The scientific objectives of a submillimetre heterodyne mission are summarized. Interstellar submillimetre wave radiation emanates from a wide diversity of "cool" astronomical objects ranging from the recombination era of the early universe to circumstellar shells of late type stars and planetary atmospheres within the solar system. The present design status of the Far-Infrared and Submillimetre Space Telescope (FIRST) is reported. Some recent new scientific observations pertaining to interstellar molecular submillimetre spectroscopy are reviewed.

This paper describes in detail the general properties of Schottky diode and SIS mixers. Their performances when used in low noise mm and sub-mm wave receivers are compared and a brief attempt is made to identify possible limitations and future improvements.

We have extended techniques used in the development of low noise quasiparticle mixers for the 3mm and 2mm wavelength ranges to include the 1.3mm range. A double sideband receiver noise temperature of below 200K has been measured using a waveguide mounted quas-particle mixer at 223 GHz. Photon assisted quasiparticle tunneling steps have been observed at 462 GHz suggesting that the type of junction used in our quasiparticle mixers may be used for low noise mixing up to this frequency.

A 145 GHz SIS mixer frontend for the Cologne 3-m-radiotelescope and the results of our first measurements are described. The mixer uses a Pb/Bi/In-Oxide-Pb/Bi junction mounted in a quarter height waveguide. It is built in split block technique with an integrated directional coupler for LO coupling. RF tuning is made via a circular non-contacting backshort. All parts of the mixer block are easy to fabricate so that the design may be scaled to frequencies in the submillimeter wave region. The mixer is cooled to 2.8 K by using a closed cycle refrigerator. The IF amplifier whose first two stages are cooled to 18 K by the second stage of the cryogenerator has a noise temperature below 10 K in a frequency band of 250 MHz around the center frequency of 1.4 GHz. First measurements show a system noise temperature of 100 - 120 K (DSB) in the 140 to 145 GHz band.

A broadband, low noise heterodyne receiver, suitable for astronomical use, has been built using a Pb alloy superconducting tunnel junction (SIS). The RF coupling is quasioptical via a bowtie antenna on a quartz lens and is accomplished without any tuning elements. In its preliminary version the double sideband receiver noise temperature rises from 205 K at 116 GHz to 815 K at 466 GHz. This is the most sensitive broadband receiver yet reported for sub-mm wavelengths. Its multi-octave sensitivity and low local oscillator power requirements make this receiver ideal for remote ground observatories or space-borne telescopes such as NASA's Large Deployable Reflector. A version of this receiver is now being built for NASA's Kuiper Airborne Observatory.

We have designed a sub-mm wave receiver using superconducting edge tunnel junctions (SIS/SIN). The mixer is subharmonically pumped at about 375 GHz, whereas the signal frequency is at twice this frequency. The local oscillator and the signal are both coupled to a constant width slot antenna (CWSA), and via an exponential tapering to an array of two junctions coupled in series across a slotline. Design and preliminary results will be discussed in this report.

Josephson junctions (JJ) mixers have been developped and in the millimeter (185-270 GHz) and in the sub-millimeter range (345 GHz, 1000 GHz). Pointcontact devices have shown the best noise temperatures (Tsys = 420 K at 230 GHz) but are unreliable for practical operations. The developped planar junctions have gown so far competitive performance with respect to cooled Schottky barrier diode mixers e.g.. Tsys = 1600 K at 345 GHz.

Experimental results on the long term behavior of this closed cycle refrigerator are presented. The long and short term stability are exceedingly good, and such a machine is powerfull enough to operate with an array of SIS detectors.

Astronomical spectroscopy in the submillimeter spectral region has been dominated by heterodyne radiometers, despite the difficulties of developing appropriate coupling structures, nonlinear mixing elements, and local oscillator sources. This is a result of the narrow linewidth of the emission produced by relatively cold portions of the interstellar medium which can be advantageously studied in this frequency range. Most molecular and atomic transitions reported to date have FWHM linewidths < 10 km/s (λ/Δλ = 3 x 104) although regions of active star formation have emission covering velocity widths as large as 100 km s -1. There is important information about the structure and dynamics of interstellar molecular regions at velocity widths down to the sound speed of 0.5 km/s which corresponds to λ/Δλ = 6 x 105. Thus, the high spectral resolution afforded by heterodyne systems is essential for extracting the full information available in spectral lines. In the short wavelength submillimeter (or far infrared) region, broadband incoherent detectors and frequency selective filters are dominant at the present time, although their resolution (approximately 30 km/s; λ/Δλ = 104) is not adequate for resolving most spectral lines Currently, the dividing line between these approaches is about 300 micrometers, but efforts are underway to increase the sensitivity of heterodyne systems at short wavelengths so that astronomers can take advantage of their superior resolution.

The first part of this paper describes the design and construction of a low noise Schottky diode waveguide mixer, operating in the 320-370 GHz band. The mixer incorporates a fundamental mode waveguide, a broadband coaxial RF-structure and a contacting tunable backshort. Construction details of the mixer will be given, as well as noise temperature and conversion loss measurements for a room temperature device. The influence of the whisker length on the mixer performance will be described. The possibility of using the same mixer design for a 490 GHz mixer will also be considered. Some preliminary measurement results will be presented. The second part of this paper describes a measurement setup for investigation of the performance of a mixer as frequency doubler with 690 GHz output frequency. Results of measurements of output power at 690 GHz will be presented, showing the capability of a single ended Schottky mixer to produce adequate local oscillator power to drive e.g. an InSb-bolometer.

Heterodyne measurements of Schottky diode performance at room temperature were carried out at wavelengths ranging from 919 μm to 372 μm. Coupling structure parameters were varied to achieve optimum coupling efficiency. First measurements on the performance of alternative beam-combining structure replacing the quasioptical diplexer are presented.

The corner cube mdxer is now in use worldwide over a wide range of submillimetre wavelengths. Its optical coupling efficiency is dependent upon many parameters so that experimental measurement of its effective tuning range is difficult. We describe a three-dimensional model which calculates the antenna efficiency for input Gaussian beams. This model is used to predict the optical performance of the corner cube antenna and to simulate its performance in various radimetric configurations so that design criteria can be evaluated.

A submm laser system, consisting of a CO2 pump, laser and a subs resonator has been developed and applied as a local oscillator in a heterodyne receiver for astronomical air-borne observations. Both lasers can be operated in the "sealed off" mode. The whole laser system, stands out for good mechanical stability and structural compactness.

Continuously tunable laser sidebands in the submillimeter and far infrared (FIR) have been generated by mixing radiation from an optically pumped FIR molecular laser with that from millimeter-wave klystrons in a Schottky-barrier diode. An enhancement in conversion efficiency over similar systems reported previously is obtained by using a Michelson interferometer to separate the sidebands from the carrier and by placing the Schottky diode in an open structure corner cube mount. With 4 mW of laser power at 693 and 762 GHz the side-band power was measured to be 101N. This is at least an order of magnitude better than the previously reported results. At higher frequencies, 22 mW of 1627 GHz laser power produced about 7.5 μW of sideband output while 3 mW of 1839 GHz laser power generated about 200 nW of sideband radiation. The lower efficiency at the higher frequencies is due primarily to the mismatch between the laser radiation and the fixed-length diode antenna. Spectral lines have been observed up to 3200 GHz. The molecular absorbtion signals are easily seen using either video or lock-in detection techniques.

A new system for wavelength calibration and stabilization of tunable lasers has been developed. Its crucial element is a newly designed internally coupled Fabry-Perot interferometer which is tunable and works over a very large wavelength range (e.g. 0.6 - 30 μM). We describe the concept of our system and its application to a tunable diode laser (TDL) including a future TDL local oscillator for IR - heterodyne spectroscopy. The possible prospects of our technique in the submillimeter region are also briefly discussed.

An 0-type carcinotron (backward-wave oscillator) operating between 850 - 1000 GHz was built with the support of the European Space Agency. It generates an RF power level of 0.5 mW minimum across the bandwidth and the maximum RF power reaches 2.3 mW. The operating high voltage varies from 4.9 to 8.8 kV. The total power consumption is less than 200 W. The spectral analysis has been made. The linewidth is less than 1 MHz in free-running operation.

Two concepts for tunable solid state oscillators in the 3-mm wavelength band are described: 1) a crossed waveguide design with a WR-19 waveguide as fundamental mode resonator and harmonic output power between 3 and 12 mW from 71 to 102 GHz, 2) a fundamental mode Gunn oscillator from 40 to 57 GHz and a frequency doubler with an output power between 3 and 9 nmW.

Imaging (multi-beam) front-end systems utilize a reflector or a lens, as well as a focal-plane array of receptor elements (beams). To be practical, such systems must utilize focal plane arrays which can be fabricated using hybrid integrated (and eventually monolithic) technology. This paper reviews alternative approaches to focussing elements, auxiliary devices required for local oscillator injection and similar functions, and focal plane arrays. It also discusses general limitations and advantages of these approaches, as well as typical applications to millimeter and submillimeter astronomical instrumentation.

The telescope effective focal length places major constraints upon the ability to design and build low loss quasi-optical components in the millimeter and submillimeter wavelength range. In this paper we discuss the principle problems involved in the design of such systems and report measurements of various quasi-optical components at 230 GHz and 460 GHz.

We have developed a prototype imaging system at 31 GHz, which employs a two-dimensional (5x5) array of tapered slot antennas, and integrated detector or mixer elements, in the focal plane of a prime-focus paraboloid reflector, with an f/D=1. The system can be scaled to shorter millimeter waves and submillimeter waves. The array spacing corresponds to a beam spacing of approximately one Rayleigh distance and a two-point resolution experiment showed that two point-sources at the Rayleigh distance are well resolved.

Large aperture parabolic reflectors are needed for space applications in different wave-length ranges including the millimetric and submillimetric ones where a high accuracy is required for the reflecting surfaces. We propose to get more accurate surfaces by replacing the preformed flexible membranes presently scheduled in the inflatable technology, by elastic disks whose thickness variation along a radius is calculated in order to obtain, after inflation, parabolic caps. As an illustration of the method, polyester and polyimide membranes are investigated. Provide a variable thickness from the center to the edge is obtainable, large aperture parabolic cap may reasonably be considered as feasible. The surface accuracy depends mainly on the elastic properties of the material and the thickness control.

A short review of the properties of various types of current and future wideband spectro-meter backends is presented. The spectrometer system with a bandwidth of 1 GHz proposed for the UK/NL mm telescope in Hawaii is discussed in more detail as an example of current developments.

We present a stable and compact Acousto-Optic spectrograph for ground applications than can accept different Bragg-cells with bandwidth of 30,150 and 500 MHz, and which is used routinely for low frequency radioastronomy. First results in the design of an A. O. S for space applications, particularly at submillimetre wavelengths are also presented. The comparison between the resolution of an A. O. S. and an filter-bank spectrograph is discussed.

Three compact acousto-optical spectrometers with a total bandwidth of 64, 250 and 500 MHz were developed. The frequency resolution (and number of detection channels) are 32 kHz (2048), 250 kHz (1510) and 500 kHz (1350) respectively. The two broadband instruments are working for the first time with a 780 nm laser diode. All instruments are easy to align and transportable; the design is modular, the electronics is standardized. The Allan-Plot has been developed and used in combination with other test methods to reliably check the performance characteristics of the AOS, such as mechanical, frequency and temperature stability. The total observational integration time used were 35000s on source without any noticeable degradation in the integration performance of the spectrometers.

Spectroscopy in the submillimeter wavelength region (1 mm to 0.1 mm) has been realised for about 20 years. This spectral region is important for molecular physics, because sub-millimeter rotational transitions of light molecules such as CO, 03, OH et al. have exceptionally high absorption coefficients in the order of 10-4 cm-1 or higher. The first rotational transitions of many high molecules, for example OH, NH3, CH, are found at sub-millimeter wavelength. In spite of many advantages of other spectroscopic methods such as microwave absorption or spectroscopy with Fourier and Michelson interferometers, the sub-millimeter heterodyne spectroscopy has the advantage of high resolution. Line profiles could be resolved and lines can be detected in absorption or in emission. Frequency measurements of K-splitting from asymmetric molecules yield new determinations of molecular constants.

In this report the essential design goals of our millimeter and submillimeter-wave laboratory spectrometer are described. The source-modulation spectrometer is mainly used to provide rest frequencies of rotational transitions of astrophysically interesting molecules in the frequency range 68 - 520 GHz. As phase-locked radiation sources millimeter-wave reflex klystrons and tunable Gunn oscillators are employed. The absorption signals are detected with a newly constructed digital lock-in amplifier. The smallest peak-absorption coefficient that could be observed is 8.10-8 cm-1 at 150 GHz and 2.10-7 cm-1 at 225 GHz in a 400-cm free-space cell. For high-resolution spectroscopy saturation techniques have been applied to resolve overlapping line profiles. The spectrometer is also used for testing new waveguide and quasioptical components and complete subsystems for the Cologne 3-m radio telescope, now located at Gornergrat near Zermatt in the Swiss Alps.

We describe an experimental sub-millimetre heterodyne receiver covering approximately 300-500GHz. This wide frequency coverage is obtained using two corcinotron local oscillators incorporated into the receiver, with two room temperature, Schottky-barrier diode corner-cube mixers. This physically compact receiver includes a PLL system and an on-board computer for controlling and supervising all components as well as communicating with a host computer.

A submillimeter receiver, to be used as common-user receiver for UKIRT and the UK-NL mm telescope at Mau-na Kea, Hawaii, is here described. The receiver operates at two submm frequency bands using one backward wave oscillator (carcinotron) and a doubler. It utilizes a bath cryostat coupled to a closed cycle cooler for cooling of the mixers and preamplifiers. All main receiver components are microprocessor controlled in order to provide reliable and efficient operation at the high altitude observatory.

The UCB submillimeter heterodyne receiver is a complete system for high resolution astronomical spectroscopy in the 350 pm and 450 pm atmospheric windows. This compact system mounts directly at the Cassegrain focus of large optical and infrared telescopes. It consists of a laser local oscillator, open structure mixer, quasi-optical coupling system, a broad band IF system, and an acousto-optical spectrometer. The local oscillator is a one meter long submillimeter laser optically pumped by a CO2 laser. The mixer is a quasi-optical corner cube antenna structure and Schottky diode. We currently operate the mixer at room temperature and are evaluating mixer performance at 77 K. The system noise temperature is less than 7000 K SSB during observations. The coupling optics allow efficient use of LO power and result in measured telescope beam efficiencies of 0.40 to 0.45. The instantaneous IF bandwidth of 1.2 GHz (450 km/s at 800 GHz) is well suited for observations of astronomically interesting broad emission lines (e.g. from the Galactic center and outflows). The back end spectrometer is a 1200 x 1 MHz channel dual acousto-optical spectrometer (0.38 km/s resolution at 800 GHz). Most of the recent work with this receiver has been on the CO J=7→6 transition at 806.6517 GHz.

Carcinotrons used as local oscillator in mm/sub-mm receivers require frequency and phase stabilisation, the latter more specifically for receivers in mm-interferometers. Locking to a phase- and frequency-stable X- or S-band reference source, using a microwave synthesizer is quite common for laboratory experiments. For use on a telescope in a common user receiver such a synthesizer is thought to be too expensive, as well as an overkill. This paper describes a tunable 11.5-12.0 GHz reference frequency system, used in a frequency and phase lock system for a 320-370 GHz carcinotron, that incorporates a relatively cheap 250-500 MHz synthesizer with relaxed phase noise specifications. Necessary phase noise specifications for the reference frequency system and synthesizer will be derived. Results on the frequency and phase locking of a carcinotron will be presented.

For the characterization of noise and stability of any measuring instrument a new and very practical method is introduced. It follows the principles of the "Allan variance" well estabilished for the characterization of the stability of frequency standards. The plot of the Allen variance versus integration time enables one to determine the different types of noise power spectra from the output of any instrument. In particular, the best range of the integration time for optimum use of the system can accurately be evaluated. Theoretical considerations and experimental results with components of the 3-m radiotelescope of Univ. of Cologne are presented.

This paper describes the design and construction of a 2k He-cryostat, using a closed cycle refrigerator to cool the cryostat radiation shields. With this configuration a 5 days hold-time has been obtained at 4.2 K. The cryostat is being used to cool a Schottky diode mixer at 350 GHz and an InSb bolometer at 690 GHz to 20 K and 4.2 K (or lower by pumping on the liquid helium) respectively. The heat load on the 4 K cold station has been minimized using a thermal model of the cryostat, partly developed on the basis of measurements on the real cryostat. Much insight in the importance of various heat loads was gained using the model and led to improvements in the original cryostat. This cryostat forms the cryogenic part of the Dutch Common User sub-mm receiver for the United Kingdom Infra Red Telescope (UKIRT) on Mauna Kea, Hawaii.

A 230 GHz mixer receiver has been constructed using (Pb/In/Au) - (Pb/Bi) SIS junctions fabricated using the Dolan bridge method and thermal oxidation. The mixer block is of conventional design with reduced height waveguide and contacting backshort. Preliminary results indicate a receiver noise temperature <1000K (DSB).

Two receiver systems are described. The first is a 220-280 GHz dual-polarization system using cryogenically cooled Schottky-diode mixers pumped with a frequency-multiplied IMPATT local oscillator. The second is a 350 GHz indium antimonide array receiver which uses a novel quasi-optical LO injection scheme.

The efficiency of a frequency multiplier depends on the varactor parameters and the embedding impedances seen by the diode at fundamental and harmonic frequencies. In this study mm-wave doublers and other multipliers were simulated by computer to find optimum embedding impedances for a given diode. A scaled model was constructed in order to search for optimum impedances for broadband multipliers in practice.