The Herschel scientific instrument complement comprises three instruments, two cameras (PACS and SPIRE) with additional imaging spectroscopy capabilities, and a very high-resolution heterodyne spectrometer (HIFI). The instruments were provided by three consortia led by their respective Principal Investigator.

Heterodyne Instrument for the Far Infrared (HIFI)

HIFI is a very high-resolution heterodyne spectrometer. The heterodyne detection principle involves translating the frequency range of the astronomical signal being observed to a lower frequency where it is easier to perform the required measurements. This is done by mixing the incoming signal with a very stable monochromatic signal, generated by a local oscillator, and extracting the difference frequency for further processing. HIFI observes in seven bands covering 480 to 1910 Ghz, or the wavelength range 157-625 µm. Bands one to five, which give continuous coverage from 480 to 1250 GHz, use superconductor-insulator-superconductor (SIS) mixers. Bands six low and six high cover 1410 to 1910 GHz and use hot electron bolometer (HEB) mixers.

The difference signal from the heterodyne process is passed to the instrument spectrometers housed in the service module.

There are four HIFI spectrometers, two Wide-Band acousto-optical Spectrometers (WBS) and two High Resolution autocorrelation Spectrometers (HRS). One of each spectrometer type is available for each polarization (vertical and horizontal polarization). All spectrometers can be used either individually or in parallel.

Photodetector Array Camera and Spectrometer (PACS)

PACS is an imaging photometer and integral field line spectrometer.  The instrument comprises two sub-instruments which offer the two basic and mutually exclusive modes:

In imaging dual-band photometry mode, PACS images a field of view of 1.75 × 3.5 arcminutes simultaneously in two bands, one of either 60 - 85 µm or 85 - 125 µm together with 125 - 210 µm, with full beam sampling in each band. The detectors for this mode are two bolometers arrays.

In integral field spectroscopy mode, PACS performs spectroscopy between 51 and 220 µm over a field of view of 47 × 47 arcseconds, resolved into 5 × 5 pixels. An image slicer employing reflective optics is used to re-arrange the two-dimensional field-of-view along a 1×25 pixels entrance slit for a grating. This PACS mode provides a resolving power between 1000 and 4000 (i.e. a spectral resolution of about 75-300 kms^-1) depending on wavelength, with an instantaneous velocity coverage of about 1500 kms^-1.  The detectors for this mode are two germanium/gallium photoconductor arrays.

Spectral and Photometric Imaging Receiver (SPIRE)

SPIRE comprises a three band imaging photometer and an imaging Fourier transform spectrometer. SPIRE employs arrays of spider-web bolometers with neutron transmutation doped (NTD) germanium temperature sensors as its detectors.

The photometer images a 4 × 8 arcminute field of view on sky in three bands simultaneously.

The photometer has three observing modes:

• Point source photometry
• Small area map; for sources or area with diameters smaller than 5 arcminutes
• Large area map; for covering large areas of sky or extended sources larger than 5 arcminutes across

The SPIRE spectrometer is based on the Mach-Zehnder configuration. One input port receives the incoming beam from the telescope while the second port accepts a signal from a calibration source. The two output ports each have a detector array, one for 194-313 µm (37 detectors) and the other for 303-671 µm (19 detectors). The spectrometer is operated in continuous scan mode. The spectral resolution can be adjusted in the range between 0.04 and 0.8 cm^-1, corresponding to λ/Δλ of 50 to 1000 at 250 µm. The SPIRE spectrometer has a circular field of view 2.6 arcminutes across.

Introduction