SOFIA, the Stratospheric Observatory for Infrared Astronomy, is a unique plane-based observatory housed inside a 747 aircraft. A joint project between NASA, Universities Space Research Association, and the German Aerospace Center-DLR since 1997, it takes advantage of the stratosphere to eliminate the absorbing effect atmospheric water has on infrared observations. Operated and maintained by NASA’s Armstrong Flight Research Center in Palmdale CA, its mission is to observe the universe and understand the formation and evolutionary mechanisms of stars, galaxies, planets and everything in between.

CSA recently spoke with Dejan Stevanovic, lead systems engineer for SOFIA, about the current experiments and status of the flying observatory. Our questions are in italics.

What are the benefits of having a plane-based reflecting telescope? What can SOFIA do that other reflecting telescopes can’t?

Most of the radiation in the universe is emitted at infrared wavelengths. Water is very good at absorbing heat—infrared radiation—so observing cosmic infrared radiation from the ground is extremely difficult—nearly impossible. You have to get above the water in the Earth’s atmosphere by either going to space or the stratosphere. Flying on a Boeing 747SP allows SOFIA to reach the stratosphere and come home every night, permitting continuous upgrades and maintenance—something not possible on space telescopes.

What challenges does SOFIA present that are unique to a stratospheric observatory?

SOFIA is the only astronomical telescope that has to accommodate turbulence in-flight. It does so by balancing on an off-axis spherical bearing, and using a suite of gyros and accelerometers to correct for essentially all the turbulent and vibrational motions. The result is the ability to maintain stabilization that could keep a laser pointer on a US quarter at roughly three miles away. Additionally, a moving observatory has to keep updating its coordinate reference frame in real time, something that ground-based observatories don’t have to do.

What role does cryogenics play in the SOFIA observatory?

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), He sorption refrigerators and adiabatic demagnetization refrigerators (ADRs). Our newest instrument, HIRMES1 (High-Resolution Mid Infrared Spectrometer), is currently in development and utilizes two PTC coolers that provide a stable 4 K environment for the optical elements. It includes an He4 sorption cooler with the sole task of cooling a detector baffle and an He3/He4 two-stage sorption refrigerator coupled with an ADR to achieve a stable detector temperature of 70mK for at least 12 hours.

What are the special considerations and challenges of a plane-based cryogenic system?

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.

Who are the key cryogenic partners of SOFIA?

A list of collaborators and partners on various SOFIA instrumentation projects is very long. Precision Cryogenics Systems of Indianapolis successfully worked on and delivered the HIRMES instrument cryostat. This instrument also uses TransMIT PTD406C PTC coolers. It is worth mentioning the unselfish and extremely generous support that we, as a program, have had from TransMIT, a cryogenic company in Giessen, Germany. This includes almost every aspect of operation and maintenance and also assistance with certifying the coolers to be installed on the aircraft—in terms of safety and airworthiness—which is always an added challenge for all OEMs that supply equipment to SOFIA.

The same coolers are used on the upGREAT2 instrument with great success. This trend of using closed-cycle coolers will most likely continue with our future instruments, moving the observatory away from the liquid cryogen dewars. Chase Research Cryogenics, of Sheffield, UK, developed various He sorption coolers on our instruments including the HAWC+3. High Precision Devices, Inc. (CSA CSM) developed the ADR for the HIRMES instrument. The observatory also uses Cryomech (CSA CSM) CP2870 He compressors.

In a follow-up phone interview, Stevanovic told Cold Facts more about current cryogenic developments at SOFIA.

Specifically, the team’s cryogenic operations begin and are “80 to 90% finished” in the lab. The cooling and recycling of the ADR systems take significant time—something in short supply during an observation flight.

“We are trying to optimize the instrument, but we’re also trying to optimize operations of the instrument to actually increase the usefulness of the whole instrument for the in-flight science operations and to maximize the time it can produce useful data for scientists,” he said. “The trick is, ‘How do we keep the instrument cold during the transportation from the lab to the plane?’ We need about 45 minutes to an hour to transfer the instrument and install it on the telescope flange in the aircraft. Then we have to reconnect it to all of the aircraft systems—so we are looking at a planned power loss. We also have to look at some of the cryogens used that must be refilled, the restarting and recycling of the ADR and the preflight checks. We look to achieve optimal temperature on the ground and maintain it for 12 to 16 hours: about eight hours of that time will be used for science.”

“With HIRMES, this is becoming more complicated,” he adds. “HAWC+ is less sensitive to power loss—liquid cryogens mean we only have to deal with boiloff of helium and topping off the systems. With HIRMES, we are looking at loss of power through the cryocoolers, which means that we have much more warming during the transit operation. After it’s installed in the aircraft, we have to restart the cryocoolers, drop the temperature down and start recycling all other coolers and refrigerators inside. While it hasn’t been attempted, we’re looking at the modeling aspects of the thermal system during the assembly process. The instrument is currently going through cooldown testing—we’re planning three tests. We hope to have good results by the end of October.”

“While this is becoming very complicated, we have some of the best people in the world on our team. Especially the people at Goddard Space Center; they have some of the latest technology when it comes to ADRs. If I have any regrets about HIRMES, it’s that Peter Shirron [CSA president] wasn’t involved earlier. He’s helped us a lot and has been a great source of knowledge transfer.”

What is a typical observation flight like on SOFIA?

SOFIA typically performs a 10-hour overnight flight with about eight hours available for science—about two hours are used for takeoff, landing, turns, etc.—with takeoff and landing in the same location, like Palmdale CA. On any given flight, different science targets are observed, resulting in somewhat of a polygon flight pattern, with roughly one to two hours per science target. Onboard there are pilots, safety crew, mission directors, observatory operators, instrument scientists, astronomers and other guests, averaging roughly 15 people per flight.

What was the most valuable observation you’ve made with SOFIA?

Two results come to mind. Primarily, the first high-resolution image of the dust and gas rings around the black hole in the center of the Milky Way that provided groundbreaking insights into the extreme environments that surround black holes. The other result is the first detection in space of helium hydride—the first molecule to form after the Big Bang—made by observing a nearby planetary nebula that provides very similar conditions to that of an early universe. SOFIA is also vastly expanding the study of magnetic fields and their role in astronomical processes, something that has been neglected due to the complexity of observations and theoretical modeling now achievable by SOFIA.

To learn more about SOFIA, visit
www.sofia.usra.edu.

References
[1] Currently in development, HIRMES will cover the 25 – 122 μm wavelength region at high (R = 100,000 – 50,000), medium (R ≈ 12,000) and low (R = 635 – 325) spectral resolution. In addition, there is a Spectral Imaging mode (R ≈ 2,000) that targets specific lines of interest at 51.8 μm, 88.3 μm [OIII], 57.3 μm [NIII] and 121.9 μm [NII]. The instrument achieves these capabilities by utilizing direct-detection Transition Edge Sensor (TES) bolometer arrays, grating-dispersive spectroscopy and Fabry-Perot tunable narrow-band filters. In spectroscopic mode, HIRMES takes full advantage of SOFIA’s diffraction limited performance by using a range of slit widths proportional to the observed wavelength. The field-of-view for the Spectral Imaging mode is ~113.0” x 106.8” covering 16 x 16 pixels. For more information on HIRMES, visit www.sofia.usra.edu/science/instruments/hirmes.

[2] GREAT is a dual channel heterodyne instrument that will provide high resolution spectra (up to R=108) in several frequency windows in the 0.490–4.747 THz range. The front-end unit consists of two independent dewars, each containing a set of mixers. Proposed is the seven-beam array upGREAT in its Low Frequency Array configuration for the [CII] line at 158 μm and High Frequency Array configuration for the [OI] line. Also available for Cycle 8 are all four 4GREAT bands at the following frequency ranges: 490–635 GHz (Herschel/HIFI band 1), 890–1100 GHz (Herschel/HIFI band 4), 1200–1500 GHz (GREAT L1 channel) and 2490–2590 GHz (GREAT M channel). The backend for each mixer is a Fast Fourier Transform spectrometer (XFFTS). Each XFFTS has 2 GHz bandwidth and 64,000 channels providing a resolution of 44 kHz. The beam size of GREAT is diffraction limited (14.1” at 158 μm). More information on GREAT and the upgraded upGREAT can be found at www.sofia.usra.edu/science/instruments/great.

[3] HAWC+ is a far-infrared camera and imaging polarimeter. It is designed to allow total and polarized flux imaging in five broad bands between wavelengths of 50 μm and 240 μm. Diffraction-limited imaging yields spatial resolutions of ~5 – 20 arcseconds with fields of view ~2 – 10 arcminutes, respectively. HAWC+ utilizes three 31×40 pixel arrays: two for the reflective component and one for the transmitted component of linear polarization. The detectors are cooled by an adiabatic demagnetization refrigerator to an operating temperature of about 0.1 – 0.2 K. Commissioning observations started in late 2016, the instrument acceptance review was completed in late 2017, and has been available for Cycle 7 observations beginning in 2018. Information on the HAWC+ can be accessed at www.sofia.usra.edu/science/instruments/hawc. ■