institute at the University of Stuttgart. The aircraft is maintained and operated by NASA’s Armstrong Flight Research Center Building 703, in Palmdale CA.