2.1.

Instrument Overview

Figures 1 and 2 show the HERO receiver. HERO covers a frequency range of 486 to 2700 GHz in only four frequency bands, in two linear polarizations. A band selection mirror in the Offner Relay directs the sky signal to two of the $3\times 3$ mixer FPA, one for each polarization. The LO signals are generated in the spacecraft bus and are sent to the mixers. Low noise amplifiers (LNA) operating at 4.5 K and 35 K amplify the IF signal. In the warm spacecraft bus, the IF signal is amplified further and digitized, and the backends carry out spectral analysis via fast Fourier transforms (FFT). The LO, instrument, and focal plane control units control all aspects of the operation of the receiver.

Fig. 2

HERO is the first heterodyne array receiver designed for space. The schematic diagram shows a relatively simple design that provides the widest frequency coverage and the highest sensitivity of any heterodyne receiver. HERO uses sensitive state-of-the-art components with low power consumption and weight.

2.2.

Optics

2.2.1.

Design of optics

Figure 3 shows the optical ray trace of HERO and Fig. 4 details of the Offner Relay, the cold receiver optics with the cold LO optics and the warm LO optics. The Origins Space Telescope has a 5.9-m on-axis primary mirror. A secondary mirror (not visible) reflects the light, to a tertiary mirror, and finally to a small field steering mirror. Around the focal plane, each of the four instruments picks up the radiation from their corresponding field-of-view. The HERO pick-off mirror is slightly in front of the Origins field of view. It directs the light into an Offner relay (1 in Figs. 3 and 4), which has a movable mirror that can send the light to one of the four frequency bands. The cold receiver optics (2a) divides the sky signal into its two linear polarizations and reimages each onto a $3\times 3$ focal plane mixer array.

Fig. 3

Ray tracing of HERO optics. The signal from the sky is collected by the Origins antenna and redirected to the HERO instrument. The Offner relay mirror assures the band selection and sends the sky singal to one of the four frequency bands, where it is refocussed on two $3\times 3$ FPAs, one for each polarization. The LO signal comes from the spacecraft bus at the botton of the picture. A set of four LO beams are superimposed onto one optics paths and send to the cold instrument bay, where the LOs are refocused, separated and sent to the FPAs.

Fig. 4

HERO optics. Details of the Offner relay, the cold receiver optics with its cold LO optics and the warm LO optics.

There are eight LO chains 2 for each of the four frequency bands. To minimize coupling of the thermal radiation from the warm spacecraft bus to the cold Cryo-payload module (CPM), the eight LOs are combined into two beams in the warm LO optics (3): dichroics superimpose bands 1 on band 3 and band 2 on band 4, and polarization grids superimpose the vertical polarization of band 1 plus 3 with the horizontal polarization of band 1 plus 3 and the same for bands 2 and 4. (A design with four dichroics would be unable to provide continuous frequency coverage or have unacceptable losses.) The combined beams transfer the LO signals to the CPM, where the cold LO optics (2b) separates the LO beams into the four frequency bands and the two polarizations. A wire grid superimposes each LO signal onto a sky signal.

The LO and sky signals have orthogonal polarizations with respect to each other. Two mirrors and a lenslet array reimage the sky and the LO signals onto the focal plane mixer array. A set of $3\times 3$ mixer feed horns captures the superimposed sky and the LO signal. A waveguide orthomode-transducer (OMT) at 45 degrees to both input signals divides the signals in sky plus LO and sky minus LO. These two signals are mixed and their outputs recombined. This method of superposition requires much less LO power than beam splitters, has no limitation on the IF bandwidth unlike Martin–Puplett interferometers, but there are some losses in the OMT.²² The HERO mixers consist of superconducting tunnel junction [superconductor-isolator-superconductor (SIS)] mixers or hot-electron bolometer (HEB) mixing junctions, depending on the target frequency band. A paper giving details of the HERO optics by Jellema is in preparation.

2.3.

Local Oscillators

Eight LO chains consisting of a combination of amplifier and multiplier stages multiply the frequency of a low noise W-band synthesizer signal to the required target LO frequency.^27,28 The LO signal is divided by waveguide power divider into nine outputs to match the mixer arrays, similar to the 16 beam example shown in Fig. 5.

Fig. 5

Compact multi-pixel electronically tunable LO systems have been demonstrated to 1.9 THz. The system shown here multiplies a $\sim 9\mathrm{GHz}$ signal to $\sim 70\mathrm{GHz}$, amplifies it, then splits it into 16 signals and then further multiplies each to $\sim 1.9\mathrm{THz}$. The output is a 16 beam LO signal that can directly pump 16 mixers in a FPA. HERO has a similar chain with 9 outputs.

The HERO LOs have bandwidth of nearly 50% each to reduce the number of LOs and hence weight and volume in the spacecraft. Broadband amplifiers and multipliers are difficult to design, but multipliers for submillimeter LOs with 40% bandwidth have been already demonstrated, e.g., Virginia Diodes (VDI).²⁹ HERO requires extremely efficient multipliers as we have only allocated 2 W of DC power to each amplifier chain. The design of amplifier multiplier chains up to 2.7 THz with sufficient power to pump an array of mixers is difficult, but the prototype chains already exist for single-pixel applications, e.g., Mehdi et al.²⁷ for an overview, VDI²⁸ with $5\mu \mathrm{W}$ at 2.7 THz. An alternative approach for the LO source is the Quantum Cascade Laser (QCL).^30–33 These components have sufficient output power, but require cooling and generally have very limited tunability. Hübers³⁴, and Hübers et al.³⁵ have packaged a QCL with 0.6 mW output power in a compact cooler requiring 85 W input power. For continuous tuning, a multimode QCL with a frequency selecting Fabry–Pérot interferometer has been developed.³⁶

2.4.

Mixer

HERO uses the most sensitive mixers available. SIS mixers^37–39 are utilized in the FPAs at the two lower frequency bands 486 to 756 GHz, and 756 to 1188 GHz. HERO’s SIS mixers will have noise temperatures of $2hf/k$ ($h=\text{Planck constant}$, $f=\text{frequency}$, and k = Boltzmann constant), which has been already achieved for HIFI/Herschel¹⁵ for band 1. HEB^40–43 mixers make up the FPAs for the bands 1188 to 1782 GHz and 1782 to 2700 GHz. The HEBs will have noise temperatures below $4hf/\mathrm{k}$. Currently, the upGREAT¹⁴ Instrument on SOFIA demonstrated the best noise temperatures of 700K double sideband (DSB) at 1.9 THz, and HEBs are continuously improving, with a 780 K receiver noise temperature (all components included) measured at 2.7 THz.⁴⁴ An overview of mixer noise temperatures is given in Fig. 6.

Fig. 6

DSB receiver noise temperature of different telescopes and the HERO goal. Below 1 THz the SIS mixers have nearly reached the requirement of $2hf/k$. HERO goal of $2hf/k$ up to 1188GHz and $\text{4\hspace{0.17em}\hspace{0.17em}}hf/k$ at higher frequencies. HEB mixers development is still required.

HERO has eight FPA (four frequency bands with two polarizations each). Each FPA consists of $3\times 3$ mixers in a square configuration. The SIS mixers that require a magnetic field have a footprint of $10\times 10{\mathrm{mm}}^2$, and the HEBs $5\times 5{\mathrm{mm}}^2$. Projected onto the sky, the arrays have 2 × full width half maximum beam spacing, which is the closest spacing that can be achieved without considerable cross-talk between pixels and low antenna efficiency. A similar FPA with $4\times 4\text{pixels}$ is shown in Fig. 7.

Fig. 7

A 16-element 1.9 THz mixer array with smooth-walled spline feed horns in a square grid configuration. The block, including circular-to-rectangular waveguide transitions and pockets to hold the hot-electron-bolometer mixers, was directly machined from copper.⁴⁵

Eight mixers of each array are DSB balanced mixers. The balanced mixer design is adopted to reduce the LO power required and to enhance the stability by suppressing LO amplitude modulation noise.⁴² One mixer in each FPA is a sideband separating (2SB) mixer to allow sideband calibration of the DSB mixers. Each junction can be individually DC biased for optimal operation.

The baseline IF range is 0.5 to 6.5 GHz (goal of 0.5 to 8.5 GHz) so that the IF covers at least $670\mathrm{km}/\mathrm{s}$ at the highest frequency, sufficient for observations of the inner Milky Way and of nearby galaxies. This IF bandwidth is typically not a problem for SIS mixers but is challenging for HEBs. A priori, NbN HEBs will be used, because NbTiN HEB mixers have narrower IF bandwidth (3 GHz), NbN HEBs need more LO power but have wider IF bandwidth. Recently, NbN HEB mixers on a GaN buffer-layer operating at $\sim 1.3\mathrm{THz}$ with 7.5 GHz IF bandwidth have been demonstrated,⁴⁶ and efforts to increase the IF bandwidth are ongoing. An alternative are ${\mathrm{MgB}}_2$ mixers⁴⁷ that have IF bandwidth of more than 8 GHz, however, they do not have the same sensitivity, yet (1400 K at 1600 THz), but progress is being made. The common Origins closed-cycle cryocoolers cool the mixers to about 4.5 K.

2.5.

Intermediate Frequency and Backend

The IF signal at the output of the mixer is immediately amplified by $\sim 20\mathrm{dB}$ by an LNA at 4.5 K physical temperature, then again by $\sim 20\mathrm{dB}$ at a physical temperature of 35 K, and finally in the warm spacecraft bus. The first stage low-noise ($<6\mathrm{K}$), low-power SiGe cryogenic amplifiers^48–50 [Fig. 8(a)] are directly behind the mixer arrays and connected to the mixers by a short transmission line on a flexible Mylar bus.^52,53 Low power dissipation of the LNAs is essential to prevent the mixers from heating up and to minimize the cooling requirements. Bardin et al.⁴⁸ have recently demonstrated a state of the art SiGe amplifier integrated with a SIS mixer having an instantaneous bandwidth of 4 GHz and with power dissipation as low as 0.3 mW. An alternative are the mature InP High-Electron-Mobility Transistor (HEMT) amplifiers,⁵⁴ which have already been used on Herschel. Normally, InP amplifiers would require around 5-mW power per pixel, but tests operating at only 0.5 mW have shown only slightly higher noise temperatures.⁵⁵ If InP amplifiers operating with less than the nominal power are stable, they could serve as a backup alternative to SiGe.

Fig. 8

(a) Low power SiGe cryogenic amplifiers.⁵⁰ (b) and (c) Extremely compact CMOS prototype amplifiers.⁵¹

Passive power combiners multiplex the IFs of different bands, such that the $2\times 9\text{pixels}$ of any two frequency bands can be operated simultaneously. The multiplexing minimizes IF cables and back-ends, thus economizing on weight and thermal conduction. Coaxial cables carry 40 IF channels to the 35 K LNAs for further amplification, and then to the spacecraft bus. Miniaturized integrated IF circuits⁵¹ [Figs. 8(b) and 8(c)] further amplify, filter, and condition the signal for the backends.

HERO requires 40 low-power ($\sim 1\mathrm{W}$) spectrometers, each covering a bandwidth of 6 to 8 GHz. Complementary Metal Oxide Semiconductor (CMOS) Application-Specific Integrated Circuit (ASIC) backends as developed by Tang et al.^56,57 and others have very low power consumption, thousands of spectral channels and are rapidly advancing in bandwidth.^58,59 The spectrometers contain analog-to-digital converters, FFT processors, and data accumulators. They digitize to 3- or 4-bit resolution, sufficient for our application. Alternatively, HERO could use backends based on field-programmable gate array (FPGA) technology.^60,61 They show excellent performance, but their power consumption has to be lowered from currently around 70 W to about 2 W per spectrometer to be usable in HERO.

To increase the frequency resolution and minimize spectral leakage, HERO will use window functions in the spectrometer such as Blackman–Harris windows that reduce the side lobes to below $-61\mathrm{dB}$. The HERO spectrometer will take full advantage of prior experiments and knowledge, described e.g., by Klein et al.⁶²

2.6.

Control Electronics

There are three different control units, one for each subsystem to make testing before integration easier. The LO control unit monitors and controls the synthesizer and the LO Unit with the amplifier multiplier chains. The focal plane control unit monitors and controls all the components mounted at 4.5 K, in particular, the mixer bias, the amplifier bias, and the Offner relay mirror that is used to select between the different frequency bands. It also monitors and regulates the temperature of the calibration loads. The instrument control unit (ICU) monitors and controls the IF and the spectrometers. This unit also reads the data from the spectrometers and compresses it before passing it to the Origins bus computer for downlinking. The ICU is the interface between the Origins bus computer and the other control units of the HERO instrument.

The control units use state of the art onboard processors such as the GR740 rad-hard quad-core LEON4FT. They use spacewire as an interface between the units and MIL1553STDB as an interface to the spacecraft. All control units are mounted on the spacecraft bus in pairs, one operating unit and one redundant spare that is normally not operating.

3.1.

Thermal Design

The thermal design contains on one hand requirements for the instrument to operate correctly and on the other hand the requirement that the instrument’s heat load on the satellite should be minimized. The HERO front-end is located in the CPM at 4.5 K, the LOs, back-end, and the control units in the warm ($\sim 300\mathrm{K}$) spacecraft bus. The CPM is cooled by four cryocoolers, which can each lift 50 mW of heat, i.e., there is only 200 mW cooling power available in the CPM for all instruments, mechanical structures, and thermal radiation.

The only components that require cryogenic temperatures for operation are the superconducting mixers. They only work below $\sim 6\mathrm{K}$ and have slightly better performance at lower temperatures. The mixer mounts have brass straps to two of the cold fingers of the ISM and hence are cooled to $\sim 4.5\mathrm{K}$. A Mylar cable bus separates the mixers from the LNAs, which have significant heat dissipation of around $0.5\mathrm{mW}/\text{amplifier}$. The second components that need to be cooled are the LNAs, which add less noise to the signal when cold. There is no strict temperature limit, so we have compromised between instrument performance and cooling demands on the satellite. The outcome is that the first stage LNAs with 20 dB gain are on the CPM at slightly above 4.5 K, the second stage with another 20 dB gain at 35 K, and the third stage with 60 dB gain in the warm spacecraft bus. The emissivity of reflective optics elements at submillimeter wavelengths is small (5%, see Table 2), and since the noise temperatures are 50 to 300 K, the temperature of the optics has only a small impact on the system noise temperatures.

The second design criterion is to keep the cooling demands of the satellite low. On the 4.5 K stage, the LNAs are the most important heat source. We have selected SiGe amplifiers,⁴⁸ which can operate at around 0.3 mW for 20 dB gain, as opposed to InP LNAs⁵³ as used in HIFI/Herschel, which had 5 mW power dissipation. All amplifiers not in use are turned off.

The passive heat load on the 4.5 K stage comes from three major sources: the warm calibration load, the two holes through which the LO beams pass, and the cable harness. We carefully designed a thermally suspended calibration load with a shutter that can be closed when HERO is not in operation. We minimized the IR radiation through the LO beam by adding five heat sunk bandpass filters that block all IR radiation except that in the observing range of HERO. The filters reduce the heat load to 2.4 mW per beam, i.e., 4.8 mW in total. In addition, the LO beams have a shutter that is closed when HERO is not in operation to minimize stray light to the other instruments. In the cable harness, the coaxial IF cables conduct the most heat. We have designed a passive multiplexing system on the 4.5 K stage so that only 40 IF signals for the $2\times 2\times (9+1)$ beams are carried back to the spacecraft bus ($+1$ because one mixer in each FPA is sideband separating and has a second IF signal). The cable harness is heat sunk on the 35 K stage.

The thermal design on the spacecraft bus is less critical, because the operating temperature range is larger between $\sim 200\mathrm{K}$ and 320 K, and heat can be removed by radiators. Nevertheless, we strive to use components with low power consumption, in particular spectrometers requiring only 2 W of power each, and efficient LO frequency multiplier chains. Alltogether HERO requires 205 W of power.

3.2.

Mechanical Design

Figure 9 shows a computer-aided design (CAD) drawing of HERO with the mechanical components in gray, brown, and green, and the ray trace in blue. The Offner relay and the cold receiver optics including the cold LO optics fit in an envelope of $1.05\times 1.47\times 0.4{\mathrm{m}}^3$. The warm LO optics requires $1.37\times 0.8\times 0.21{\mathrm{m}}^3$. Given the large space of the CPM under the 5.9 m primary mirror HERO fits comfortably between the other instruments as can be seen in Fig. 10.

Fig. 9

CAD drawing of HERO with ray trace in blue. The Offner relay, the cold receiver optics, and the LO separation optics are at 4.5 K and are in the CPM below the primary mirror in the upper part of the figure on the left and in the enlarged drawing on the right. To minimize the cryogenic heat load, the LO signals are generated in the spacecraft bus. The warm LO optics (lower part of figure) superimpose the 8 LO signals onto two beams and transfer them from the warm spacecraft bus the cold CPM.

Fig. 10

HERO fits comfortably into the spacecraft between the other three instruments, MISC, FIP, and OSS.

Despite using lightweight mechanical structures, HERO still weighs $\sim 180\mathrm{kg}$. The mirrors for the LO relay and their support structure are the biggest contributors weighing 40 kg.

3.3.

Mechanisms

HERO has only four mechanisms (Table 3). The Offner Relay^63–66 directs the sky beam to the respective frequency array with the help of a band selection mirror (see Fig. 4, insert 1) that has two axis of rotation. The tip-tilt mirror (Fig. 4, insert 3) on the warm side of the LO relay compensates for any mechanical displacement between the warm spacecraft bus and the focal plane. Finally, the calibration load shutter and the LO door [location of LO door see Fig. 3, for the design see light tight shutter design for SPace Infrared telescope for Cosmology and Astrophysics (SPICA)⁶⁷] can be closed when HERO is not in use to minimize heat transfer and to ensure a dark environment for the other instruments. All mechanisms are based on prior designs and are hence relatively low-risk.

Table 3

HERO uses two proven mechanisms and two shutters.

3.4.

HERO Tests, Integration, Alignment, and Calibration

The Herschel/HIFI testing, integration, alignment, and calibration procedures (see PhD thesis by Jellema⁶⁸) will be used for HERO. The HERO instrument team tests, calibrates, and internally aligns the instrument in existing cryostats prior to delivery. Origins is designed such that each instrument can be removed separately for maintenance and upgrades. Consequently, the National Aeronautics and Space Administration (NASA) team can integrate the instruments in any order.

The usual procedures are used to calibrate HERO in flight: HERO has two internal calibration loads to calibrate the receiver response, one of the mixers in each array is sideband separating to allow a calibration of the sideband ratio of the DSB mixers, and calibration on astronomical continuum sources (such as planets) are used to obtain a bandpass and allow the determination of the optical coupling/ aperture efficiency. Observations of point sources are used to determine the main beam efficiency.

HERO has two calibration loads, a hot load at a few tens of K and a cold load at 4.5 K. The loads are made of stycast or a similar material that looks like a black body at THz frequencies. The loads are cone shaped to have many internal reflections and to avoid any backscattering into the receiver. To minimize any heating of the CPM, the loads have infrared filters. The filters are mounted at an angle to avoid reflection and standing waves. The loads also have a shutter that will be closed when HERO is not in operation.

3.5.

Resource Requirements

Table 4 summarizes the resources required by HERO.

Table 4

The HERO instrument summary.