2.1.

Ground-Based

Far-infrared astronomy from the ground faces the fundamental limitation of absorption in Earth’s atmosphere, primarily by water vapor. The atmosphere is mostly opaque in the midinfrared through far-infrared, with only a few narrow wavelength ranges with modest transmission. This behavior is shown in Fig. 1, which compares atmospheric transmission for ground-based observing, observing from Stratospheric Observatory for Infrared Astronomy (SOFIA) (Sec. 2.2), with two higher altitudes that are accessible by balloon-based platforms. The difficulties of observing from the ground at infrared wavelengths are evident. Moreover, the transmissivity and widths of these windows are heavily weather-dependent. Nevertheless, there do exist spectral windows at 34, 350, 450, 650, and $850\mu \mathrm{m}$ with good, albeit weather-dependent transmission at dry, high-altitude sites, with a general improvement toward longer wavelengths. At wavelengths longward of about 1 mm, there are large bands with good transmission. These windows have enabled an extensive program of ground-based far-infrared astronomy, using both single-dish and interferometer facilities.

Fig. 1

Atmospheric transmission over 1 to $1000\mu \mathrm{m}$.⁷¹ The curves for ALMA and SOFIA are computed with a 35-deg telescope zenith angle. The two balloon profiles are computed with a 10-deg telescope zenith angle. The PWV for ALMA (5060 m), SOFIA (12,500 m), and the 19,800- and 29,000-m altitudes are 500, 7.3, 1.1, and $0.2\mu \mathrm{m}$, respectively. The data are smoothed to a resolution of $R=2000$.

2.2.

Stratospheric Observatory for Infrared Astronomy

The SOFIA⁷⁷ is an effective 2.5-m diameter telescope mounted within a Boeing 747SP aircraft that flies at altitudes of 13,700 m to get above over 99.9% of the Earth’s atmospheric water vapor. The successor to the Learjet observatory and NASA’s Kuiper Airborne Observatory (KAO), SOFIA saw first light in May 2010, began prime operations in May 2014, and offers $\sim 600\mathrm{h}$ per year for community science observations.⁷⁸ SOFIA is the only existing public observatory with access to far-infrared wavelengths inaccessible from the ground, though CMB polarization studies at millimeter wavelengths have also been proposed from platforms at similar altitudes to SOFIA.⁷⁹

SOFIA’s instrument suite can be changed after each flight and is evolvable with new or upgraded instruments as capabilities improve. SOFIA is also a versatile platform, allowing for (1) continuous observations of single targets for up to 5 h, (2) repeated observations over multiple flights in a year, and (3) in principle, observations in the visible though millimeter-wavelength range. Example flight paths for SOFIA are shown in Fig. 2. Each flight path optimizes observing conditions (e.g., elevation, percentage of water vapor, and maximal on-target integration time). SOFIA can be positioned to where the science needs, enabling all-sky access. Annually, SOFIA flies from Christchurch, New Zealand, to enable Southern Hemisphere observations.

Fig. 2

(a) The world’s largest flying infrared astronomical observatory, SOFIA (Sec. 2.2). (b) Two flight plans, originating from SOFIA’s prime base in Palmdale, California. In a typical 8- to 10-h flight, SOFIA can observe one to five targets.

SOFIA’s instruments include the 5- to $40\text{-}\mu \mathrm{m}$ camera and grism spectrometer FORCAST,^80,81 the high-resolution (up to $R=\lambda /\mathrm{\Delta }\lambda =\mathrm{100,000}$) 4.5- to $28.3\text{-}\mu \mathrm{m}$ spectrometer EXES,⁸² the 51- to $203\text{-}\mu \mathrm{m}$ integral field spectrometer FIFI-LS,⁸³ the 50- to $203\text{-}\mu \mathrm{m}$ camera and polarimeter HAWC,⁸⁴ and the $R\sim {10}^8$ heterodyne spectrometer GREAT.^85,86 The first-generation HIPO⁸⁷ and FLITECAM⁸⁸ instruments retired in early 2018. The sensitivities of these instruments as a function of wavelength are presented in Fig. 3. Upgrades to instruments over the past few years have led to new science capabilities, such as adding a polarimetry channel to HAWC (HAWC+⁸⁹), and including larger arrays and simultaneous channels on GREAT (upGREAT⁹⁰ & 4GREAT, commissioned in 2017), making it into an efficient mapping instrument. Figure 4 shows early polarimetry measurements from HAWC+.

Fig. 3

The continuum sensitivities, as a function of wavelength, of SOFIA’s mid- to far-infrared instrument suite. Shown are the $4\sigma$ minimum detectable continuum flux densities for point sources in janskys for 900 s of integration time.

Fig. 4

The $89\mu \mathrm{m}$ image (intensity represented by color) with polarization measurements at the same wavelength (black lines), taken using HAWC+ on SOFIA, of $\rho$-ophiucus (courtesy of Fabio Santos, Northwestern University, Illinois). The length of the line is the degree of polarization. The SOFIA beam size is 7.8″, which is indicated by the black circle in the lower right.

Given the versatility and long-term nature of SOFIA, there is a continuous need for more capable instruments throughout SOFIA’s wavelength range. However, the unique niche of SOFIA, given its warm telescope and atmosphere, the imminent era of the James Webb Space Telescope (JWST), and ever more capable ground-based platforms, is high-resolution spectroscopy. This is presently realized with two instruments (GREAT and EXES). An instrument under development, the high-resolution midinfrared spectrometer (HIRMES), scheduled for commissioning in 2019, will enhance SOFIA’s high-resolution spectroscopy capabilities. HIRMES covers 25 to $122\mu \mathrm{m}$, with three spectroscopic modes ($R=600$, $R=\mathrm{10,000}$, and $R=\mathrm{100,000}$), and an imaging spectroscopy mode ($R=2000$).

As SOFIA can renew itself with new instruments, it provides both new scientific opportunities and maturation of technology to enable future far-infrared space missions. SOFIA offers a 20-kVA cryocooler with two compressors capable of servicing two cold heads. The heads can be configured to operate two cryostats or in parallel within one cryostat to increase heat-pumping capacity, with second-stage cooling capacity of $\mathrm{Q}2\ge 800\mathrm{mW}$ at 4.2 K and first-stage cooling capacity of $\mathrm{Q}1\ge 15\mathrm{W}$ at 70 K. Instruments aboard SOFIA can weigh up to 600 kg, excluding the instrument electronics in the counterweight rack and PI Rack(s), and can draw power up to 6.5 kW. Their volume is limited by the aircraft’s door access and must fit within the telescope assembly constraints.

Capabilities that would be invaluable in a next-generation SOFIA instrument include, but are not limited to, the following:

Other possible improvements to the SOFIA instrument suite include: (1) upgrading existing instruments (e.g., replacing the FIFI-LS germanium photoconductors to achieve finer spatial sampling through higher multiplexing factors), and (2) instruments that observe in current gaps in SOFIA wavelength coverage (e.g., 1 to $5\mu \mathrm{m}$, 90 to $150\mu \mathrm{m}$, and 210 to $310\mu \mathrm{m}$).

More general improvements include the ability to swap instruments faster than a 2-day timescale, or the ability to mount multiple instruments. Mounting multiple instruments improves observing efficiency if both instruments can be used on the same source, covering different wavelengths or capabilities. This would also allow for flexibility to respond to targets of opportunity, time domain, or transient phenomena, and increase flexibility as a development platform to raise technology readiness levels (TRLs^91,92) of key technologies.

2.3.

Scientific Ballooning

Balloon-based observatories allow for observations at altitudes of up to $\sim \mathrm{40,000}\mathrm{m}$ (130,000 ft). At these altitudes, $<1\%$ of the atmosphere remains above the instrument, with negligible water vapor. Scientific balloons, thus, give access, relatively cheaply, to infrared discovery space that is inaccessible to any ground-based platform, and in some cases even to SOFIA. For example, several key infrared features are inaccessible even at aircraft altitudes (Fig. 1), including low-energy water lines and the [N II] $122\text{-}\mu \mathrm{m}$ line. Scientific ballooning is, thus, a valuable resource for infrared astronomy. Both standard balloons, with flight times of $\lesssim 24\mathrm{h}$, and long duration balloons (LDBs) with typical flight times of 7 to 15 days (though flights have lasted as long as 55 days) have been used. Balloon projects include the Balloon-borne Large Aperture Submillimeter Telescopes (BLAST^93–95), PILOT,⁹⁶ the Stratospheric Terahertz Observatory⁹⁷ (Fig. 5), and Far-Infrared Interferometric Telescope Experiment (FITE)⁹⁹ and Balloon Experimental Twin Telescope for Infrared Interferometry (BETTII),¹⁰⁰ both described in Sec. 4.1. Approved future missions include GUSTO, scheduled for launch in 2021. With the development of ultra-LDBs (ULDBs), with potential flight times of over 100 days, new possibilities for far-infrared observations have become available.

Fig. 5

(a) The STO balloon observatory and science team, after the successful hang test in the Columbia Scientific Balloon Facility in Palestine, Texas, in August 2015. This image originally appeared on the Netherlands Institute for Space Research (SRON) STO website. (b) The second science flight of the STO took place from McMurdo in Antarctica on December 8, 2016, with a flight time of just under 22 days. This image was taken from Ref. 98.

A further advantage of ballooning, in a conceptually similar manner to SOFIA, is that the payloads are typically recovered and available to refly on $\sim 1$-year timescales, meaning that balloons are a vital platform for technology development and TRL raising. For example, far-infrared direct-detector technology shares many common elements (detection approaches, materials, and readouts) with CMB experiments, which are conducted on the ground,^101–103 from balloons,^104–106 and in space. These platforms have been useful for developing bolometer and readout technology applicable to the far-infrared.

All balloon projects face challenges, as the payload must include the instrument and all of the ancillary equipment needed to obtain scientific data. For ULDBs, however, there are two additional challenges:

Payload mass: Whereas zero-pressure balloons (including LDBs) can lift up to about 2700 kg, ULDBs have a mass limit of about 1800 kg. Designing a payload to this mass limit is nontrivial, as science payloads can have masses in excess of 2500 kg. For example, the total mass of the GUSTO gondola is estimated to be 2700 kg.

Cooling: All far-infrared instruments must operate at cryogenic temperatures. Liquid cryogens have been used for instruments on both standard and LDBs, with additional refrigerators (e.g., ${}^3\mathrm{He}$, adiabatic demagnetization) to bring detector arrays down to the required operating temperatures, which can be as low as 100 mK. These cooling solutions typically operate on timescales commensurate with LDB flights. For the ULDB flights, however, it is not currently possible to achieve the necessary cryogenic hold times. Use of mechanical coolers to provide first-stage cooling would solve this problem, but current technology does not satisfy the needs of balloon missions. Low-cost cryocoolers for use on the ground are available, but have power requirements that are hard to meet on balloons, which currently offer total power of up to about 2.5 kW. Low-power cryocoolers exist for space use, but their cost (typically $\gtrsim \$1\mathrm{M}$) does not fit within typical balloon budgets. Cryocoolers are discussed in detail in Sec. 5.5.

In addition to addressing the challenges described above, there exist several avenues of development that would enhance many balloon experiments. Three examples are as follows:

2.4.

Short Duration Rocket Flights

Sounding rockets inhabit a niche between high-altitude balloons and fully orbital platforms, providing 5 to 10 min of observation time above the Earth’s atmosphere, at altitudes of 50 to $\sim 1500\mathrm{km}$. They have been used for a wide range of astrophysical studies, with a heritage in infrared astronomy stretching back to the 1960s.^107–110

Though an attractive way to access space for short periods, the mechanical constraints of sounding rockets are limiting in terms of size and capability of instruments. However, sounding rockets observing in the infrared are flown regularly,¹¹¹ and rockets are a viable platform for both technology maturation and certain observations in the far-infrared. In particular, measurements of the absolute brightness of the far-infrared sky, intensity mapping, and development of ultra-low-noise far-infrared detector arrays are attractive applications of this platform.

Regular access to sounding rockets is now a reality, with the advent of larger, more capable Black Brant XI vehicles to be launched from southern Australia via the planned Australian NASA deployment in 2019–2020. Similarly, there are plans for recovered flights from Kwajalein Atoll using the recently tested NFORCE water recovery system. Long-duration sounding rockets capable of providing limited access to orbital trajectories and $>30\text{-}\min$ observation times have been studied,¹¹² and NASA is continuing to investigate this possibility. However, no missions using this platform are currently planned, and as a result the associated technology development is moving slowly.

3.1.

Space Infrared Telescope for Cosmology and Astrophysics

First proposed by JAXA scientists in 1998, the SPICA^160–164 garnered worldwide interest due to its sensitivity in the mid- and far-infrared, enabled by the combination of the actively cooled telescope and the sensitive far-infrared detector arrays. Both ESA and JAXA have invested in a concurrent study, and an ESA–JAXA collaboration structure has gelled. ESA will provide the 2.5-m telescope, science instrument assembly, satellite integration and testing, and the spacecraft bus. JAXA will provide the passive and active cooling systems (supporting a telescope cooled to below 8 K), cryogenic payload integration, and launch vehicle. JAXA has indicated commitment to their portion of the collaboration, and the ESA selected SPICA as one of the three candidates for the Cosmic Visions M5 mission. The ESA phase-A study is underway now, and the downselect among the three missions will occur in 2021. Launch is envisioned for 2031. An example concept design of SPICA is shown in Fig. 11.

Fig. 11

A concept image for the proposed SPICA satellite (Sec. 3.1).

SPICA will have three instruments. JAXA’s SPICA MIRI will offer imaging and spectroscopy from 12 to $38\mu \mathrm{m}$. It is designed to complement JWST-MIRI with wide-field mapping (broadband and spectroscopic), $\mathrm{R}\sim \mathrm{30,000}$ spectroscopy with an immersion grating, and an extension to $38\mu \mathrm{m}$ with antimony-doped silicon detector arrays. A polarimeter from a French-led consortium will provide dual-polarization imaging in 2 to 3 bands using high-impedance semiconductor bolometers similar to those developed for Herschel-PACS, but modified for the lower background and to provide differential polarization. A sensitive far-infrared spectrometer, SAFARI, is being provided by an SRON-led consortium.^165,166 It will provide full-band instantaneous coverage over 35 to $230\mu \mathrm{m}$, with a longer wavelength extension under study, using four $R=300$ grating modules. A Fourier transform module, which can be engaged in front of the grating modules, will offer a boost to the resolving power, up to $R=3000$. A U.S. team is working in collaboration with the European team and aims to contribute detector arrays and spectrometer modules to SAFARI¹⁶⁷ through NASA’s Mission of Opportunity.

3.2.

Probe-Class Missions

Recognizing the need for astronomical observatories beyond the scope of Explorer-class missions but with a cadence more rapid than flagship observatories, such as the Hubble Space Telescope (HST), JWST, and the Wide-Field Infrared Survey Telescope, NASA announced a call for Astrophysics Probe concept studies in 2017. Ten probe concepts were selected in Spring 2017 for 18-month studies. Probe study reports will be submitted to NASA and to the Astro 2020 Decadal Survey to advocate for the creation of a probe observatory line, with budgets of $400 million to $1 billion.

Among the probe concepts under development is the far-infrared Galaxy Evolution Probe (GEP), led by the University of Colorado Boulder and the Jet Propulsion Laboratory. The GEP concept is a two-meter-class, mid/far-infrared observatory with both wide-area imaging and follow-up spectroscopy capabilities. The primary aim of the GEP is to understand the roles of star formation and black hole accretion in regulating the growth of stellar and black hole mass. In the first year of the GEP mission, it will detect $\ge {10}^6$ galaxies, including $\gtrsim {10}^5$ galaxies at $z>3$, beyond the peak in redshift of cosmic star formation, by surveying several hundred square degrees of the sky. A unique and defining aspect of the GEP is that it will detect galaxies by bands of rest-frame midinfrared emission from polycyclic aromatic hydrocarbons (PAHs), which are indicators of star formation, while also using the PAH emission bands and silicate absorption bands to measure photometric redshifts.

The GEP will achieve these goals with an imager using $\sim 25$ photometric bands spanning $10\mu \mathrm{m}$ to at least $230\mu \mathrm{m}$, giving a spectral resolution of $R\simeq 8$ (Fig. 12). Traditionally, an imager operating at these wavelengths on a 2-m telescope would be significantly confusion-limited, especially at the longer wavelengths (see e.g., the discussion in the introduction to Sec. 3). However, the combination of many infrared photometric bands, and advanced multi-wavelength source extraction techniques, will allow the GEP to push significantly below typical confusion limits. The GEP team is currently simulating the effects of confusion on their surveys, with results expected in early 2019. The imaging surveys from the GEP will, thus, enable new insights into the roles of redshift, environment, luminosity, and stellar mass in driving obscured star formation and black hole accretion over most of the cosmic history of galaxy assembly.

Fig. 12

A mid/far-infrared galaxy spectrum, the GEP photometric bands, and notional survey depths. The spectrum is a model of a star-forming galaxy¹⁶⁸ exhibiting strong PAH features and far-infrared dust continuum emission. The black spectrum is the galaxy at a redshift of $z=0$, but scaled vertically by a luminosity distance corresponding to $z=0.1$ to reduce the plot range. The same spectrum is shown at redshifts $z=1$, 2, and 3. The vertical dashed lines mark the GEP photometric bands. As the galaxy spectrum is redshifted, the PAH features move through the bands, enabling photometric redshift measurements. This figure does not include the effects of confusion noise.

In the second year of the GEP survey, a grating spectrometer will observe a sample of galaxies from the first-year survey to identify embedded active galactic nucleus (AGN). The current concept for the spectrometer includes four or five diffraction gratings with $R\simeq 250$, and spectral coverage from $23\mu \mathrm{m}$ to at least $190\mu \mathrm{m}$. The spectral coverage is chosen to enable detection of the high-excitation [NeV] $24.2\text{-}\mu \mathrm{m}$ line, which is an AGN indicator, over $0<z<3.3$, and the [OI] $63.2\text{-}\mu \mathrm{m}$ line, which is predominantly a star formation indicator, over $0<z\lesssim 2$.

Recent advances in the far-infrared detector array technology have made an observatory such as the GEP feasible. It is now possible to fabricate large arrays of sensitive kinetic inductance detectors (KIDs; see Sec. 5.1.2) that have a high-frequency multiplex factor. The GEP concept will likely employ Silicon blocked impurity band arrays (similar to those used on JWST-MIRI) for wavelengths from 10 to $24\mu \mathrm{m}$ and KIDs at wavelengths longer than $24\mu \mathrm{m}$. Coupled with a cold ($\sim 4\mathrm{K}$) telescope such that the GEP’s sensitivity would be photon-limited by astrophysical backgrounds (Fig. 7), the GEP will detect the progenitors of Milky Way-type galaxies at $z=2(\ge {10}^{12}{\mathrm{L}}_{\odot })$. Far-infrared KID sensitivities have reached the NEPs required for the GEP imaging to be background-limited ($3\times {10}^{-19}\mathrm{W}/{\mathrm{Hz}}^{-0.5}$^169,170), although they would need to be lowered further, by a factor of at least 3, for the spectrometer to be background-limited. The GEP would serve as a pathfinder for the OST (Sec. 3.3), which would have a greater reach in redshift by virtue of its larger telescope. SOFIA and balloons will also serve as technology demonstrators for the GEP and OST.

The technology drivers for the GEP center on detector array size and readout technology. Whereas KID arrays with ${10}^4-{10}^5\text{pixels}$ are within reach, investment must be made for the development of low-power-consumption readout technology (Sec. 5.1.4). Large KID (or other direct-detection technology) arrays with low-power readouts on SOFIA and balloons would raise their respective TRLs, enabling the GEP and OST.

3.3.

Origins Space Telescope

As part of the preparations for the 2020 Decadal Survey, NASA is supporting four studies of flagship astrophysics missions. One of these studies is for a far-infrared observatory. A Science and Technology Definition Team (STDT) is pursuing this study with support from NASA Goddard Space Flight Center (GSFC). The STDT has settled on a single-dish telescope, and coined the name “Origins Space Telescope.” The OST will trace the history of our origins, starting with the earliest epochs of dust and heavy-element production through to the search for extrasolar biomarkers in the local universe. It will answer textbook-altering questions, such as “How did the universe evolve in response to its changing ingredients?” and “How common are planets that support life?”

Two concepts for the OST are being investigated, based on an Earth-Sun L2 orbit, and a telescope and instrument module actively cooled with 4 K-class cryocoolers. Concept 1 (Fig. 13) has an open architecture, similar to that of JWST. It has a deployable segmented 9-m telescope with five instruments covering the mid-infrared through the submillimeter. Concept 2 is smaller and simpler and resembles the Spitzer Space Telescope architecturally. It has a 5.9-m diameter telescope (with the same light collecting area as JWST) with no deployable components. Concept 2 has four instruments, which span the same wavelength range and have comparable spectroscopic and imaging capabilities as the instruments in concept 1.

Fig. 13

A concept image for the proposed OST (Sec. 3.3). This image shows a design for the more ambitious “concept 1.” The design includes nested sunshields and a boom, in which the instrument suite is located. The color coding of the image gives a qualitative indication of telescope temperature.

Because OST would commence in the middle of the next decade, improvements in far-infrared detector arrays are anticipated, both in per-pixel sensitivity and array format, relative to what is currently mature for spaceflight (Sec. 5.1). Laboratory demonstrations, combined with initial OST instrument studies which consider the system-level readout requirements, suggest that total pixel counts of 100,000 to 200,000 will be possible, with each pixel operating at the photon background limit. This is a huge step forward over the 3200 pixels total on Herschel PACS and SPIRE, and the $\sim 4000\text{pixels}$ anticipated for SPICA.

The OST is studying the impact of confusion on both wide- and deep-survey concepts. Their approach is as follows. First, a model of the far-infrared sky is used to generate a three-dimensional (3-D) hyperspectral data cube. Each slice of the cube is then convolved with the telescope beam, and the resulting cube is used to conduct a search for galaxies with no information given on the input catalogs. Confusion noise is then estimated by comparing the input galaxy catalog to the recovered galaxy catalog. The results from this work are not yet available, but this approach is a significant step forward in robustness compared to prior methods.¹⁴⁴

3.4.

CubeSats

CubeSats are small satellites built in multiples of 1U ($10\mathrm{cm}\times 10\mathrm{cm}\times 10\mathrm{cm}$, $<1.33\mathrm{kg}$). Because they are launched within containers, they are safe secondary payloads, reducing the cost of launch for the payload developer. In addition, a large ecosystem of CubeSat vendors and suppliers is available, which further reduces costs. CubeSats, thus, provide quick, affordable access to space, making them attractive technology pathfinders and risk mitigation missions toward larger observatories. Moreover, according to a 2016 National Academies report,¹⁷¹ CubeSats have demonstrated their ability to perform high-value science, especially via missions to make a specific measurement, and/or that complement a larger project. To date, well over 700 CubeSats have been launched, most of them 3Us.

Within general astrophysics, CubeSats can produce competitive science, although the specific area needs to be chosen carefully.^172,173 For example, long-duration pointed monitoring is a unique niche. So far, the Astrophysics division within NASA’s Science Mission Directorate has funded four CubeSat missions: in $\gamma$-rays (BurstCube¹⁷⁴), in x-rays (HaloSat¹⁷⁵), and in the ultraviolet (SPARCS;¹⁷⁶ CUTE¹⁷⁷).

For the far-infrared, the CubeSat technology requirements are daunting. Most far-infrared detectors require cooling to reduce the thermal background to acceptable levels, to 4 K or even 0.1 K, although CubeSats equipped with Schottky-based instruments that do not require active cooling may be sufficiently sensitive for certain astronomical and Solar System applications (see also e.g., Ref. 178). CubeSat platforms are, thus, constrained by the lack of low-power, high-efficiency cryocoolers. Some applications are possible at 40 K, and small Stirling coolers can provide 1 W of heat lift at this temperature (see also Sec. 5.5). However, this would require the majority of the volume and power budget of even a large CubeSat (which typically have total power budgets of a few tens of watts), leaving little for further cooling stages, electronics, detector systems, and telescope optics.

CubeSats are also limited by the large beam size associated with small optics. A diffraction-limited 10-cm aperture operating at $100\mu \mathrm{m}$ would have a beam size of about 3.5′. There are concepts for larger, deployable apertures,¹⁷⁹ up to $\sim 20\mathrm{cm}$, but none has been launched.

For these reasons, it is not currently feasible to perform competitive far-infrared science with CubeSats. However, CubeSats can serve to train the next generation of space astronomers, as platforms for technology demonstrations that may be useful to far-infrared astronomy, and as complements to larger observing systems. For example, the CubeSat Infrared Atmospheric Sounder (CIRAS) is an Earth Observation 6U mission with a 4.78 to $5.09\mu \mathrm{m}$ imaging spectrograph payload. The detector array will be cooled to 120 K, using a Lockheed Martin Coaxial MPT Cryocooler, which provides a 1-W heat lift (Fig. 14). At longer wavelengths, the Aerospace Corporation’s CUMULOS¹⁸¹ has demonstrated 8 to $15\mu \mathrm{m}$ Earth imaging with an uncooled bolometer from a CubeSat. CubeSats can also serve as support facilities. In the submillimeter range, CalSat uses a CubeSat as a calibration source for CMB polarization observatories.¹⁸²

Fig. 14

The Lockheed Martin Coaxial Micro Pulse Tube Cryocooler, which will provide cooling up to 120 K for the CIRAS, scheduled for launch in 2019.¹⁸⁰ This cooler weighs $<0.4\mathrm{kg}$ and has reached TRL of $\ge 6$.

3.5.

International Space Station

The International Space Station (ISS) is a stable platform for both science and technology development. Access to the ISS is currently provided to the U.S. astronomical community through Mission of Opportunity calls, which occur approximately every two years and have $\sim \$60\mathrm{M}$ cost caps. Several payload sites are available for hosting U.S. instruments, with typically $1{\mathrm{m}}^3$ of volume, at least 0.5 and up to 6 kW of power, wired and wireless Ethernet connectivity, and at least 20 kbps serial data bus downlink capability.¹⁸³

In principle, the ISS is an attractive platform for astrophysics, as it offers a long-term platform at a mean altitude of 400 km, with the possibility for regular instrument servicing. Infrared observatories have been proposed for space station deployment at least as far back as 1990.¹⁸⁴ There are, however, formidable challenges in using the ISS for infrared astronomy. The ISS environment is, for infrared science, significantly unstable, with 16 sunrises in every 24-h period, “glints” from equipment near the FoV, and vibrations and electromagnetic fields from equipment in the ISS. Furthermore, the external instrument platforms are not actively controlled and are subject to various thermal instabilities over an orbit, which would require active astrometric monitoring.

Even with these challenges, there are two paths forward for productive infrared astronomy from the ISS:

Efforts, thus, exist to enable infrared observing from the ISS. For example, the Terahertz Atmospheric/Astrophysics Radiation Detection in Space is a proposed infrared experiment that will observe both in the upper atmosphere of Earth and in the ISM of the Milky Way.

4.1.

Interferometry

Most studies of future far-infrared observatories focus on single-aperture telescopes. There is, however, enormous potential for interferometry in the far-infrared (Fig. 15). Far-infrared interferometry is now routine from the ground (as demonstrated by ALMA, NOEMA, and the SMA) but has been barely explored from space- and balloon-based platforms. However, the combination of access to the infrared without atmospheric absorption and angular resolutions that far exceed those of any single-aperture facility and enables entirely new areas of investigation.^185–187

Fig. 15

The angular resolutions of selected facilities as a function of wavelength. Very high spatial resolutions are achievable at millimeter to radio wavelengths using ground-based interferometers, whereas current and next-generation large-aperture telescopes can achieve high spatial resolutions in the optical and near-infrared. In the mid/far-infrared, however, the best achievable spatial resolutions still lag several orders of magnitude behind those achievable at other wavelengths. Far-infrared interferometry from space will remedy this, providing an increase in spatial resolution shown by the yellow arrow. A version of this figure originally appeared in the FISICA report, courtesy of Thijs de Graauw.

In our Solar System, far-infrared interferometry can directly measure the emission from icy bodies in the Kuiper belt and Oort cloud. Around other stars, far-infrared interferometry can probe planetary disks to map the spatial distribution of water, water ice, gas, and dust, and search for structure caused by planets. At the other end of the scale, far-infrared interferometry can measure the rest-frame near/midinfrared emission from high-redshift galaxies without the information-compromising effects of spatial confusion. This was recognized within NASA’s 2010 long-term roadmap for Astrophysics, Enduring Quests/Daring Visions,¹⁸⁸ which stated that, within the next few decades, scientific goals will begin to outstrip the capabilities of single-aperture telescopes. For example, imaging of exo-Earths, determining the distribution of molecular gas in protoplanetary disks, and directly observing the event horizon of a black hole, all require single-aperture telescopes with diameters of hundreds of meters, over an order of magnitude larger than is currently possible. Conversely, interferometry can provide the angular resolution needed for these goals with much less difficulty.

Far-infrared interferometry is also an invaluable technology development platform. Because certain technologies for interferometry, such as ranging accuracy, are more straightforward for longer wavelengths, far-infrared interferometry can help enable interferometers operating in other parts of the electromagnetic spectrum (interferometer technology has, however, been developed for projects outside the infrared; examples include the Keck Interferometer, CHARA, LISA Pathfinder, the Terrestrial Planet Finder, and several decades of work on radio interferometry). This was also recognized within Enduring Quests/Daring Visions: “the technical requirements for interferometry in the far-infrared are not as demanding as for shorter wavelength bands, so far-infrared interferometry may again be a logical starting point that provides a useful training ground while delivering crucial science.” Far-infrared interferometry, thus, has broad appeal, beyond the far-infrared community, as it holds the potential to catalyze development of space-based interferometry across multiple wavelength ranges.

The 2000 Decadal Survey¹⁸⁹ recommended development of a far-infrared interferometer (FIRI), and the endorsed concept [the submillimeter probe of the evolution of cosmic structure (SPECS)] was subsequently studied as a “vision mission.”¹⁹⁰ Recognizing that SPECS was extremely ambitious, a smaller, structurally connected interferometer was studied as a potential origins probe—the Space Infrared Interferometric Telescope (SPIRIT,¹⁹¹ Fig. 16). At around the same time, several interferometric missions were studied in Europe, including FIRI¹⁹² and the heterodyne interferometer Exploratory Submm Space Radio-Interferometric Telescope.¹⁹³ Another proposed mission, TALC,^194,195 is a hybrid between a single-aperture telescope and an interferometer and, thus, demonstrates technologies for a structurally connected interferometer. There are also concepts using nanosats.¹⁹⁶ Recently, the European community carried out the Far-Infrared Space Interferometer Critical Assessment (FP7-FISICA), resulting in a design concept for the Far-Infrared Interferometric Telescope. Finally, the “double Fourier” technique that would enable simultaneous high spatial and spectral observations over a wide FoV is maturing through laboratory experimentation, simulation, and algorithm development.^197–201

Fig. 16

The SPIRIT structurally connected interferometer concept.¹⁹¹ SPIRIT is a spatio-spectral “double Fourier” interferometer that has been developed to Phase A level. SPIRIT has sub-arcsecond resolution at $100\mu \mathrm{m}$, along with $R\sim 4000$ spectral resolution. The maximum interferometric baseline is 36 m.

Two balloon payloads have been developed to provide scientific and technical demonstration of interferometry. They are the FITE⁹⁹ and the BETTII,¹⁰⁰ first launched in June 2017. The first BETTII launch resulted in a successful engineering flight, demonstrating nearly all of the key systems needed for future science flights. Unfortunately, an anomaly at the end of the flight resulted in complete loss of the payload. A rebuilt BETTII should fly before 2020.

Together, BETTII and FITE will serve as an important development step toward future space-based interferometers, while also providing unique scientific return. Their successors, taking advantage of many of the same technologies as other balloon experiments (e.g., new cryocoolers and lightweight optics), will provide expanded scientific capability while continuing the path toward space-based interferometers.

FIRIs have many of the same technical requirements as their single-aperture cousins. In fact, an interferometer could be used in “single aperture” mode, with instruments similar to those on a single-aperture telescope. However, in interferometric mode, the development requirements for space-based far-infrared interferometry are as follows:

Finally, we comment on the temporal performance requirements. The temporal performance requirements of different parts of an interferometer depend on several factors, including the FoV, sky and telescope backgrounds, rate of baseline change, and desired spectral resolution. We do not discuss these issues in detail here, as they are beyond the scope of a review paper. We do, however, give an illustrative example; a 1′ FoV, with a baseline of 10 m, spectral resolution of $R=100$, and 16 points per fringe results in a readout speed requirement of 35 Hz. However, increasing the spectral resolution to $R=1000$ (at the same scan speed) raises the readout speed requirement to 270 Hz. These correspond to detector time constants of 17 and 3 ms. A baseline requirement for a relatively modest interferometer (e.g., SHARP-IR²⁰²) is, thus, a detector time constant of a few milliseconds. The exact value is, however, tied tightly to the overall mission architecture and operation scheme.

4.2.

Time-Domain and Rapid-Response Astronomy

Time-domain astronomy is an established field at x-ray through optical wavelengths, with notable observations including Swift’s studies of transient high-energy events and the Kepler mission using optical photometry to detect exoplanets. Time-domain astronomy in the far-infrared holds the potential for similarly important studies of phenomena on timescales of days to years, namely, (1) searching for infrared signatures of (dust-obscured) $\gamma$-ray bursts, (2) monitoring the temporal evolution of waves in debris disks to study the earliest stages of planet formation, and (3) monitoring supernovae light curves to study the first formation stages of interstellar dust. To date, however, such capabilities in the far-infrared have been limited. For example, Spitzer was used to measure secondary transits of exoplanets,²⁰³ but only when the ephemeris of the target was known.

The limitations of far-infrared telescopes for time-domain astronomy are twofold. First, to achieve high photometric precision in the time domain, comparable to that provided by Kepler, the spacecraft must be extremely stable, to requirements beyond those typically needed for cameras and spectrographs. This is not a fundamental technological challenge, but the stability requirements must be taken into consideration from the earliest design phase of the observatory. Second, if the intent is to discover transient events in the far-infrared (rather than monitor known ones), then the FoV of the telescope must be wide, as most transient events cannot be predicted and, thus, must be found via observations of a large number of targets.

5.1.

Direct-Detector Arrays

A key technical challenge for essentially any future far-infrared space observatory (whether single aperture or interferometer) is the development of combined direct-detector + multiplexer readout systems. These systems are not typically developed by the same industrial teams that build near-infrared device and midinfrared device. Instead, they are usually developed by dedicated groups at universities or national laboratories. These systems have two core drivers:

There are also the challenges of interference and dynamic range (Sec. 3).

The world leaders in far-infrared detector technology include SRON in the Netherlands, Cambridge and Cardiff in the U.K., and NASA in the USA, with at least three approaches under development. In order of technical readiness they are as follows:

All are potentially viable for future far-infrared missions. We consider each one in turn, along with a short discussion of multiplexing.

5.1.1.

Transition edge sensors

A TES (Fig. 17) consists of a superconducting film operated near its superconducting transition temperature. This means that the functional form of the temperature dependence of resistance, $R(T)$, is very sharp. The sharpness of the $R(T)$ function allows for substantially better sensitivity than semiconducting thermistors (though there are other factors to consider, such as readout schemes; see Sec. 5.1.4). Arrays of TES bolometers have been used in CMB experiments^{206,207–210} and in calorimeters in the $\gamma$-ray,²¹¹ x-ray,^212,213 ultraviolet, and optical. They are also anticipated for future x-ray missions, such as Athena.^214,215

Fig. 17

TES bolometers developed at (a) SRON and (b) JPL, targeting high sensitivity for far-infrared spectroscopy from cold telescopes. These are silicon-nitride suspensions, similar to the Herschel and Planck bolometers, but they feature long ($\sim 1\mathrm{mm}$), narrow ($\sim 0.4\mu \mathrm{m}$) suspension legs and are cooled to below 100 mK. Both programs have demonstrated NEPs of $1\text{-}3\times {10}^{-19}\mathrm{W}{\mathrm{Hz}}^{-1/2}$.²⁰⁵ (c) An example NEP measurement of the JPL system.

In the infrared, TES bolometers are widely used. A notable ground-based example is SCUBA2 on the JCMT²¹⁶ (Table 1). Other sub-orbital examples include HAWC+ and the upcoming HIRMES instrument, both on SOFIA. TES bolometers are also planned for use in the SAFARI instrument for SPICA.^217–220 In terms of sensitivity, groups at SRON and Jet Propulsion Laboratory (JPL) have demonstrated TES sensitivities of $1\times {10}^{-19}\mathrm{W}{\mathrm{Hz}}^{-1/2}$.^219,221,222

The advantages of TES arrays over KIDs and QCD arrays are technological maturity and versatility in readout schemes (see Sec. 5.1.4). However, TES detector arrays do face challenges. The signal in TES bolometers is a current through a (sub-$\mathrm{\Omega }$) resistive film at sub-kelvin temperatures, so conventional amplifiers are not readily impedance matched to conventional low-noise amplifiers (LNAs) with high-input impedance. Instead, superconducting quantum interference devices (SQUIDs) are used as first-stage amplifiers and SQUID-based circuits have been fashioned into a switching time-domain multiplexers (the TDMs, from NIST and UBC²²³), which has led to array formats of up to $\sim {10}^4\text{pixels}$. Although this time-domain multiplexing system is mature and field-tested in demanding scientific settings, it is not an approach that can readily scale above $\sim {10}^4\text{pixels}$, due primarily to wire count considerations. Other issues with TES arrays include (1) challenging array fabrication, (2) relatively complex SQUID-based readout systems, and (3) no on-chip multiplexing (yet).

5.1.2.

Kinetic inductance detectors

The simplest approach to high-multiplex-factor frequency-domain multiplexing (FDM; see also Sec. 5.1.4) thus far is the KID^224,225 (Fig. 18). In a KID, photons incident on a superconducting film break Cooper pairs, which results in an increase in the inductance of the material. When embedded in a resonant circuit, the inductance shift creates a measureable frequency shift, which is encoded as a phase shift of the probe tone. KIDs originated as far-infrared detector arrays, with on-telescope examples, including MAKO²²⁶ and MUSIC²²⁷ at the CSO, A-MKID²²⁸ at APEX, NIKA/NIKA2^229–231 at IRAM, the extremely compact $\mu$-Spec,^232,233 SuperSpec,²³⁴ and the submillimeter wave imaging spectrograph DESHIMA.²³⁵ KIDs were later adapted for the optical/near-infrared,²³⁶ where they provide advances in time resolution and energy sensitivity. Examples include ARCONS,²³⁷ DARKNESS and MEC,^238,239 the KRAKENS IFU,²⁴⁰ and PICTURE-C.²⁴¹ KIDs are also usable for millimeter-wave/CMB studies,^242–246 although there are challenges in finding materials with suitably low $T_c$’s when operating below 100 GHz. KIDs are now being built in large arrays for several ground-based and sub-orbital infrared observatories, including the BLAST-Pol2 balloon experiment.

Fig. 18

Kinetic inductance detectors. The schematic representation in the left is reprinted from Ref. 224. (a) Photons are absorbed in a superconducting film operated below its transition temperature, breaking Cooper pairs to create free electrons; (b) the increase in free electron density increases the inductance (via the kinetic inductance effect) of an RF or microwave resonator, depicted schematically here as a parallel LC circuit which is capacitively coupled to a through line. (c) On resonance, the LC circuit loads the through line, producing a dip in its transmission. The increase in inductance moves the resonance to lower frequency ($f\sim 1/\sqrt{L}$), which produces a phase shift (d) of a RF or microwave probe signal transmitted through the circuit. Because the resonators can have high quality factor, many hundreds to thousands can be accessed on a single transmission line. (e) The 432-pixel KID array in the Caltech/JPL MAKO camera, and (f) shows an image of SGR B2 obtained with MAKO at the CSO.

There exist three primary challenges in using KIDs in space-based infrared observatories:

Sensitivity: Sub-orbital far-infrared observatories have relatively high backgrounds and thus have sensitivities that are 2 to 3 orders of magnitude above those needed for background-limited observations from space. For space-based KID instruments, better sensitivities are needed. The state of the art is from SPACEKIDs, for which NEPs of $3\times {10}^{-19}\mathrm{W}{\mathrm{Hz}}^{-1/2}$ have been demonstrated in aluminum devices coupled via an antenna.^169,247,248 This program has also demonstrated 83% yield in a 961-pixel array cooled to 120 mK. A further important outcome of the SPACEKIDs program was the demonstration that the effects of cosmic ray impacts can be effectively minimized.^169,249 In the U.S., the Caltech/JPL group and the SuperSpec collaboration have demonstrated sensitivities below $1\times {10}^{-18}\mathrm{W}{\mathrm{Hz}}^{-1/2}$ in a small-volume titanium nitride devices at 100 mK, also with radiation coupled via an antenna.

Structural considerations: KIDs must have both small active volume (to increase response to optical power) and a method of absorbing photons directly without using superconducting transmission lines. Options under development include:

• • devices with small-volume meandered absorbers/inductors, potentially formed via electron-beam lithography for small feature widths; and

• • thinned substrate devices, in which the KID is patterned on a very thin (micron or submicron) membrane which may help increase the effective lifetime of the photo-produced quasiparticles, thereby increasing the response of the device.

Antenna coupling at high frequencies: Although straightforward for the submillimeter band, the antenna coupling becomes nontrivial for frequencies above the superconducting cutoff of the antenna material (e.g., $\sim 714\mathrm{GHz}$ for Nb and 1.2 THz for NbTiN). To mitigate this, one possible strategy is to integrate the antenna directly into the KID, using only aluminum for the parts of the detector that interact with the THz signal. This approach has been demonstrated at 1.55 THz, using a thick aluminum ground plane and a thin aluminum central line to limit ground plane losses to 10%.^169,170 This approach does not rely on superconducting stripline technology and could be extended to frequencies up to $\sim 10\mathrm{THz}$.

A final area of research for KIDs, primarily for CMB experiments, is the KID-sensed bolometer, in which the thermal response of the KID is used to sense the temperature of a bolometer island. These devices will be limited by the fundamental phonon transport sensitivity of the bolometer and so are likely to have sensitivity limits comparable to TES bolometers, but may offer advantages, including simplified readout, on-array multiplexing, lower sensitivity to magnetic fields, and larger dynamic range.

5.1.3.

Quantum capacitance detectors

The QCD^250–254 is based on the single Cooper-pair box (SCB), a superconducting device initially developed as a qubit for quantum computing applications. The SCB consists of a small island of superconducting material connected to a ground electrode via a small ($100\mathrm{nm}\times 100\mathrm{nm}$) tunnel junction. The island is biased with respect to ground through a gate capacitor, and because it is sufficiently small to exhibit quantum behavior, its capacitance becomes a strong function of the presence or absence of a single free electron. By embedding this system capacitively in a resonator (similar to that used for a KID), a single electron entering or exiting the island (via tunneling through the junction) produces a detectable frequency shift.

To make use of this single-electron sensitivity, the QCD is formed by replacing the ground electrode with a superconducting photon absorber. As with the KIDs, photons with energy larger than the superconducting gap breaks Cooper pairs, thereby establishing a density of free electrons in the absorber that then tunnel onto (and rapidly back out of) the island through the tunnel junction. The rate of tunneling into the island, and thus the average electron occupation in the island, is determined by the free-electron density in the absorber, set by the photon flux. Because each photo-produced electron tunnels back and forth many times before it recombines, and because these tunneling events can be detected individually, the system has the potential to be limited by the photon statistics with no additional noise.

This has indeed been demonstrated. QCDs have been developed to the point where a 25-pixel array yields a few devices which are photon-noise-limited for $200\text{-}\mu \mathrm{m}$ radiation under a load of ${10}^{-19}\mathrm{W}$, corresponding to a NEP of $2\times {10}^{-20}{\mathrm{WHz}}^{-1/2}$. The system seems to have good efficiency as well, with inferred detection of 86% of the expected photon flux for the test setup. As an additional demonstration, a fast-readout mode has been developed which can identify individual photon arrival events based on the subsequent increase in tunneling activity for a timescale on order of the electron recombination time (Fig. 19).

Fig. 19

The quantum capacitance detector. (a) Schematic representation showing the mesh absorber QCD with its LC resonator coupling it to the readout circuit. The SCB island is formed between the tunnel junction and the gate capacitor. The tunnel junction is connected to the mesh absorber which in turn is connected to the ground plane. The SCB presents a variable capacitance in parallel with an LC resonator. (b) Optical microscope picture of a device, showing the feedline, the inductor, the interdigitated capacitor, all fabricated in Nb and the Al mesh absorber. (c) SEM picture of mesh absorber consisting of 50-nm wide aluminum lines on a $5\mu \mathrm{m}$ pitch grid. (d) Detail of the SCB showing the aluminum island (horizontal line) in close proximity to the lowest finger of the interdigitated capacitor and the tunnel junction (overlap between the island and vertical line connecting to the mesh absorber below). (e) Optical setup schematic representation showing temperature-tunable blackbody, aperture, and filters that define the spectral band. This device has demonstrated an optical NEP of $2\times {10}^{-20}\mathrm{W}{\mathrm{Hz}}^{-1/2}$ at $200\mu \mathrm{m}$, as well as the ability to count individual photons.^254,255

With its demonstrated sensitivity and natural FDM, the QCD is promising for future far-infrared space systems. Optical NEPs of below ${10}^{-20}{\mathrm{WHz}}^{-1/2}$ at $200\mu \mathrm{m}$ have been demonstrated, with the potential for photon counting at far-infrared wavelengths.²⁵⁵ However, QCDs are some way behind both TES and KID arrays in terms of technological maturity. To be viable for infrared instruments, challenges in (1) yield and array-level uniformity, (2) dark currents, and (3) dynamic range must all be overcome. The small tunnel junctions are challenging, but it is hoped that advances in lithography and processing will result in improvements.

5.2.

Medium-Resolution Spectroscopy

A variety of spectrometer architectures can be used to disperse light at far-infrared wavelengths. Architectures that have been successfully used on air-borne and space instruments include grating dispersion such as FIFI-LS on SOFIA²⁶⁰ and PACS on Herschel,¹³³ Fourier transform spectrometers such as the Herschel/SPIRE-FTS,¹³⁴ and Fabry–Pérot etalons such as FIFI on the KAO telescope.²⁶¹ These technologies are well understood and can achieve spectral resolutions of $R={10}^2-{10}^4$. However, future spectrometers will need to couple large FoVs to many thousands of imaging detectors, a task for which all three of these technologies have drawbacks. Grating spectrometers are mechanically simple devices that can achieve $R\sim 1000$ but are challenging to couple to wide FoVs as the spectrum is dispersed along one spatial direction on the detector array. FTS systems require moving parts and suffer from noise penalties associated with the need for spectral scanning. They are also not well suited for studying faint objects because of systematics associated with long-term stability of the interferometer and detectors.²⁶² Fabry–Pérot systems are also mechanically demanding, requiring tight parallelism tolerances of mirror surfaces, and typically have restricted free spectral range due to the difficulty of manufacturing sufficiently precise actuation mechanisms.²⁶³ A new technology that can couple the large FoVs anticipated in next-generation far-infrared telescopes to kilo-pixel or larger detector arrays would be transformative for far-infrared spectroscopy.

A promising approach to this problem is the far-infrared filter bank technology.^264,265 This technology has been developed as a compact solution to the spectral dispersion problem and has potential for use in space. These devices require the radiation to be dispersed to propagate down a transmission line or waveguide. The radiation encounters a series of tuned resonant filters, each of which consists of a section of transmission line of length ${\lambda }_i/2$, where ${\lambda }_i$ is the resonant wavelength of channel $i$. These half-wave resonators are evanescently coupled to the feedline with designable coupling strengths described by the quality factors $Q_{\text{feed}}$ and $Q_{\det }$ for the feedline and detector, respectively. The filter bank is formed by arranging a series of channels monotonically increasing in frequency, with a spacing between channels equal to an odd multiple of ${\lambda }_i/4$. The ultimate spectral resolution $R=\lambda /\mathrm{\Delta }\lambda$ is given as

Two ground-based instruments are being developed that make use of filter banks. A prototype transmission-line system has been fabricated for use in SuperSpec^266,267 for the LMT. SuperSpec will have $R\sim 300$ near 250 GHz and will allow photon-background-limited performance. A similar system is WSPEC, a 90-GHz filter bank spectrometer that uses machined waveguide to propagate the radiation.²⁶⁸ This prototype instrument has five channels covering the 130- to 250-GHz band. Though neither instrument is optimized for space applications, this technology can be adapted to space, and efforts are underway to deploy it on suborbital rockets.

5.3.

High-Resolution Spectroscopy

Several areas of investigation in mid/far-infrared astronomy call for spectral resolution of $R\ge {10}^5$, higher than can be achieved with direct-detection approaches. At this very high spectral resolution, heterodyne spectroscopy is routinely used,^269,270 with achievable spectral resolution of up to $R\simeq {10}^7$. In heterodyne spectroscopy, the signal from the “sky” source is mixed with a spectrally pure, large-amplitude, locally generated signal, called the “local oscillator (LO),” in a nonlinear device. The nonlinearity generates the sum and difference of the sky and LO frequencies. The latter, the “intermediate frequency (IF),” is typically in the 1- to 10-GHz range and can be amplified by LNAs and subsequently sent to a spectrometer, which now is generally implemented as a digital signal processor. A heterodyne receiver is a coherent system, preserving the phase and amplitude of the input signal. Although the phase information is not used for spectroscopy, it is available and can be used in, e.g., interferometry.

The general requirements for LOs are as follows: narrow linewidth, high stability, low noise, tunability over the required frequency range, and sufficient output power to couple effectively to the mixer. For far-infrared applications, LO technologies are usually one of the two following types: multiplier chain and quantum cascade laser (QCL). Multiplier chains offer relatively broad tuning, high spectral purity, and known output frequency. The main limitation is reaching higher frequencies ($>3\mathrm{THz}$). QCLs are attractive at higher frequencies, as their operating frequency range extends to 5 THz and above, opening up the entire far-infrared range for high-resolution spectroscopy.

For mixers, most astronomical applications use one or more of the following three technologies: Schottky diodes, SIS mixers, and hot electron bolometer (HEB) mixers.²⁷¹ Schottky diodes function at temperatures of $>70\mathrm{K}$, can operate at frequencies as high as $\sim 3\mathrm{THz}$ ($100\mu \mathrm{m}$), and provide large IF bandwidths of $>8\mathrm{GHz}$, but offer sensitivities that can be an order of magnitude or more poorer than either SIS or HEB mixers. They also require relatively high LO power, in the order of 1 mW. SIS and HEB mixers, in contrast, have operating temperatures of $\sim 4\mathrm{K}$ and require LO powers of only $\sim 1\mu \mathrm{W}$. SIS mixers are most commonly used at frequencies up to about 1 THz, whereas HEB mixers are used over the 1 to 6 THz range. At present, the SIS mixers offer IF bandwidths and sensitivities both a factor of 2 to 3 better than the HEB mixers. All three mixer types have been used on space-flown hardware: SIS and HEB mixers in the Herschel HIFI instrument,^272,273 and Schottky diodes on instruments in SWAS and Odin.

Heterodyne spectroscopy can currently achieve spectral resolutions of $R\simeq {10}^7$, and in principle the achievable spectral resolution is limited only by the purity of the signal from the LO. Moreover, heterodyne spectroscopy preserves the phase of the sky signal as well as its frequency, lending itself naturally to interferometric applications. Heterodyne arrays are used on SOFIA, as well as many ground-based platforms. They are also planned for use in several upcoming observatories, including GUSTO. A further example is FIRSPEX, a concept study for a small-aperture telescope with heterodyne instruments to perform several large-area surveys targeting bright far-infrared fine-structure lines, using a scanning strategy similar to that used by Planck.²⁷⁴

There are, however, challenges for the heterodyne approach. We highlight five here:

On a final note, for the higher frequency ($>3\mathrm{THz}$) arrays, high-power (5 to 10 mW) QCL LOs are a priority for development, along with power division schemes (e.g., Fourier phase gratings) to utilize QCLs effectively.^277–279 At $<3\mathrm{THz}$, frequency-multiplied sources remain the system of choice and have been successfully used in missions including SWAS, Herschel-HIFI, STO2, and in GREAT and upGREAT on SOFIA. However, to support large-format heterodyne arrays, and to allow operation with reduced total power consumption for space missions, further development of this technology is necessary. Further valuable developments include SIS and HEB mixers that can operate at temperatures of $>20\mathrm{K}$ and integrated focal planes of mixers and low-noise IF amplifiers.

5.4.

Fabry–Pérot Interferometry

Fabry–Pérot Interferometers (FPIs) have been used for astronomical spectroscopy for decades, with examples, such as FIFI,²⁸⁰ KWIC,²⁸¹ ISO-SWS/LWS,^282,283 and SPIFI.²⁸⁴ FPIs similar to the one used in ISO have also been developed for balloon-borne telescopes.²⁸⁵

FPIs consist of two parallel, highly reflective (typically with reflectivities of $\sim 96\%$), very flat mirror surfaces. These two mirrors create a resonator cavity. Any radiation whose wavelength is an integral multiple of twice the mirror separation meets the condition for constructive interference and passes the FPI with high transmission. As the radiation bounces many times between the mirrors before passing, FPIs can be fabricated very compactly, even for high spectral resolution, making them attractive for many applications. In addition, FPIs allow for large FoVs, making them an excellent choice as devices for spectroscopic survey instruments.

Observations with FPI are most suitable for extended objects and surveys of large fields, where moderate-to-high spectral resolution ($R\sim {10}^2-{10}^5$) is required. For example:

FPIs do, however, face challenges. We highlight four examples here:

5.5.

Small and Low-Power Coolers

For any spaceborne observatory operating at mid/far-infrared wavelengths, achieving high sensitivity requires that the telescope, instrument, and detectors be cooled, with the level of cooling dependent on the detector technology, the observation wavelength, and the goals of the observations. Cooling technology is thus fundamentally enabling for all aspects of mid/far-infrared astronomy.

The cooling required for the telescope depends on the wavelengths being observed (Fig. 7). For some situations, cooling the telescope to 30 to 40 K is sufficient. At these temperatures, it is feasible to use radiative (passive) cooling solutions if the telescope is space-based and if the spacecraft orbit and attitude allow for a continuous view of deep space.²⁸⁷ Radiative coolers typically resemble a set of thermal/solar shields in front of a black radiator to deep space (Fig. 6). This is a mature technology, having been used on Spitzer, Planck, and JWST (for an earlier proposed example, see Ref. 288).

For many applications, however, cooling the telescope to a few tens of kelvins is suboptimal. Instead, cooling to an order of 4 K is required, e.g., zodiacal background limited observations (see also Sec. 3). Moreover, detector arrays require cooling at least to this level. For example, SIS and HEB mixers need cooling up to 4 K, whereas TES, KID, and QCD arrays need cooling to 0.1 K or below. Achieving cooling at these temperatures requires a cooling chain—a staged series of cooling technologies selected to maximize the cooling per-mass and per-input power.

To achieve temperatures below $\sim 40\mathrm{K}$, or where a continuous view of deep space is not available, cryocoolers are necessary. In this context, the Advanced Cryocooler Technology Development Program (ACTDP²⁸⁹), initiated in 2001, has made excellent progress in developing cryogen-free multiyear cooling for low-noise detector arrays at temperatures of 6 K and below (Fig. 20). The state of the art for these coolers include those onboard Planck, JWST, and Hitomi.²⁹⁰ Similar coolers that could achieve 4 K are at TRL 4-5, having been demonstrated as a system in a laboratory environment²⁹¹ or as a variant of a cooler that has a high TRL (JWST/MIRI). Mechanical cryocoolers for higher temperatures have already demonstrated impressive on-orbit reliability (Table 2). The moving components of a 4 K cooler are similar (expanders) or the same (compressors) as those that have flown. Further development of these coolers to maximize cooling per input power for small cooling loads ($<100\mathrm{mW}$ at 4 K) and lower mass is however needed. There is also a need to minimize the vibration from the cooler system. The miniature reverse-Brayton cryocoolers under development by Creare are examples of reliable coolers with negligible exported vibration. These coolers are at TRL 6 for 80 K and TRL 4 for 10 K operation.

Fig. 20

Three cryocoolers for 6 K cooling developed through the ACTDP.

Table 2

Long-life space cryocooler operating experiences as of May 2016.

For cooling to below 0.1 K, adiabatic demagnetization refrigerators (ADRs) are currently the only proven technology, although work has been funded by ESA to develop a continuously recirculating dilution refrigerator [continuous adiabatic demagnetization refrigerator (CADR)]. A single-shot DR was flown on Planck producing $0.1\mu \mathrm{W}$ of cooling at 100 mK for about 1.5 years, whereas a three-stage ADR was used on Hitomi producing $0.4\mu \mathrm{W}$ of cooling at 50 mK with an indefinite lifetime. In contrast, a TRL 4 CADR has demonstrated $6\mu \mathrm{W}$ of cooling at 50 mK with no life-limiting parts²⁹² (Fig. 21). This technology is being advanced toward TRL 6 by 2020 via funding from the NASA SAT/TPCOS program.²⁹³ Demonstration of a 10-K upper stage for this machine, as is planned, would enable coupling to a higher temperature cryocooler, such as that of Creare, that has near-zero vibration. The flight control electronics for this ADR are based on the flight-proven Hitomi ADR control and has already achieved TRL 6. ADR coolers are the current reference design for the Athena x-ray observatory. For the OST, all three of the above technologies are required to maintain the telescope at near 4 K and the detector arrays at near 50 mK.

Fig. 21

The CADR under development at NASA GSFC. This will provide $6\mu \mathrm{W}$ of cooling at 50 mK. It also has a precooling stage that can be operated from 0.3 to 1.5 K. The picture also shows a notional enclosing magnetic shield for a $<1\mu \mathrm{T}$ fringing field.

Continuous development of 0.1 and 4 k coolers with cooling powers of tens of milliwatt, high reliability, and lifetimes of 10+ years is of great importance for future far-infrared observatories. Moreover, the development of smaller, lighter, vibration-resistant, power-efficient cryocoolers enables expansion of infrared astronomy to new observing platforms. An extremely challenging goal would be the development of a 0.1 K cooler with power, space, and vibration envelopes that enable its use inside a 6U CubeSat, while leaving adequate resources for detector arrays, optics, and downlink systems (see also Sec. 3.4). More generally, the ubiquity of cooling in infrared astronomy means that the development of low-mass, low-power, and low-cost coolers will reduce mission costs and development time across all observational domains.

5.6.

High Surface Accuracy Lightweight Mirrors

As far-infrared observing platforms mature and develop, there emerge new opportunities to use large aperture mirrors for which the only limitations are (1) mirror mass and (2) approaches to active control and correction of the mirror surface. This raises the possibility of a high-altitude, long-duration far-infrared-observing platform with a mirror factors of 2 to 5 larger than on facilities such as SOFIA or Herschel.

The key enabling technology for such an observing platform is the manufacturing of lightweight, high surface accuracy mirrors, and their integration into observing platforms. This is especially relevant for ULDBs, which are well suited for this activity. Lightweight mirrors with apertures of 3 m to several tens of meters are ideal for observations from balloon-borne platforms. Carbon-fiber mirrors are an attractive option; they have low mass and can offer high sensitivity in the far-infrared, at low cost of manufacture. Apertures of 2.5 m are used on projects, such as BLAST-TNG.⁹⁵ Apertures of up to $\sim 10\text{-}\mathrm{m}$ are undergoing ground-based tests, including the phase 2 NIAC study for the large balloon reflector.^294–296

A conceptually related topic is the physical size and mass of optical components. The physical scale of high-resolution spectrometers in the far-infrared is determined by the optical path difference required for the resolution. For resolutions of $R\gtrsim {10}^5$, this implies scales of several meters for a grating spectrometer. This scale can be reduced by folding, but mass remains a potentially limiting problem. Moreover, larger physical sizes are needed for optical components to accommodate future large format arrays, posing challenges for uniformity, thermal control, and antireflection coatings. The development of low-mass optical elements suitable for diffraction-limited operation at $\lambda \ge 25\mu \mathrm{m}$ would open the range of technical solutions available for the highest performance instruments.

5.7.

Other Needs

There exist several further areas for which technology development would be beneficial. We briefly summarize them below:

Lower-loss THz optics: Lenses, polarizers, filters, and duplexers.

Digital backends: Low-power (of order a few watts or less) digital backends with $>1000$ channels covering up to several tens of gigahertz of bandwidth.

Wide-field imaging Fourier transform spectrometers: Expanding on the capabilities of, e.g., SPIRE on Herschel, balloon- or space-based imaging Fourier transform spectrometer with FoVs of tens of square arcminutes.²⁹⁷ Examples include the concept H2EX.²⁹⁸

Deployable optics: Development of deployable optic schemes across a range of aperture sizes would be enabling for a range of platforms. Examples range from 20- to 50-cm systems for CubeSats to 5- to 10-m systems for JWST.

Data downlinking and archiving: The advent of infrared observatories with large-format detector arrays presents challenges in downlinking and archiving. Infrared observatories have, to date, not unduly stressed downlinking systems, but this could change in the future with multiple instruments each with ${10}^4$ to ${10}^5\text{pixels}$ on a single observatory. Moreover, the increasing number and diversity of PI and facility-class infrared observatories poses challenges to data archiving, in particular for enabling investigators to efficiently use data from multiple observatories in a single study. One way to mitigate this challenge is by increasing the use of onboard data processing and compression, as is already done for missions operating at shorter wavelengths.

Commonality and community in instrument software: Many tasks are similar across a single platform, and even between platforms (e.g., pointing algorithms, focus, and data download). Continuous adherence to software development best practices, code sharing via repositories via GitHub, and fully open-sourcing software, will continue to drive down associated operating costs, speed up development, and facilitate ease of access.