Aims. This paper describes the Heterodyne Instrument for the Far-Infrared (HIFI) that was launched onboard ESA's Herschel Space Observatory in May 2009. Methods. The instrument is a set of 7 heterodyne receivers that are electronically tuneable, covering 480−1250 GHz with SIS mixers and the 1410−1910 GHz range with hot electron bolometer (HEB) mixers. The local oscillator (LO) subsystem comprises a Ka-band synthesizer followed by 14 chains of frequency multipliers and 2 chains for each frequency band. A pair of auto-correlators and a pair of acousto-optical spectrometers process the two IF signals from the dual-polarization, single-pixel front-ends to provide instantaneous frequency coverage of 2 × 4 GHz, with a set of resolutions (125 kHz to 1 MHz) that are better than 0.1 km s −1 . Results. After a successful qualification and a pre-launch TB/TV test program, the flight instrument is now in-orbit and completed successfully the commissioning and performance verification phase. The in-orbit performance of the receivers matches the pre-launch sensitivities. We also report on the in-orbit performance of the receivers and some first results of HIFI's operations.

Abstract. Some recent results associated with the development of tantalum based photon counting superconducting tunnel junctions (STJ) suitable for use as broadband low resolution spectrometers for optical and ultraviolet astronomy are presented. A 20 × 20 µm square tantalum based STJ, operated at a temperature of 0.3 K, has demonstrated a limiting resolution of ∼ 8 nm at 200 nm and ∼ 80 nm at 1000 nm. The device is extremely linear in response with photon energy, and covers the waveband from 200 nm to 2 µm while measuring the individual photon wavelength and arrival time. The short wavelength limit is currently constrained by the current experimental configuration (a fibre optic) as well as to some extent the sapphire substrate. The estimated quantum efficiency for single photons is over ∼ 50% between 200 and 700 nm with a maximum of ∼ 75% at 550 nm. Such an STJ when packaged into an array could contribute significantly to many fields of near infrared, optical and ultraviolet astronomy being able to provide efficiently and simultaneously the broad band spectrum and photon arrival time history of every single object in the field over a very wide dynamic range.

In the last two years, in the context of the Geant4 toolkit, a package for the Monte Carlo simulation of atomic relaxation processes has been developed, and tested against experimental data. It includes models for the simulation of Auger electron and fluorescence photon emission. Results from a comparison with test beam data, presented here, show a very good statistical coincidence of the simulation and of experimental measurements. The package allows to fully exploit the power of other Geant4 components, such as geometry and materials modeling. This gives, for the first time, the chance of simulating fluorescence and Auger emission of complex material as rocks, whatever is the physical process inducing it, thanks to advanced object-oriented software techniques. Application results from the studies for the design of BepiColombo ESA mission to Mercury are presented.

Abstract. We discuss the capabilities of superconducting tunnel junctions as detectors for ultraviolet, optical, and near-infrared astronomy. Such junctions have recently been shown to allow the detection of individual optical and ultraviolet photons with an inherent spectral resolution related to the critical temperature of the absorbing superconductor. Limiting resolutions at 500 nm ranging from 5 -40 nm (for materials with critical temperatures between 0.1 to 10 K) should be achievable. These detectors should have a high quantum efficiency (> 50 per cent) over a very broad wavelength range from the ultraviolet to the near infrared (100 -2000 nm). The overall efficiency is limited by reflection from the superconducting film, and should be improved significantly by appropriate anti-reflection coatings. The devices function at very high incident photon rates-with count rates of order 10 kHz or higher being feasible, and photon arrival time datation possible to microsec-level accuracy. It is realistic in the future to envisage that these devices, of a size typically 20 − 50 µm 2 , could be packaged into imaging arrays. These key characteristics imply that many areas of optical and ultraviolet astronomy could benefit significantly from their further development.

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