G. de Lange

In-orbit performance of Herschel-HIFI

Aims. In this paper the calibration and in-orbit performance of the Heterodyne Instrument for the Far-Infrared (HIFI) is described. Methods. The calibration of HIFI is based on a combination of ground and in-flight tests. Dedicated ground tests to determine those instrument parameters that can only be measured accurately using controlled laboratory stimuli were carried out in the instrument level test (ILT) campaign. Special in-flight tests during the commissioning phase (CoP) and performance verification (PV) allowed the determination of the remaining instrument parameters. The various instrument observing modes, as specified in astronomical observation templates (AOTs), were validated in parallel during PV by observing selected celestial sources. Results. The initial calibration and in-orbit performance of HIFI has been established. A first estimate of the calibration budget is given. The overall in-flight instrument performance agrees with the original specification. Issues remain at only a few frequencies.

The first-light APEX submillimeter heterodyne instrument FLASH

Development of a dual-color heterodyne instrument for use with the Atacama Pathfinder EXperiment. Commissioning of the APEX began in mid 2004, and regular science operation has been performed since July 2005. Verification of the telescope required a dualchannel receiver operating at (short) submillimeter wavelengths. It was important for the characterization of the telescope to observe at the highest possible frequency at which routine observations can be performed. For pointing, focus, and tracking verification (simultaneous) operation at lower frequencies was requested. We developed FLASH operating on two channels simultaneously – at orthogonal polarizations – in the 460 GHz and 810 GHz atmospheric windows. The system performs with a wide tuning range (420–500 GHz, 780–880 GHz) and intermediate frequency bandwidths of 2 and 4 GHz, respectively. As backends, we operate two fast-Fourier transform spectrometers (FFTS) with 2 × 1 GHz bandwidth each and a maximum of 16 384 channels. The receiver has been in continuous operation since June 2004. While first used for the telescope commissioning, since the middle of last year the instrument has served as the high-frequency workhorse on APEX. Simultaneous observations of the rotational transitions of warm carbon monoxide (J = 4–3 and J = 7–6) and of the two fine-structure lines of atomic carbon are scientifically attractive. FLASH is a principal investigator instrument, available to the APEX-user community on a collaborative basis with MPIfR. A state-of-the-art dualchannel heterodyne instrument has been developed, which made timely commissioning of the APEX possible. Most of the scientific results presented in this special issue rely on data derived with FLASH.

ASUR-an airborne SIS receiver for atmospheric measurements of trace gases at 625 to 760 GHz

A heterodyne receiver making use of a SIS waveguide mixer with integrated horn and single backshort tuner has been built. It has been used for a series of airborne measurements of atmospheric trace gases, such as HCl and ClO, above northern Europe during the winter of 1993 and 1994. The receiver is suitable for measurements in the range of 625-760 GHz and shows stable operation in the airplane together with high sensitivity. Best achieved noise temperatures are T/sub DSB/=310 K at 708 GHz in the laboratory and T/sub SSB/=1500 K at 625 GHz for the complete system in the airplane. >

IF impedance and mixer gain of NbN hot electron bolometers

The intermediate frequency (IF) characteristics, the frequency dependent IF impedance, and the mixer conversion gain of a small area hot electron bolometer (HEB) have been measured and modeled. The device used is a twin slot antenna coupled NbN HEB mixer with a bridge area of 1×0.15??m2, and a critical temperature of 8.3?K. In the experiment the local oscillator frequency was 1.300?THz, and the (IF) 0.05–10?GHz. We find that the measured data can be described in a self-consistent manner with a thin film model presented by Nebosis et al. [Proceedings of the Seventh International Symposium on Space Terahertz Technology, Charlottesville, VA, 1996 (unpublished), pp. 601–613] , that is based on the two temperature electron-phonon heat balance equations of Perrin-Vanneste [J. Phys. (Paris) 48, 1311 (1987)] . From these results the thermal time constant, governing the gain bandwidth of HEB mixers, is observed to be a function of the electron-phonon scattering time, phonon escape time, and the electron temperature. From the developed theory the maximum predicted gain bandwidth for a NbN HEB is found to be 5.5–6?GHz. In contrast, the gain bandwidth of the device under discussion was measured to be ? 2.3?GHz which, consistent with the outlined theory, is attributed to a somewhat low critical temperature and nonoptimal film thickness (6?nm).

Niobium titanium nitride-based superconductor-insulator-superconductor mixers for low-noise terahertz receivers

Integrating NbTiN-based microstrip tuning circuits with traditional Nb superconductor-insulator-superconductor (SIS) junctions enables the low-noise operation regime of SIS mixers to be extended from below 0.7?to?1.15?THz. In particular, mixers incorporating a NbTiN/SiO2/NbTiN microstrip tuning circuit offer low-noise performance below 0.8–0.85?THz, although their sensitivities drop significantly at higher frequencies. Furthermore, a microstrip geometry in which NbTiN is used as the ground plane material only (NbTiN/SiO2/Al) yields significant improvements in the sensitivities of SIS mixers operating up to 1.15?THz, with an upper operating frequency that depends upon the quality of the NbTiN layer, and thus its deposition process. Films deposited at room temperature have Tc = 14.4?K and ?n,20?K ? 60????cm, and offer low-noise performance up to 1?THz, whereas films deposited at 400?°C have Tc = 16?K and ?n,20?K ? 110????cm, and offer low-noise performance up to 1.15?THz. Taken together, these results demonstrate that the high-frequency surface resistance of a NbTiN layer depends upon the film’s structural properties. Most significantly, the drop in performance that is seen at F>1?THz in mixers incorporating NbTiN ground planes deposited at room temperature is attributed to nonhomogeneities in the structural and electrical properties of these films, as is the poor performance of mixers that incorporate NbTiN wiring layers at F>0.85?THz. The development of these NbTiN-based microstrip tuning circuits will enable the production of low-noise SIS mixers for the 0.8–0.96- and 0.96–1.12-THz frequency bands of the Heterodyne Instrument for the Far Infrared on board the European Space Agency’s Herschel Space Observatory.

NbTiN/SiO/sub 2//Al tuning circuits for low-noise 1 THz SIS mixers

Waveguide SIS mixers in which Nb/Al-AlO/sub x//Nb tunnel junctions are integrated with NbTiN/SiO//sub 2//Al tuning circuits are shown to yield receiver noise temperatures as low as 565 K at 970 GHz. Analyzing the noise and gain of one such receiver, it is shown that the NbTiN ground plane is low-loss (<0.6 dB) at 970 GHz. These results are in good agreement with results obtained previously with a quasi-optical mixer incorporating a similar tuning circuit. A decrease in sensitivity above 1 THz is attributed to increasing loss in the NbTiN.

TES arrays for the short wavelength band of the SAFARI instrument on SPICA

SPICA is an infra-red (IR) telescope with a cryogenically cooled mirror (~5K) with three instruments on board, one of which is SAFARI that is an imaging Fourier Transform Spectrometer (FTS) with three bands covering the wavelength of 34-210 μm. We develop transition edge sensors (TES) array for short wavelength band (34-60 μm) of SAFARI. These are based on superconducting Ti/Au bilayer as TES bolometers with a Tc of about 105 mK and thin Ta film as IR absorbers on suspended silicon nitride (SiN) membranes. These membranes are supported by long and narrow SiN legs that act as weak thermal links between the TES and the bath. Previously an electrical noise equivalent power (NEP) of 4×10-19 W/√Hz was achieved for a single pixel of such detectors. As an intermediate step toward a full-size SAFARI array (43×43), we fabricated several 8×9 detector arrays. Here we describe the design and the outcome of the dark and optical tests of several of these devices. We achieved high yield (<93%) and high uniformity in terms of critical temperature (<5%) and normal resistance (7%) across the arrays. The measured dark NEPs are as low as 5×10-19 W/√Hz with a response time of about 1.4 ms at preferred operating bias point. The optical coupling is implemented using pyramidal horns array on the top and hemispherical cavity behind the chip that gives a measured total optical coupling efficiency of 30±7%.

Balloon-Borne Superconducting Integrated Receiver for Atmospheric Research

A Superconducting Integrated Receiver (SIR) was proposed more than 10 years ago and has since then been developed up to the point of practical applications. We have demonstrated for the first time the capabilities of the SIR technology for heterodyne spectroscopy both in the laboratory and at remote operation under harsh environmental conditions for atmospheric research. Within a SIR the main components needed for a superconducting heterodyne receiver such as an SIS-mixer with quasi-optical antenna, a Flux-Flow oscillator (FFO) as the local oscillator, and a harmonic mixer to phase-lock the FFO are integrated on a single chip. Light weight and low power consumption combined with broadband operation and nearly quantum limited sensitivity make the SIR a perfect candidate for future airborne and space-borne missions. The noise temperature of the SIR was measured to be as low as 85 K, with an intermediate frequency band of 4-8 GHz in double sideband operation; the spectral resolution is well below 1 MHz. The SIR was implemented in the three-channel balloon-borne instrument TELIS (TErahertz and submillimeter LImb Sounder) that detects spectral emission lines of stratospheric trace gases (like ClO and BrO). These gases even in small quantities can have a significant impact on the atmosphere because they speed up certain chemical processes, such as ozone depletion.

Performance of the Flight Model HIFI band 3 and 4 mixer units

We describe the performance of the Band 3 and Band 4 Flight Model mixer units for Herschel/HIFI Instrument. These units are part of the Focal Plane Unit of HIFI. The band 3 and 4 mixer units cover the 800-960 GHz and 960-1120 GHz frequency range and have a 4-8 GHz IF frequency band. The sensitivities of the mixers within the HIFI setting are excellent and are the best reported to date. The DSB receiver noise performance in the HIFI FPU environment ranges from 150 K at 800 GHz to 350 K at 1120 GHz. This sensitivity and the absence of atmospheric attenuation will reduce the necessary observation time for astronomical observations in this frequency range by at least two orders of magnitude compared to ground based facilities.

The CHESS spectral survey of star forming regions : Peering into the protostellar shock L1157-B1. II. Shock dynamics

Context. The outflow driven by the low-mass class 0 protostar L1157 is the prototype of the so-called chemically active outflows. The bright bowshock B1 in the southern outflow lobe is a privileged testbed of magneto-hydrodynamical (MHD) shock models, for which dynamical and chemical processes are strongly interdependent. Aims: We present the first results of the unbiased spectral survey of the L1157-B1 bowshock, obtained in the framework of the key program “Chemical HErschel Surveys of star forming regions” (CHESS). The main aim is to trace the warm and chemically enriched gas and to infer the excitation conditions in the shock region. Methods: The CO 5-4 and o-H2O 110-101 lines have been detected at high-spectral resolution in the unbiased spectral survey of the HIFI-band 1b spectral window (555-636 GHz), presented by Codella et al. in this volume. Complementary ground-based observations in the submm window help establish the origin of the emission detected in the main-beam of HIFI and the physical conditions in the shock. Results: Both lines exhibit broad wings, which extend to velocities much higher than reported up to now. We find that the molecular emission arises from two regions with distinct physical conditions : an extended, warm (100 K), dense (3 × 105 cm-3) component at low-velocity, which dominates the water line flux in Band 1; a secondary component in a small region of B1 (a few arcsec) associated with high-velocity, hot (>400 K) gas of moderate density ((1.0-3.0) × 104 cm-3), which appears to dominate the flux of the water line at 179μm observed with PACS. The water abundance is enhanced by two orders of magnitude between the low- and the high-velocity component, from 8 × 10-7 up to 8 × 10-5. The properties of the high-velocity component agree well with the predictions of steady-state C-shock models. Herschel is an ESA space observatory with science instruments provided by European-led Principal Investigator consortia and with important participation from NASA.