The Infrared Space Observatory (ISO) is an astronomical satellite, which will operate at infrared wavelengths (2.5 to 200 microns) for a period of at least 18 months. Imaging, spectroscopic, photometric and polarimetric observations will be obtained by four scientific instruments in the focal plane of its 60-cm diameter, cryogenically-cooled telescope. Two-thirds of ISO's observing time will be available to the astronomical community. ISO is a fully approved and funded project of the European Space Agency (ESA) with a foreseen launch date of May 1993

Kessler, M. F.

Sibille, F.

English

NASA Technical Reports Server

N90-21340
INTRODUCTION TO THE INFRARED SPACE OBSERVATORY (ISO)
M. F. Kessler 1 and F. Sibille 2
1 Space Science Department of ESA, Noordwijk, the Netherlands.
20bservatoire de Lyon, Saint Genis Laval, France.
Abstract: The Infrared Space Observatory (ISO) is an astronomical satellite, which will op-
erate at infrared wavelengths (2.5 - 200pro) for a period of at least 18 months. Imaging,
spectroscopic, photometric and polarimetric observations will be obtained by four scientific in-
struments in the focal plane of its 60-cm diameter, cryogenically-cooled telescope. Two-thirds
of ISO's observing time will be available to tile astronomical community. ISO is a fully approved
and funded project of the European Space Agency (ESA) with a foreseen launch date of May
1993.
1. Introduction
IRAS, during its brief 10-month in-orbit existence, provided astronomers with their first un-
obscured glimpse of the infrared universe. Such was the wealth of the data - catalogues, sky
maps and spectra - of this highly successful mission that now, five years later, they are still
being digested, interpreted and producing fresh science. With the end of the IRAS mission, due
to depletion of its liquid-helium coolant, observations of the majority of these sources are no
longer possible. The Infrared Space Observatory (ISO, shown in figure 1) will give astronomers
the capability of routinely making high sensitivity observations at all these wavelengths, but
for detailed study of individual objects rather than as a survey mission. Compared to It{AS,
ISO will have higher sensitivity - in part from improvements that have been made in detectors
since IRAS, wider wavelength coverage, better angular resolution, a longer lifetime and more
sophisticated instruments.
ISO results from a mission proposM submitted to ESA in 1979. After various assesslnents and
studies, particularly a phase _ study in 1981-2, ISO was chosen in March 1983 to be the next
new start in the ESA Scientific Programme. The focal plane instruments were selected in mid-
1985 and tile industrial phase B study of the spacecraft started ill December 1986. Currently,
ISO is in its main development phase (C/D).
This paper presents, firstly, an overview of the ISO spacecraft (section 2) and mission operations
(section 3) and secondly, a synthesis of the scientific capabilities (section 4) and detector systems
of the four instruments (section 5).
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2. Overview of Spacecraft
The ISO satellite (5.3 m high, 2.3 m wide and 2400 kg at launch) is dominated by its so-called
"Payload Module", the upper cylindrical part seen in figure 1. This module, shown in section in
figure 2, is essentially a large cryostat, containing the telescope and the scientific instruments.
Inside the vacuum vessel is a toroidal tank filled with about 2300 litres of superfluid helium,
which will provide an in-orbit lifetime of at least 18 months• Some of the infrared detectors
are directly coupled to this helium tank and are at a temperature of around 2 K. Apart from
these, all other units are cooled using the cold boil-off gas from the liquid helium. This gas is
first routed through the optical support structure, where it cools the telescope and the scientific
instruments to temperatures of 3-4 K. It is then passed along the baffles and radiation shields
before being vented to space. A small auxiliary tank, containing about 60 litres of normal liquid
helium, fulfils all of ISO's cooling needs for tile last 72 hours before launch. Mounted on tile
outside of the vacuum vessel is a sunshield, which prevents the sun from shining directly on the
cryostat. The solar cells are carried by this sunshield.
• " -O-
O"
Figure 1: Computer Graphic of lSO in Orbit
338
The "classical"spacecraftfunctionsareprovidedby a so-called"ServiceModule", which can
beseenunderneaththe payloadmodulein figure1. Thesefunctionsincludethe structureand
the loadpath to the launcher,the solararraymountedon the sunshield,and sub-systemsfor
thermal control, data handling,powerconditioning,telemetryand telecommand(using two
antennas),and attitude andorbit control. Thelast providesthe three-axisstabilisationto an
accuracyof a few arc secondsand alsothe rasterpointingfacilitiesneededfor the mission.It
consistsof sun and earthsensors,star trackers,a quadrantstar sensoron the telescopeaxis,
gyros,reactionwheelsand usesa hydrazinereactioncontrol system.The nominal down-link
bit rate is 33kbpsof whichabout 24kbpsarededicatedto the scientificinstruments.
Thedefinitionstudy(phaseB) for the ISOspacecraftwassucessfullycompletedearlyin 1988.
The detaileddesign,development,integrationandtest phase(phaseC/D) wasstartedon 15
March1988by an industrial consortiumled by Aerospatiale(F). The structureof this teamis
shownin figure3. Thenext major milestoneis the System Design Review, to be held in April
1989, with the aim of releasing (i) the structural and thermal model for integration and (ii) the
qualification model units for manufacture. The scheduled launch date is May 1993.
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339
3. Launcher, Orbit and Operations
ISO will be launched into a transfer orbit by an Ariane-4 vehicle. Its hydrazine reaction control
system will then be used to attain the operational orbit, which has a 24-hour period, a perigee
height of 1000 km and an apogee height of 70000 km. The inclination to the equator will
be between 5 ° and 20 ° (to be finalised later). ESA plans to supply only one ground station,
enabling ISO to make astronomical observations during the best 14 hrs of each orbit. The
mission's scientific return could be greatly increased by the addition of a second ground station,
which would permit ISO to be operated for the entire time that it spends outside the main part
of the Earth's radiation belts. An international collaboration is being sought by ESA to provide
this second ground station.
Observing time on ISO will be allocated on a "per object" basis, as was the case for EXOSAT,
rather than on a "per shift" basis as is done with IUE. Two-thirds of ISO's observing programme
will be determined by the scientific community via the submission and selection (by peer review)
of proposals. The remaining time will be reserved for the groups who provide the instruments,
for the Mission Scientists and for the Observatory Team who operate the satellite. Details
of observations to be carried out in this reserved time will be included in the first Call for
Observing Proposals, to be issued to the scientific community 18 months before launch.
The in-orbit operations of the spacecraft and instruments will be carried out by a team of sci-
entists and engineers located at the ISO Control Centre in Villafranca, Spain. During scientific
use, the satellite will always be in contact with the ground segment; however, it is planned to
minimise real time modifications to the observing programme in order to maximise the overall
efficiency of the satellite. A "quick-look" output, adequate for judging the scientific quality
of the data, will be produced within a few hours of an observation being completed. A final
product with more detailed data reduction and calibration will be supplied later. This product
will be the one from which the guest observers make their astronomical analyses.
4. Scientific Capabilities of Instruments
The ISO scientific payload consists of four instruments: a camera (ISOCAM, PI: C.J. Cesarsky),
an imaging photo-polarimeter (ISOPHOT, PI: D. Lemke), a long wavelength spectrometer
(LWS, PI: P.E. Clegg) and a short wavelength spectrometer (SWS, PI: Th. de Graauw). Each
instrument is being developed by a consortium of institutes, using national funding, under
the authority of the Principal Investigator (PI). The scientific capabilities, provided jointly by
these instruments, can be summarised in three categories: photometry/polarimetry, imaging
and spectroscopy.
Photometry and Polarimetry will be possible in broad and narrow spectral bands across
ISO's entire wavelength range from 2.5 to 200pm. Work using multiple apertures will be possible
out to ll0pm.
340
Suspended in the middle of the main helium tank is the telescope, which has a Ritchey-Chr_tien
configuration with an effective aperture of 60 cm and an overall f/ratio of 15. A weight-relieved
fused-silica primary mirror and a solid fused-silica secondary mirror have been selected as
the telescope optics. The optical quality of these mirrors is adequate for diffraction-limited
performance at a wavelength of 5 #m. Stringent control of stray light, particularly from bright
infrared sources outside the telescope's field of view, is necessary in order to ensure that the
system sensitivity is not degraded. This control is accomplished by imposition of viewing
constraints and by means of the sunshade, tile cassegraln and main baffles, and an additional
light-tight shield around the instruments.
SUNSHADE _l/\ ['_'-, - CRYO COVER
00T,CALW,N00WS
PYRAMID MIRROR '"I _ QUADRANT STAI_ SENSOR
Figure 2: Schematic of the Payload Module
The scientific instruments are mounted on the opposite side of the optical support structure
to the primary mirror, each one occupying an 80 ° segment of the cylindrical volume available.
The 20 arc minute total unvignetted fieht of view of the telescope is split up between the
four instruments by a pyramidal mirror. Thus, each instrument simultaneously receives a 3 arc
minute unvignetted field centred on an axis at an angle of 8.5 arc minutes to the telescope optical
axis. To view the same target with different instruments, the satellite has to be repointed.
341
ISOwill becapableof imaging in broadandnarrowspectralbandsacrossits entirewavelength
rangefrom 2.5to 200#m.The numberof pixelsand the pixel field of view vary asa function
of wavelength.Detailsaregivenin table 1 below.
WavelengthRange No. of Pixels Pixelf.o.v.
(arc secs)
2.5-5
5-17
18-28
30-60
60 120
120-200
32x32
32x32
8x8
3x5
3x3
2x2
1.5, 3, 6, 12
1.5, 3, 6, 12
19
31
44
90
Table 1: ISO's hnaging Capability.
For Spectroscopy, various resolutions will be available at different wavelengths. Details are
given in table 2 below.
Wavelength Range Resolution Instrument
(itm) (A/AA)
2.5-12
2.5-16.5
2.5-45
14-30
45 180
45-180
90
50
2000
20000
200
10000
ISOPIIOT-S
ISOCAM-CVF
SWS-Grating
SWS-FP
LWS-Grating
LWS-FP
Table 2: ISO's Spectroscopic Capability.
In principle, only one of the four instruments will be operated at a time; however, when ISOCA M
is not the main instrument, it can be used in a so-called "parallel" mode either to gain extra
astronomical data or to assist the other instruments in acqniring and tracking their targets.
Whenever possible during satellite slews, ISOPItOT will be operated in its normal 200#m mode
so as to make a partial sky survey ("serendipity mode") at these wavelengths which were not
covered by IRAS.
5. Outline Description of Scientific Instruments and Detector Systems
The ISOCAM instrument consists of two optical channds, each with a 32 x 32 element detector
array, operating, respectively, in the wavelength ranges 2.5-5ttm and 5-17#m. Each channel
contains a wheel for selecting various filters (including circular variable filters, CVF) and a
second wheel for choosing a pixel field of view of 1.5, 3, 6, or 12. Polarisers are mounted on a,n
entrance wheel common to both channels. A sixth wheel carries mirrors for selecting between
the channels, of which only one is operational at a time.
342
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Table 3: Baseline ISO Detector System,s
343
The ISOPHOT instrument consists of four sub-systems:
- ISOPHOT-C: a photopolarimeter which also provides imaging capability at close to the
diffraction limit in the wavelength range from 30#m to 200#m,
- ISOPHOT-P: a multi-band, multi-aperture photopolarimeter for the wavelength range from
30#m to ll0#m,
ISOPHOT-A: an multi-band array of 8 x 8 discrete elements for the wavelength range from
,_12/zm to 28#m,
ISOPHOT-_: a dual grating spectrophotometermeter which simultaneously provides a resolv-
ing power of ,-,90 in two wavelength bands 2.5-5pm and 6-12_um.
A focal plane chopper with a beam throw of up to 3' is also included in ISOPHOT.
The LWS instrument consists of a reflection diffraction grating used in two orders with an array
of discrete detectors to provide a spectral resolving power of ,-,200 over the wavelength range
from 45/zm to 180#m. Either of two Fabry-P6rot interferometers can be rotated into the beam
to increase the resolving power to ,,_104 across the entire wavelength range.
The SWS instrument provides a resolving power of between 1000 and 2000 across the wavelength
range from 2.4#m to 45#m by means of two reflection diffraction gratings used in 1st, 2nd and
3rd orders. Filters for order-sorting are placed at the instrument's various entrance apertures.
Over a part (14-30#m) of the SWS's operating range, the resolution can be increased to
,-_2 x 104 by directing the incident radiation through either of two Fabry-P6rot interferometers.
Each of the instruments contains a variety of detector and read-out systems from various man-
ufacturers. A summary is given in table 3 below; further details may be found in companion
papers in this volume.
The first models of all the instruments (alignment, mass and thermal model) have been com-
pleted and are either already delivered to ESA or are undergoing final checks (vibration testing
at liquid-helium temperatures) prior to delivery. The production of the second instrument
models (engineering qulaification model) is well advanced and the due date for delivery to ESA
is spring 1990.
6. Conclusion
ISO is a fully-approved and funded mission, which will offer astronomers unique and unprecen-
dented observing opportunities in the infrared spectral region for a period of at least 18 months.
Both the spacecraft and its selected complement of instruments are in their main development
phase and the scheduled launch date is May 1993.
344


Introduction to the Infrared Space Observatory (ISO)

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