HIFI - The Heterodyne Instrument for the Far Infrared

HIFI was designed to obtain spectra with very high resolution (up to 10⁷) in the far-infrared and sub-millimetre wavelenghts not directly observable from the ground-based telescopes. The HIFI instrument is an heterodyne receiver which provides spectroscopy in the continuos frequency range 480-1250 GHz (625-240 microns) and in the frequency range 1410-1910 GHz (213-157 micron).

The Heterodyne spectometer

At the sub-millimeter frequencies no amplifier exist and heterodyne tecniques are necessary. The Heterodyne instrument converts a high frequency signal to lower frequency, by mixing the incoming frequency with a similar frequency generated by a local oscillator (LO). The lowest frequencies (480-1250 GHz) of the HIFI spectral range are divided in 5 mixers based on the Superconductor-Insulator-Superconductor (SIS) technologies. The highest frequencies (1410-1910 GHz) are separated in 2 bands and incorporate Hot-Electron Bolometer (HEB) mixers. For each band two mixers exist, both for the horizontal and vertical polarization of the sky signal. The mixed frequency is called 'Intermediate Frequency' (IF) and it is related to the observed signal by the following equation: |F_{signal}-F_{LO}|. This quantity can be amplified and detected by the HIFI double sideband system (Fig.1).
For the 5 lower bands the bandwidth is 4GHz. The Upper Side Band (USB) extends from F_{LO}+4GHz to F_{LO}+8GHz, while the Lower Side Band (LSB) ranges between F_{LO}-4GHz and F_{LO}-8GHz. The higher mixer bands provide a bandwidth of 2.4GHz.

Fig.1 - Double Sideband System
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HIFI Sub-Systems

HIFI consists of five hardware (or sub-systems): the Local Oscillator Unit (LOU), the Instrument Control Unit, the Wide Band and the High Resolution Spectometers, and the Focal Plane Sub-System. The LOU generates a spectrally pure signal (24-36GHz) and manipulates it to obtain the desidered frequency for the LO.
The Wide Band and High Resolution Spectometers have a warm check unit controlled by the single Instrument Control Unit interconnected with the satellite control system. In the Focal Plane sub-system there is the Focal Plane Unit (FPU, Fig.2) located on the optical bench in the Herschel cryostat. In the Common Optics Assembly of the FPU, 6 mirrors relay the signal into a diplexer box (Fig.3). Successively the signal is splitted in 7 beams by using 7 sets of 3 mirrors (Fig.4). Each beam is then combined with its corresponding local oscillator, creating two linearly polarized (Horizontal and Vertical) beams per channel. Each of the 14 beams is then combined with the local oscillator and amplified into a Mixer Sub-Assembly.

Fig.2 - HIFI Focal Plane Unit
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Fig.3 - HIFI Beamsplitter
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Fig.4 - Channel
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HIFI spectometers

HIFI Documentation.

The Herschel Observers' Manuals (Version 2.1) provides general information about the Herschel Observatory.
The HIFI Observers' Manual (Version 2.3) describes the instrument characteristics, the available observing modes, the HIFI scientific capabilities, how to perform observations and an overview of the data processing.
For an overview of the HIFI instrument see the paper.
Click here to download the HIFI flyer.

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