HIFI is the Heterodyne Instrument for the Far Infrared. It is designed to provide spectroscopy at
high to very high resolution over a frequency range of approximately 480-1250 and 1410-1910 GHz
(625-240 and 213-157 µm). This frequency range is covered by 7 mixer bands, with dual horizontal
and vertical polarizations, which can be used one pair at a time. The mixers act as detectors that
feed either, or both, the two spectrometers on HIFI. An instantaneous frequency coverage of 2.4
GHz is provided with the high frequency band 6 and 7 mixers, while for bands 1 to 5 a frequency
range of 4 GHz is covered. The data is obtained as dual sideband data which means that each
channel of the spectrometers reacts to two frequencies (separated by 4.8 to 16 GHz) of radiation at
the same time. The beam size (HPBW) ranges from 12 arcsec at 1900 GHz to 42 arcsec at 480
There are four spectrometers on board HIFI, two Wide-Band Acousto-Optical Spectrometers
(WBS) and two High Resolution Autocorrelation Spectrometers (HRS). One of each spectrometer
type is available for each polarization. They can be used either individually or in parallel. The WBS
covers the full intermediate frequency bandwidth of 2.4 GHz in the highest frequency bands (bands
6 and 7) and 4 GHz in all other bands at a single resolution (1.1 MHz). The HRS have variable
resolution from 0.125 to 1.00 MHz with subbands sampling up to half the 4 GHz intermediate
frequency range. Subbands have the flexibility of being placed anywhere within the 4 GHz range.
The HRS can be split up to allow the sampling of more than one part of the available range.
The overall HIFI frequency resolutions are summarised in Table 1 for the WBS and HRS in two of
Table 1. HIFI resolutions using the WBS and HRS in two of its modes.
Spectrometer observing Modes
The four spectrometers, one WBS and one HRS per polarization, may all be used simultaneously.
When all spectrometers are in use frame times are 4 seconds each. Shorter frame times are possible
when only one type of spectrometer is used (1 or 2 seconds).
Three Astronomical Observing Templates (AOTs) are available:
• Single Point:(AOT I) for observing science targets at one position on the sky;
• Mapping: (AOT II) for covering extended regions;
• Spectral Scanning: (AOT III) for surveying a single position on the sky over a continuous range
of frequencies selected within the same LO band by the user.
Each AOT can be used in a variety of different modes of operation, providing the widest range of
options for performing spectroscopic science observations in different astronomical that HIFI and
the Observatory will allow, in terms of reference measurements and calibration. In other words, the
three AOTs come with Observing Modes where the user may select from different calibration
The observing reference modes are:
Position Switch: with the telescope a single pixel HIFI beam is pointed alternately at a target
position and at a reference position. The reference position is usually chosen to be a nearby area
of the sky that is devoid of emission in the band being used.
Dual Beam Switch (DBS): in this mode an internal chopper mirror within HIFI is used to move
the beam to a reference Off position on the sky. The reference Off position can be set up to 3 arc
minutes away from the On-target position. Since moving the internal mirror changes the light
path for the incoming waves the possibility of residual standing waves exist. By moving the
telescope in such a way that the source appears in both (On-Off) chop positions, the impact of
standing wave differences is expected to be eliminated to a large extent. The low dead time in
moving a small distance with the telescope and in the internal chopper motion makes this mode
the most efficient and in particular for small astronomical targets. There are two chopper speeds.
The faster chop is available for observations for low spectral resolutions where effects of
instrumental drifts might be expected to distort baselines and increase noise.
Frequency Switch: in this mode, following an observation at a given On frequency, the local
oscillator frequency is changed by a small amount (a few tens of MHz). The shift in frequency
is small enough that the lines of interest remain observable at the two LO frequencies.
Effectively, therefore, this makes for a very efficient mode since target emission lines are
observed in both ON and OFF positions. Subtraction of the Off spectrum from the On means
that we remove the baseline, but significant ripples may still remain in the On – Off
Load Switch: in this reference scheme, an internal cold source is used as a reference. The
chopping mirror alternately looks at the target on the sky and an internal source of radiation.
This is particularly useful when there are no emission-free regions near the target that can be
used as reference in either dual beam switch or position switch mode or where frequency switch
cannot be used due to the frequency structure of the source. Since the optical path differs
between source and reference, a residual standing wave structure may remain. Addition of an
Off measurement of a (relatively distant) emission-free region can be used to reduce baseline
ripple. Such a scheme is robust but has relatively low efficiency.
Figure 4.1. Overview of available AOT observing modes. The numbering scheme of the observing
modes represents an association between the AOT class (in Roman numerals) and four possible
modes of reference treatment (Arabic numerals) that are foreseen. The dual beam switch modes
further split into two separate modes using a slow chopper speed (Mode I-2) and a fast chopper
speed (Mode I-2a).
Spectrometer Sensitivity Estimates
Figure 1 summarises the current status of measured mixer performance in each of the HIFI mixer
bands. These values are good for the currently measured best polarization for each band. Some
variation in sensitivity does occur across the IF frequency band and some deterioration of sensitivity
occurs towards band edges, notably for the situation where diplexers are used for beam combining
(bands 3 and 4).
Figure 1. Double sideband system temperatures of HIFI mixers. System temperatures are based on
a mixture of pre-FM ILT, FM-ILT and extrapolated FM-ILT results.
An overview of the expected sensitivities is given in Table 2. These are 5- values for an hour of
integration except for the frequency scans. Here 1- values are given for integration times of 4
hours or 10 hours of observing time spent per band for bands 1-5 or bands 6 L/H.
Table 2. Expected performance for the various HIFI bands as derived from the mixer unit tests and
the first Instrument Level Tests carried out in April/May 2006. For bands 5 and 6 only a few spot
frequencies have been measured so far.