HIFI

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45
HERSCHEL-HIFI: THE HETERODYNE INSTRUMENT FOR THE FAR-INFRARED
Th. de Graauw
1
,
2
and F.P. Helmich
1
on behalf of the HIFI consortium
1
Space Research Organization Netherlands, P.O.Box 800, 9700 AV Groningen, The Netherlands
2
Kapteyn Astronomical Institute, P.O.Box 800, 9700 AV Groningen, The Netherlands
Abstract
The science requirements for and the capabilities of
the Heterodyne Instrument for the Far-Infrared (HIFI)
are given. It is shown that the HIFI design can satisfy the
science requirements and will be able to do, unhindered
by telluric lines and varying atmosphere, many unique ob-
servations.
windows, whereas the thermally stable location of L2, im-
plies that calibration can be better than ever achievable
from the ground.
All these factors together stress the need for a cryo-
genicspace-borneheterodyneinstrumentwithalargespec-
tral working range. Here, we will list the science require-
ments on the instrument and its compliance with these.
The conceptual design, its operating modes and its ex-
pected sensitivity will be given.
Key words: Missions: Herschel – Instrumentation: spec-
trographs – Techniques: spectroscopic – Infrared: general
– Radio lines: general – Submillimetre
2. Science Objectives
1. Introduction
A number of key scientific themes of modern astrophysics
is related to understanding the cyclical interrelation of
stars and the interstellar medium of galaxies (Fig. 1). On
the one hand, stars and planetary systems are formed
throughthe gravitationalcollapse of interstellarmolecular
clouds, on the other hand, the interstellar medium is for-
med from the ashes, enriched by newly nucleo-synthetised
elements, of dying stars. This complex interplay between
stars and the ISM drives the evolution and, thus, deter-
mines the observational characteristics of the Milky Way
and other nearby and far away galaxies.
The unifying aspect of these research areas is the pres-
ence of copious amounts of cool molecular gas and dust,
which reprocessesessentially all radiation from central ob-
jects to far-infrared (FIR) and sub-millimetre (sub-mm)
wavelengths. Furthermore, most molecules possess many
rotational transitions in the sub-mm range. Here the im-
portant light hydride molecules are often uniquely observ-
able, as are the fine-structure lines of atoms, ions, and
their isotopes. These transitions providean effective probe
of the physical conditions (density, temperature) of the
emitting gas. A wide spectral coverage is needed for such
studies because it allows the simultaneous analysis of a
large number of transitions of a species and its isotopes,
thereby confirming its identification and determining its
abundance. Such studies will also be greatly assisted by
the complete absence of telluric absorption and good rel-
ative calibration, which are prerequisites for accurate de-
terminations of line ratios. In particular there is a need
for space-borne observations to detect many transitions of
H
2
O, since this molecule plays such a dominant role in
the energy balance as well as the chemical evolution of a
wide variety of objects and is almost unobservable due to
telluric emission.
Over the last decades there has been a growing use of
the submillimetre region as the region where physical and
chemical processes in dusty places can be best studied.
Starting from the millimetre region where the lowest ro-
tational line of CO lies up to about 1 THz, observations
of many molecules are possible and nowadays common
practice. However, the submillimetre wavelength region,
as viewed from the Earth’s surface, only consists of small
windows in which the transmission varies between one to
zero, with generally less transmission at higher frequen-
cies. At these frequencies, atomic and ionic fine-structure
lines and a plethora of rotational molecular lines (in par-
ticular those of light hydrides) can be found. Together,
these lines form excellentprobes of the physicaland chem-
ical conditions in many regions, varying from places of
planet formation to the surroundings of nuclear engines of
active galaxies.
The advancements in heterodyne mixing devices have
provided low-noise, high-gain instrumentation with a nat-
ural, very high spectral resolution. This is of prime impor-
tance when to avoid line confusion, within the expected
forests of lines, and to study dynamically evolving regions.
Ground-based observatories have the advantage of large
mirrors and/or the possibility of interferometric observa-
tions. Space-borne observations will, however, have the
advantage that e.g. water can be studied in astrophysi-
cally important environments, which is impossible from
even the best submillimetre sites. The absence of telluric
lines allows for spectral line-surveys in the whole submil-
limetre region instead of the relatively small atmospheric
Proc. Symposium
‘The Promise of the Herschel Space Observatory’
12–15 December 2000, Toledo, Spain
ESA SP-460, July 2001, eds. G.L. Pilbratt, J. Cernicharo, A.M. Heras, T. Prusti, & R. Harris
46
Th. de Graauwet al.
the originandevolutionof the generalISM, galactic nuclei
to nearby and distant dusty galaxies.
HIFI willdirectlyaddressawide rangeofkeyquestions
including the following:
Figure 1. Gas and dust are cycled through interstellar matter
in the Milky Way, a variety of physical and chemical processes
continuously drive the chemical evolution of the Galaxy; small
photographs adapted from D. Malin and IPAC.
1. HIFI will probe the physics, kinematics and energet-
ics of star forming regions through their cooling lines,
including H
2
O, the major coolant.
2. HIFI will survey the molecular inventory of such di-
verseregionsasshockedmolecularclouds,densePhoto-
Dissociation Regions (PDRs), diffuse atomic clouds,
HotCoresandproto-planetarydisksaroundnewlyform-
edstars,windsfromdyingstarsandtoroidsinteracting
with AGN engines.
3. HIFI is particularly suited to search for low-lying ro-
vibrationaltransitionsofcomplexspeciessuchasPAHs
and,thus, to investigatethe originand evolutionof the
molecular universe.
4. HIFIcanprovidetheout-gassingrateofcometsthrough
H
2
O rotational lines and determine the vertical distri-
bution of H
2
O in the giant planets.
5. HIFI can measure the mass-loss history of stars which,
ratherthan nuclearburning,regulatesstellar evolution
after the main sequence, and dominates the gas and
dust mass balance of the ISM.
6. HIFI will measure the pressure of the interstellar gas
throughout the Milky Way and will resolve the prob-
lem of the origin of the intense galactic [CII] 158
In addition, as confirmed by COBE (Cosmic Back-
ground Explorer), the powerful FIR emission of distant
dusty galaxies is red-shifted to sub-mm wavelengths.
Many studies also require the very high spectral reso-
lution permitted by heterodyne instruments as this is es-
sential to overcome spectral confusion and line blending.
Moreover, when studying dynamically evolving regions,
velocity information is often indispensable in unravelling
the contributions from the multiple components present.
This resolution is also needed for the detailed study of
planetary atmospheres and cometary material.
These science driverscanbe translatedin the following
requirements on HIFI:
–
HIFI should have high (10
6
−
7
) spectral resolution to
overcome spectral confusion and line blending, and to
preserve and detect the kinematic information in line
profiles
–
HIFI should have a large instantaneous bandwidth to
measure lines from extra-galactic objects in one fre-
quency setting and to allowfor fast spectral surveys
–
HIFI should have state-of-the art mixing devices or
better to detect faint (isotopic) lines
–
HIFI shouldbe ableto measureatleastupto 1910GHz
(158
µ
m
emission measured by COBE.
7. HIFI can determine the
12
C/
13
C, and
14
N/
15
Nisotope
ratiosas a function of galactic radiusin the Milky Way
and other galaxies, through the hyperfine splitting of
atomic fine-structure lines. One can thus constrain the
parameters of the Big Bang and explore the nuclear
processes that enrich the ISM.
8. HIFI will measure the FIR line spectrum of nearby
galaxies as templates for distant, possibly primordial
galaxies.
3. The HIFI instrument
3.1. Introduction
m) to measure the [C II] line within the Milky
Way and in nearby galaxies
Suchaninstrumentnotonly wouldfulfill these require-
ments but is also particularly apt for studies ranging from
collapsing molecular clouds forming newstars and plan-
etary systems, stellar winds associated with dying stars,
µ
The Heterodyne Instrument for the Far-Infrared, HIFI,
has been optimised to address the astronomical key ques-
tions discussed above which all require high spectral re-
solvingpowersandhighsensitivity.By combiningthe high
spectralresolvingpowerof the radioheterodyne technique
with quantum noise limited detection from superconduc-
tor physics and the state-of-the-art in microwave technol-
ogy, it is nowpossible to develop an instrument with the
following capabilities:
–
continuous frequency coverage from 480 to 1250 GHz
in five bands, while a sixth and seventh band will pro-
vide coverage for 1410-1910 GHz,
–
resolving powers up to 10
7
(300 - 0.03 km/s)
Herschel-HIFI: The Heterodyne Instrument for the Far-Infrared
47
–
detection sensitivity within a factor 3 of the theoretical
quantum noise limit. Both polarisations of the astro-
nomical signal will be detected for maximum sensitiv-
ity.
–
calibration accuracy within 10%, with a goal of 3%
In order to cover the wide frequency range with high
sensitivity, HIFI is designed to have 7 mixer bands and
14 LO subbands. The first five frequency-bands will each
contain a pair of mixers using superconductor-insulator-
superconductor(SIS)tunneljunctions.Channels6Lowand
6High will contain two mixers based on the recently de-
veloped fast hot-electron bolometers (HEB). The instru-
ment will operate at one frequency at a time in both po-
larisations; i.e. only one of the frequency bands will be
active. For the Local Oscillator (LO), solid-state varac-
tor/varistor frequency multipliers will be used. HIFI will
have an instantaneous bandwidth of 4 GHz analysed in
parallelby two types of spectrometers:acousto-opticspec-
trometers and autocorrelators.
resolutionof1MHzanda bandwidthof4 GHzfor each
of the two polarisations. It is located in the SVM.
4. The high-resolution spectrometer sub-system (HRS)
consists of a pair of auto-correlator spectrometers and
will provide several combinations of bandwidth and
resolution. It is also located in the SVM.
5. Theinstrumentcontrolunitsub-system(ICU), located
in the SVM, interprets commands from the satellite
tele-command system, controlsthe operation of the in-
strument, and returns science and housekeeping data
to the satellite telemetry system.
Inordertomaterializethe highsensitivityofthe super-
conducting mixers a high opto-mechanical, thermal and
electrical stability, throughout the whole instrument is re-
quired.
3.2. Focal Plane Sub-System
In order to cover the challengingly wide frequency range,
the FPU employs a highly modular design consisting of:
–
a common optics assembly (COA) which contains the
optical elements which are common to the 7 optical
beamsandservesasthe supportstructureforthechop-
per, calibration assembly, and local oscillator optics
–
7 mixer assemblies (MA) containing the optical ele-
ments, mixers and IF components specific to each of
the 7 frequency bands.
Figure 2. Block diagram of the HIFI instrument
HIFI consists therefore of five major sub-systems:
1. The focal-plane sub-system comprises the focal-plane
unit (FPU) inside the cryostat and the FPU control
unit (FCU) located in the service module (SVM). The
FPU contains a.o. mixers and low-noiseIF HEMT pre-
amplifiers.TheFCU suppliesthe biasvoltagesfor mix-
ers and IF preamplifiers in the FPU and controls the
FPU.
2. The local oscillator sub-system comprises the Local
Oscillator Unit (LOU) located on the outside of the
cryostat,theLocalOscillatorreferencefrequencySource
Unit(LSU)andaLocalOscillatorControlUnit(LCU),
both located in the service module. The LOU gener-
ates the LO signals which are coupled into the FPU
via windows in the cryostat wall.
3. The wide-band spectrometer sub-system (WBS) con-
sists of a pair of AOS spectrometers with a frequency
Figure 3. View of the common optics assembly in the instru-
ment
48
Th. de Graauwet al.
Acalibrationassemblyislocatedjustneartheentrance
opening of the common optics assembly. It provides two
black body sources with adjustable temperature in the
range 15-100 K. The end-to-end calibration of the system
including the telescope will be accomplished by observa-
tion of astronomical sources of known strength. The ac-
curacy achieved will depend upon pointing accuracy but
should be better than 10%. The goal is to achieve an in-
strumental calibration accuracy of 3%.
The LO beams are coupled through vacuum windows
in the Herschel cryostat wall and directed into the respec-
tive MA’s by a set of folding mirrors. We have chosen to
use 7 separate sub-windows, each optimised for the trans-
mission of its LO band. The Local Oscillator Unit (LOU)
itself is located outside the dewar at an optical distance
of more than 650 mm from the HIFI FPU.
Figure 4. View of the beams in the common optics assembly
3.3. Mixer Assemblies
The Common Optics Assembly (COA) contains the
opticsfrom mirrorM3 in the telescopefocal planethrough
to but excluding the Mixer Assemblies (MA). The COA
mechanical structure provides also the support for the
chopper, the calibration assembly and some optics for the
local oscillators. The telescope focal plane mirror (M3)
acts as a folding mirror. The telescope focal plane is re-
imaged in the main optics by means of a Gaussian tele-
scope at unit magnificationimplemented by anOffner sys-
tem. Between its two mirror sections a flat chopper mirror
is positioned in the pupil plane. After the imaging mirror
a flat mirror folds the beam towards a stack of 7 field-
splitting mirrors placed at an image of the focal plane.
These 7 mirrors differ in orientation so that the seven re-
sulting beams are separated in direction creating seven 50
mm equidistant optical axes for the 7 mixer assemblies.
The focal length of the individual band splitting mirrors
can be chosen to alter the system exit pupil location while
keeping the focal plane image in the same position.
Finally, the beams from the band splitting field mir-
rors (7x) are re-imaged into the mixer assemblies, which
are mounted onto the Main Optics structure in a stack.
The housing of the COA and the mirrorswill be machined
from a single block of aluminium giving rigidity and di-
mensional stability.
Most observations with HIFI will be made using beam
switching. A focal-plane chopper within the instrument
will switch the telescope beam between the astronomical
source and a nearby reference position. The focal-plane
chopper in HIFI will have a maximum beam throw (sep-
aration between source and reference position) of 3 arc-
minutes on the sky and will chop at frequencies up to 5
Hz. The mechanism moves a mirror between four posi-
tions: two positions on the sky and two calibration posi-
tions. The required rotation for the present optical design
is 4.6
◦
.
The HIFI mixers are located in Mixer Assemblies (MA).
There will be 7 MA’s, each covering a certain frequency
range with two mixers, yet only one MA will operate at
any time. The pair of mixers in individual MA’s will oper-
ate at orthogonalpolarisation.The MA’s contain mechan-
ical supports, mixers, diplexers as well as IF amplifiers,
and are mechanically mounted on the FPU. The optical
input to an MA consists of a signal beam and a LO beam.
In the MA box the signal beam will be split into 2 polar-
isations for the 2 mixers. The LO beam will also be split
into 2 beams with suitable linear polarisation to be cou-
pled to the mixers. The combining of the signal and LO
beams will be done by a beam-splitter for the two lowest
frequency bands, and by tuneable diplexers in the higher
bands where less LO power is available. This gives rise to
two different optics layouts for the MA boxes, but they
will be identical externally.
Each mixer is followed by a dedicated IF preamplifier
at an IF centre frequency of 6 GHz and a bandwidth of 4
GHz. Sinceonlyonepairofmixerswill operateatanytime
power combiners are used to feed the signals from bands 1
to 4 into a single pair of coaxial cables, as is also done for
the signals from bands 5, 6 and 7. The choice of band is
made by activation of the required pair of preamplifiers.
The first stages of the IF preamplifiers are to be cooled to
15 K as well as the second stages.
Mixer technologies for fabricating sensitive heterodyne
mixers favour the use of wave-guide mixers for the lower
frequencybands,while thehigherfrequencieswilluselens-
es and planar antennas such as double slot lines. However
both solutions are compatible with the chosen mechani-
cal and optical configurations. The HIFI frequency bands,
sensitivities and foreseen mixer elements are given in Ta-
ble 1.
Herschel-HIFI: The Heterodyne Instrument for the Far-Infrared
49
contain two LO multiplier chains and their feeding
power amplifiers/triplers
–
The LO control electronics (LCU), which is sited in
the SVM and which also supplies the electrical and
microwave signals needed by the LOU, monitors the
LO system, and reports its status to the ICU.
The LOA’s are arranged with a spacing of 50 mm be-
tween the optical axes of adjacent LOA’s yielding one op-
tical plane per mixer assembly. The LOA’s are mounted
onto the LOU mechanical support structure.
Figure 5. View of a mixer assembly
Table 1. HIFI DSB base line receiver noise temperature for the
probable mixer types. The last column indicates the baseline
mixer type: WG - waveguide, QO - quasi-optical.
Band Range
T
noise
Mixer Mixer
(GHz) (DSB) (K) Technology Type
1
480
70
Nb-SIS WG
640
110
2
640
110 NbTiN-SIS WG
800
150
3
800
150 NbTiN-SIS WG
960
190
Figure 6. Block diagram of LO signal generation for the 7 LO
bands. The source unit and control unit are located in the SVM.
4
960
190 NbTiN-SIS WG
1120
230
5
1120
230 NbTiN-SIS QO
1250
510
6Low 1410
650 NbN-HEB QO
1700
780
3.4.1. Multipliers
6High 1700
790 Nb-HEB QO
1920
870
The LOA’s consist of optics with a focusing/re-imaging
mirror and a wire grid beam combiner and two almost
identical LO sources oriented with orthogonal polarisa-
tions. Together these two sources are to provide full fre-
quency coverage of a particular mixer band. The chosen
arrangement of two LO sources pumping two mixers on
orthogonal polarisation can be implemented using polar-
ising beam splitters. Note that only one LO source is op-
erational at any given time and that this LO pumps two
mixers operating on orthogonal polarisation.
The LO frequency bands are given in Table 2. The
listed tuning ranges can be achieved when a broadband,
high-power mm-wave sourceis used to drive the frequency
multiplierchain(Huangetal.1997).High-powermm-wave
amplifiers, have been successfully applied in LO sources
at 100 GHz. The demonstrated output powers of over 400
mWinthe75-115GHzfrequencyrangeexceedbyfarwhat
is available with Gunn devices.
3.4. Local Oscillator Sub-System
A block diagram of the LO signal generation is given in
Fig. 6. The local oscillator sub-system (LO S/S) consists
of the following units:
–
The LO Source Unit (LSU), which contains the stable
LO reference frequency source and whose output is
split over 14 waveguides,each feeding one of the chains
in the LOA’s with signals in the 26-41 GHz range.
–
The local oscillator unit (LOU), which is located out-
side the cryostat and supplies the LO signal for the
mixers inside the FPU. It consists of a mechanical sup-
port structure to support a cooler radiator and seven
LO multiplier assemblies. The 7 Local Oscillator As-
semblies (LOA) for bands 1-5, 6-Lowand 6-High, each
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