This paper describes the Heterodyne Instrument for the Far-Infrared (HIFI), to be launched onboard of ESA's Herschel Space Observatory, by 2008. It includes the first results from the instrument level tests. The instrument is designed to be electronically tuneable over a wide and continuous frequency range in the Far Infrared, with velocity resolutions better than 0.1 km/s with a high sensitivity. This will enable detailed investigations of a wide variety of astronomical sources, ranging from solar system objects, star formation regions to nuclei of galaxies.
The instrument comprises 5 frequency bands covering 480-1150 GHz with SIS mixers and a sixth dual frequency band, for the 1410-1910 GHz range, with Hot Electron Bolometer Mixers (HEB). The Local Oscillator (LO) subsystem consists of a dedicated Ka-band synthesizer followed by 7 times 2 chains of frequency multipliers, 2 chains for each frequency band. A pair of Auto-Correlators and a pair of Acousto-Optic spectrometers process the two IF signals from the dual-polarization front-ends to provide instantaneous frequency coverage of 4 GHz, with a set of resolutions (140 kHz to 1 MHz), better than < 0.1 km/s. After a successful qualification program, the flight instrument was delivered and entered the testing phase at satellite level. We will also report on the pre-flight test and calibration results together with the expected in-flight performance.
The THIS instrument (Tuneable Heterodyne Infrared Spectrometer) is a versatile heterodyne receiver with a sensitivity
close to theoretical prediction. It uses a Quantum Cascade Laser (QCL) as local oscillator and a HgCdTe
photo-voltaic detector as mixer. The IF-spectrum is analyzed by means of a new broadband Acousto-Optical Spectrometer
(AOS) with 3 GHz bandwidth and 1 MHz resolution. A dual sideband (DSB) system noise temperature has
been measured with 2300 K at 10 μm wavelength, which is only 60% above the quantum limit. The stability of the
system has been determined at an Allan variance minimum time of 50 seconds. Below this integration time the performance
is purely radiometric. Also, the frequency stability has been measured with 1 MHz rms error within several
hours. The quality of the instrument has been demonstrated by a few observing campaigns at the McMath-Pierce
observatory on Kitt Peak. Measurements of Winds on Mars and Venus have been carried out and molecular line signals
in sunspots have been detected. We propose to develop THIS as a second generation instrument for future astronomical
observations on SOFIA.
In this paper, the design, manufacturing and testing of the optical subassembly specifically tailored for the acousto-optical Wide Band Spectrometer (WBS), subsystem of HIFI (Heterodyne Instrument for Herschel), is presented.
The WBS optical sub assembly consists of a laser source module with two collimated lasers and a prism beamsplitter, and imaging optic modules. The light source is a near-infrared laserdiode operating at 785 nm. The outgoing beam from the collimating unit is elliptical with 8 mm width and splitted by a prism device into four beams. The quadruplets are focussed in the vertical direction by means of a cylindrical element thus achieving a four "sticks" like focussed pattern in an intermediate focus where the acoustic channels of a Bragg cell are positioned. A combination of scanoptic and cylindrical lens is used to image the deflected light on a four line linear CCD. The laser source unit has been designed to operate under paraxial working conditions.
Despite the conceptually simple optical configuration, the system has represented a technological challenge, being of the order of few micrometres the integration scale for the optics and for the tight tolerance set requested in terms of degree of collimation and for the alignment precisions and stabilities over a wide range of temperatures and other environmental conditions.
A new type of geometry for terahertz traveling-wave photomixers, vertically pumped free-space by two detuned continuous-wave diode lasers, is proposed and experimentally verified for devices based on low-temperature-grown GaAs (LT-GaAs). It combines the advantages of conventional interdigitated small area structures and traveling-wave devices. An output power of 1 μW at the mixing frequency of 1 THz was measured as a first result, which meets power requirements for superconducting heterodyne mixer devices.
None of the existing spectrometer technologies currently fulfil the demands for larger IF-bandwidth up to 10 GHz for future THz heterodyne systems without using hybrid-solutions.
At KOSMA (Koelner Observatorium fuer Sub-Millimeter Astronomie) we are investigating the idea of a laser side-band-spectrometer.
We use laser modulation method to generate optical sidebands near of the
laser frequency, which can then be frequency analyzed by standard optical
methods like a high finesse Fabry-Perot (FP) etalon.
First laboratory results of a prototype setup with 9 GHz Bandwidth will be presented.
The Cologne spectrometer THIS (Tuneable Heterodyne Infrared Spectrometer) opens the mid-infrared wavelength region from 5 to 17 µm for ultra-high-resolution spectroscopy. With a current bandwidth of 14 km/sec and a frequency resolution of R=2*10<sup>7</sup> it is the only widely tuneable and transportable infrared heterodyne receiver. A Quantum-Cascade laser is used as local oscillator (LO). To provide optimum beam combination a Fabry-Perot ringresonator is used to superimpose the LO and the radiation. Frequency mixing is done by a Mercury-Cadmium-Telluride photomixer and spectral analysis is performed by an Acousto-Optical spectrometer. The system noise temperature is about three times the quantum limit giving THIS a sensitivity equivalent to CO<sub>2</sub>-laser based heterodyne systems. Various measurements at different ground based telescopes including the analysis of trace gases in Earth's atmosphere, observations of molecular features in sunspots, and detection of non-LTE CO2 emission from the atmosphere of Venus' have been performed and demonstrate the instrument's capabilities for astronomical observations at ground based telescopes and the stratospheric observatory SOFIA in the near future. Possible targets for future observations with THIS will be discussed.
GREAT - a heterodyne instrument for high-resolution spectroscopy aboard SOFIA is developed by a consortium of German research institutes. The first-light configuration will allow parallel observations in two far-infrared frequency bands. We will have a choice of back-ends, including a broad-band acousto-optical array and a high-resolution chirp transform spectrometer. We describe the structural and quasi-optical design of the receiver, update on the front-end and back-end developments and discuss the data acquisition system.
We present the concept for KOSMA's 16 element 1.9 THz heterodyne array
STAR (SOFIA Terahertz Array Receiver) which is being developed for
SOFIA. The instrument will consist of two interleaved sub-arrays of 8
pixels each. Together we will have a 4 × 4 pixel array with a beam spacing on the sky of approximately 1.5 times the beam size of 15 arcsec (FWHM). The receiver is mainly targeted at measuring the fine structure transition of ionized atomic carbon at 1.9 THz (158 microns). STAR's optics setup is modeled after the successful design used in KOSMA's SMART receiver. It will contain a K-mirror type beam rotator, a Martin-Puplett diplexer for LO coupling and an LO multiplexer using imaging Fourier gratings. Complete optical sub-assemblies will be machined monolithically as integrated optics units, to reduce the need for optical alignment. STAR will probably use waveguide mixers with diffusion cooled hot electron bolometers, which are being developed at KOSMA. The receiver backends will be KOSMA Array-AOSs. Local oscillator power will be provided by a backward wave oscillator (BWO), followed by a frequency tripler.
We present the first results obtained with our new dual frequency SIS array receiver SMART The instrument is operational since September 2001 at the KOSMA 3m telescope on Gornergrat near Zermatt/Switzerland. The receiver consists of two 2×4 pixel subarrays. One subarray operates at a frequency of 490 GHz, the other one at 810 GHz. Both subarrays are pointed at the same positions on the sky. We can thus observe eight spatial positions in two frequencies simultaneously. For the first year of operation we installed only one half of each subarray, i.e. one row of 4 mixers at each frequency.
The receiver follows a very compact design to fit our small observatory. To achieve this, we placed most of the optics at ambient temperature, accepting the very small sensitivity loss caused by thermal emission from the optical surfaces. The optics setup contains a K-mirror type image rotator, two Martin-Puplett diplexers and two solid state local oscillators, which are multiplexed using collimating Fourier gratings. To reduce the need for optical alignment, we machined large optical subassemblies monolithically, using CNC milling techniques. We use the standard KOSMA fixed tuned waveguide SIS mixers with Nb junctions at 490 GHz, and similar Nb mixers with Al tuning circuits at 810 GHz.
We give a short description of the front end design and present focal plane beam maps, receiver sensitivity measurements, and the first astronomical data obtained with the new instrument.
The present status of AOS development at KOSMA is discussed. A study of a new generation of AOS using the new Bragg-cell material "Rutil" is on the way, which is supposed to lead to spectrometers in the range of 4 GHz total bandwidth at an resolution of 2-3 MHz. A second alternative for a 4 GHz bandwidth spectrometer has been developed as an engineering model for the HIFI instrument aboard the ESA cornerstone mission "Herschel". It consists of an array-AOS with 1 GHz bandwidth of each of the four AOS bands at a resolution of 1 MHz. A hybrid system for an input between 4 and 8 GHz is setup, and various laboratory tests have demonstrated that this system is well suited for large bandwidth applications like with HIFI. For eventual future demand of even larger bandwidth, details of a new optical method for Rf-analysis are discussed. It consists of a modulated laser with one or two Fabry-Perot etalons to analyze the frequency distribution of the resulting laser sidebands. A bandwidth of several 10 GHz at moderate resolution can be achieved.
We present the newly developed transportable setup of the Cologne Tuneable Heterodyne Infrared Spectrometer (THIS) designed for astronomical observations aboard the Stratospheric Observatory For Infrared Astronomy (SOFIA). With THIS a competitive tuneable heterodyne spectrometer for the mid-infrared is available that will allow measurements in a wide field of astronomical applications. Frequency tuneability over a wide range provided by the use of semiconductor lasers as local oscillators (LO) allows a variety of molecules in the mid infrared to be observed under very high frequency resolution. With the use of newly developed quantum-cascade lasers (QCL) a sensitivity close to the quantum limit will be in reach.
The wideband acousto-optical spectrometer (WBS) for HIFI- FIRST is comprised of two array-AOS with 4 times 1 GHz bands each. There are some advantages to this design, the most important one is that relative frequency and amplitude variations between the 4 bands are rather unlikely. This is demonstrated by laboratory tests, which verify also that fairly slow beam-switching at 0.5 Hz may be a sufficient chop speed for HIFI. The performance of array-AOS has also been demonstrated during measurements at ground-based observatories. WBS consists of three independent units, one IF-, one optics-, and one electronics-unit. Some of the details of the WBS design are described, and the present performance estimates are given.
A consortium of German research laboratories has been established for the development of a modular dual-channel heterodyne instrument (GREAT: German Receiver for Astronomy at Terahertz Frequencies) for high-resolution spectroscopy aboard SOFIA. The receiver is scheduled to be available in time for SOFIA's very first astronomical mission in late 2002. The first-flight version will offer opportunities for parallel observations in two frequency bands. We will have a choice of backends, including an acousto-optic array (4 X 1 GHz) and a high-resolution chirp transform spectrometer.
In this paper, we present the design for a 16-channel heterodyne array receiver for use on SOFIA. The array will be capable of using either hot-electron bolometers or membrane mounted Schottky diodes in efficient, low-cost waveguide mounts. Focal plane arrays will be constructed to target astrophysically important lines between approximately 1.9 and 3 THz. Due to the prevailing physical conditions in the interstellar medium, this frequency range is one of the richest in the FIR portion of the spectrum. An array receiver designed for this wavelength range will make excellent use of the telescope and the available atmospheric transmission, and will provide a new perspective on stellar, chemical, and galaxy evolution in the present as well as past epochs. The proposed system uses the most sensitive detectors available in an efficient optical system.
We describe the receiver concept for KOSMA's planned second generation SOFIA instrument STAR (SOFIA Terahertz Array Receiver). The receiver will contain a 4 X 4 element heterodyne mixer array for the frequency range from 1.7 to 1.9 THz (158 to 176 microns). Its main scientific goal is large scale mapping of the 158 micron fine structure transition of singly ionized carbon. The design frequency range covers this line out to moderate red shifts and also allows to observe a variety of other spectral lines.
On December 5, 1998, the Submillimeter Wave Astronomy Satellite has been launched with a PEGASUS carrier after more than 3 years delay. SWAS is observing molecular line signals (H<SUB>2</SUB>O, <SUP>13</SUP>CO, Cl, O<SUB>2</SUB> and H<SUB>2</SUB> <SUP>18</SUP>O) from astronomical sources at frequencies between 487 and 557 GHz. SWAS is the first sub-millimeter heterodyne space mission, and, for the spectral analysis of the received signals, it carries the first acousto-optical spectrometer (AOS) in space. The AOS has been built at University of Cologne, and it covers 1.4 GHz bandwidth with approximately 1400 frequency channels. The total weight is 7.5 kg and the power consumption is 5.5 Watts only. The very stable temperature conditions on SWAS allow longtime integrations at total observing times far above 100 hours still with radiometric performance. So far, the AOS- spectra have not been degraded by particle hits, particularly the CCD detector of the AOS does not exhibit any visible effect due to cosmic rays. SWAS has already observed many interstellar sources in our galaxy. Emission of water is seen to be very abundant, while signals of molecular oxygen seem to be too weak to be observable.
The Kolner Observatorium fur Submillimeter-Astronomie (KOSMA) has recently been equipped with a new 3 m submm telescope. The new telescope dish has a CFRP backstructure and aluminum panels with a mean surface accuracy of the individual panels of well below 10 micrometer. The 18 panels of the primary reflector have been adjusted to a surface rms of at present about 30 micrometer with the help of a holographic phase retrieval algorithm developed for and previously used at the JCMT. The present main beam efficiency derived from observations of Jupiter and Saturn is approximately 45% at 660 GHz. The new telescope features a chopping secondary and 2 Nasmyth ports. The excellent atmospheric transmission during winter time at the telescope site, Gornergrat near Zermatt, Switzerland, allow us flexible operation up to the highest atmospheric submm windows. We present the current status of the new telescope, in particular with regard to its surface alignment, and first astronomical results at 660 and 690 GHz.
Large bandwidth acousto-optical spectrometers have now reached a very high level of maturity. They achieve very compatible results in comparison with other spectrometer types like filterbanks, autocorrelators, and chirp transform spectrometers. In addition, AOS are rather simple in design, have little complexity and can be designed for space applications very easily. A new generation of broad-band AOS, the array-AOS, consists of four parallel 1 GHz spectrometers built into one optical unit. Tests results in the laboratory as well at a radio-observatory are very promising. For example, the Allan variance minimum time has been found above 1000 seconds. In comparison it can be shown that the AOS spectra are less affected by instrumental noise or baseline distortions due to platforming effects as they are visible with most hybrid autocorrelators. For future applications of acousto-optics the development of cross-correlators seems to be feasible. First steps in this new direction are on the way.
We describe frontend concepts for the future heterodyne array instruments of the KOSMA 3-m telescope and for SOFIA. For KOSMA we are currently developing a dual frequency (400 - 500, 800 - 900 GHz) SIS mixer array of four elements per frequency band. For SOFIA, we are planning an up to 4 X 4 element array for 1.6 - 2.0 THz using superconducting hot-electron bolometers. The small number of pixels allows us to keep the optics relatively compact. For the same reason, a single sideband filter is not included. The local oscillator power will be distributed using Dammann gratings. Motivated by the excellent beam characteristics of waveguide horns we are planning to extend the range of our waveguide mixers to 2 THz. The mixers are based on the wideband tunerless mixers that have been successfully used in single element telescope receivers at KOSMA. The mixers will be standard building blocks mounted at the back of waveguide horns integrated into the optical setup. Local oscillators for 400 - 900 GHz are solid state sources, for the Terahertz array we are developing several alternative local oscillator concepts.
We describe the preliminary design of the proposed Heterodyne Instrument for FIRST (HIFI). The instrument will have a continuous frequency coverage over the range from 480 to 1250 GHz in five bands, while a sixth band will provide coverage for 1410 - 1910 GHz and 2400 - 2700 GHz. The first five bands will use SIS mixers and varactor frequency multipliers while in the sixth band a laser photomixer local oscillator will pump HEB mixers. HIFI will have an instantaneous bandwidth of 4 GHz, analyzed in parallel by two types of spectrometers: a pair of wide-band spectrometers (WBS), and a pair of high- resolution spectrometer (HRS). The wide-band spectrometer will use acousto-optic technology with a frequency resolution of 1 MHz and a bandwidth of 4 GHz for each of the two polarizations. The HRS will provide two combinations of bandwidth and resolution: 1 GHz bandwidth at 200 kHz resolution, and at least 500 MHz at 100 kHz resolution. The HRS will be divided into 4 or 5 sub-bands, each of which can be placed anywhere within the full 4 GHz IF band. The instrument will be able to perform rapid and complete spectral line surveys with resolving powers from 10<SUP>3</SUP> up to 10<SUP>7</SUP> (300 - 0.03 km/s) and deep line observations.
Acousto optical spectrometers (AOS) have become an attractive alternative to filterbanks or autocorrelators for applications in radioastronomy and in heterodyne as well as in laboratory spectroscopy. Due to continuous improvements, AOSs have now achieved a performance and reliability level that makes this technology applicable for airborne or spaceborne missions. A first fully space qualified AOS was built at the University of Cologne for the Submillimeter Wave Astronomy Satellite (SWAS) to be launched in fall 1995. The SWAS-AOS has a large bandwidth of 1.4 GHz covered by 1365 channels, with a center frequency of 2.1 GHz. Only 11 mW rf white noise input power is required for simultaneous saturation of all channels. The design is optimized for very high stability and allows operation within a temperature range from minus 5 to plus 30 degrees Celsius at temperature variations of up to 2 degrees Celsius/hour. The total weight is 7.2 kg including electronics, the power consumption is 5.4 watts including data pre-averaging electronics and dc-dc converter losses. The performance was verified also after complete vibrational and thermal vacuum environmental testing. For future projects with large bandwidth requirements or with multichannel systems the AOS technology can also be used to fabricate array spectrometers. Such array AOS offers the unique option to multiply the available bandwidth without multiplying the hardware accordingly. Especially for spaceborne applications this is an extremely useful development because weight, power consumption as well as costs increase only very moderately. At present the first prototype with four independent 1 GHz channels is in development. This array AOS will have a total bandwidth of 4 GHz covered by 4000 channels, and will be available in 1996.
The 3-m KOSMA telescope at Gornergrat (Switzerland) is dedicated to millimeter- and submillimeter-wave astrophysics with observations mainly of interstellar atomic and molecular lines. This includes measurement of the large-scale structure and dynamics of molecular clouds in our galaxy for a better understanding of the processes connected with star formation. The present research activity of the observatory is described with a special scope on the description of the techniques and instrumentation used. The site at Gornergrat has superb atmospheric transparency, thus allowing operation up into the highest-frequency atmospheric window at around 850 GHz. We discuss the rapid development of sensitive superconductor-insulator-superconductor mixer elements, now covering frequencies well up into the submillimterwave range with close to quantum-limit detection. As frequency-analyzing devices we exclusively use acousto-optical spectrometers at KOSMA. Progress into the submillimeter spectral range is complemented by the now mature technique of building radiotelescopes with sufficient surface accuracy to guarantee operation into the terahertz region (λ = 200 μm). A new 3-m reflector for the KOSMA telescope with an expected surface accuracy of 10 μm is under construction.
The Submillimeter Wave Astronomy Satellite (SWAS) mission will study galactic star formation and interstellar chemistry. To carry out this mission, SWAS will survey dense (n<sub>H2</sub> > 10<SUP>3</SUP> cm<SUP>-3</SUP>) molecular clouds within our galaxy in either the ground-state or a low- lying transition of five astrophysically important species: H<SUB>2</SUB>O, H<SUB>2</SUB><SUP>18</SUP>O, O<SUB>2</SUB>, CI, and <SUP>13</SUP>CO. By observing these lines SWAS will: (1) test long-standing theories that predict that these species are the dominate coolants of molecular clouds during the early stages of their collapse to form stars and planets and (2) supply heretofore missing information about the abundance of key species central to the chemical models of dense interstellar gas. During its two-year mission, SWAS will observe giant and dark cloud cores with the goal of detecting to setting an upper limit on the water abundance of 3 X 10<SUP>-6</SUP> (relative to H<SUB>2</SUB>) and on the molecular oxygen abundance of 2 X 10<SUP>-6</SUP> (relative to H<SUB>2</SUB>). SWAS is designed to carry all elements of a ground based radiotelescope. The telescope is a highly efficient 54 X 68-cm off-axis Cassegrain antenna with an aggregate surface error less than or equal to 11 micrometers rms. The receiver system consists of two independent heterodyne receivers with second harmonic Schottky diode mixers, passively cooled to approximately equals 150 K. The spectrometer is a single acousto-optical spectrometer (AOS) with 1400 1-MHz channels enabling simultaneous observations of the H<SUB>2</SUB>O, O<SUB>2</SUB>, CI, and <SUP>13</SUP>CO lines.
The first fully space qualified acousto-optical spectrometer (AOS) is described. It is built for the Submillimeter Wave Astronomy Satellite (SWAS) to be launched in July 1995. It has a very large bandwidth from 1400 to 2800 MHz covered by 1365 channels. This corresponds to a nearly 1 MHz channel spacing. The design is optimized for very high stability, which is demonstrated by means of Allan variance stability test. The Allan plot minimum time was found well above 800 seconds. The AOS can operate within a temperature range from -5 to +30 degree(s)C (+5 to +25 degree(s)C nominal) and with temperature variations of up to 2 degree(s)C/h. The performance was verified also after environmental testing such as random vibration (10.2 G rms) and thermal cycling of -30 to +50 degree(s)C. The lightweight mechanical design resulted in a total weight of 7.2 kg including electronics. A detailed optical design study was performed in order to achieve diffraction limited channel resolution, high efficiency and low sensitivity to mechanical distortion. The RF input power needed for full scale is 11 mW. The power consumption is 5.4 Watts (including data pre-averaging and DC-DC converter losses). The development has shown that AOSs are well suited for spaceborne applications.