Infrared astronomy with the ISO satellite1

Cryogenics 39 (1999) 125–133

Infrared astronomy with the ISO satellite1 D. Lemke

*

Max-Planck-Institut fu¨r Astronomie, Ko¨nigstuhl 17, 69117 Heidelberg, Germany Received 1 July 1998; received in revised form 1 July 1998; accepted 1 January 1999

Abstract Four versatile focal plane instruments made ESA’s Infrared Space Observatory capable of analysing the long wavelength radiation emitted by the cold and optically hidden universe. The instruments were operated at 1.8 to 3 K and were all equipped with numerous mechanisms, drives and sensor electronics. All systems worked until the end of the mission, which gave a wealth of scientific data for 29 months—11 months longer than anticipated. The early scientific highlights of the mission include contributions to the earliest stages of star formation, the star formation history in the universe and the discovery of the ubiquitous presence of water. Future cooled space observatories will follow up ISO’s scientific findings from the data analysis ongoing for the next few years and they will apply many of the technical innovations of this mission.  1999 Elsevier Science Ltd. All rights reserved. Keywords: Cryogenic mechanisms; Drives; Electronics; Infrared radiation

1. Technical challenges Infrared astronomy explores the cold universe. In order to detect fluxes as low as 10−18 W emitted from cool and distant objects like a collapsing cloud of interstellar matter as cold as 14 K, which will eventually form a new star, the detectors of an astronomical observatory have to be cooled to T ⬍ 2 K. This is sufficient to reduce the thermally generated noise in these extrinsic photoconductor devices. But also the whole observatory has to be cooled to avoid photon noise generated by the statistical arrival of background photons from a warm telescope. Cooling of the optics to T ⬍ 4 K is usually sufficient to reduce this noise below the natural sky background noise in the far infrared at wavelength ␭ > 100 ␮m for broadband imaging. Following the pioneering IRAS sky survey mission in 1983, the European Space Agency decided in 1985 to develop an Infrared Observatory, equipped with several instruments to fully analyse the radiation of cosmic objects. They were planned to carry out low and high resolution spectroscopy, photometry, polarimetry and imaging over a large wavelength range 2.5–240 ␮m, i.e. * Tel.: 0049-6221-528259; fax: 0049-6221-528246; e-mail: [email protected] 1 Revised version of a presentation at the “1998 Space Cryogenics Workshop, ESTEC, Noordwijk, NL, July 20–21, 1998”.

far beyond the IRAS limit of 110 ␮m. Such versatile observatory instruments require in particular cryomechanisms in order to change optical elements in the beam or to scan spectra. Dozens of new devices and techniques had to be developed in Europe to meet the challenge of ISO and its 4 scientific instruments: low noise electronics operational at 1.8 K, black paint with high absorption in the FIR without causing particle contamination, reliable electrical connections between thin wires and strange material, etc. to mention only a few. Numerous European institutes and industrial companies involved in the development of the ISO satellite have mastered all the difficulties. Thorough monitoring and careful preflight testing under ESA supervision have resulted in the outstanding success of the mission.

2. ISO—11 months overtime ISO was launched by a European Ariane 4 rocket on 17 Nov 1995. The mission was planned to be for 18 months, consequently the end, defined by the evaporation of the coolant, was expected in summer 1997. During the mission every new measurement of the remaining liquid helium on board had forecast an increased lifespan (Fig. 1). As a result hopes were high during a conference at the beginning of April 1998 that ISO would last until June. At the end of March 1998 fluctuations in tempera-

0011-2275/99/$ - see front matter  1999 Elsevier Science Ltd. All rights reserved. PII: S 0 0 1 1 - 2 2 7 5 ( 9 9 ) 0 0 0 0 8 - 9

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Table 1 ISO’s four scientific focal plane instruments

Fig. 1. The lifetime predictions were based on measurements of the temperature increase ( 苲 mK) in the superfluid helium after a definite electrical heating of the liquid. Every measurement resulted in a longer but better determined evaporation end date. The actual end date of the LHe, 8 April, was very close to the date of 10 April, predicted almost a year before (ESA).

ture of only about one thousandth of a degree in the ISOPHOT far infrared sensors were the first signs of a change in the otherwise stable behaviour of the ISO satellite. These minor temperature changes were the writing on the wall: on 8 April 1998, very close to the 10 April date predicted almost a year before, the last drops of helium evaporated and within a few hours the temperature of the sensors rose by several degrees (see figure in A. Seidel’s paper, this volume). This made any further measurements in the far infrared impossible. The first reactions of the scientists were a mixture of disappointment and satisfaction. The work on mapping the coldest matter in the Small Magellanic Cloud and the Orion B molecular cloud had to be stopped. These incomplete 200 ␮m maps will remain a reminder of that April day. In the end, however, satisfaction won the day: the mission had lasted a total of 29 months—11 months longer for observations than originally calculated. The final year in particular was very rewarding scientifically, because by then those involved had learnt the best observing techniques.

Instrument

Wavelength

ISOCAM: PI: Catherine Cesarsky, Saclay, F— imaging and polarimetry ISOPHOT: PI: Dietrich Lemke, Heidelberg, D— imaging spectrophotopolarimeter SWS: PI: Thijs de Graauw, Groningen, NL—short wavelength spectrometer LWS: PI: Peter Clegg, London, UK—long wavelength spectrometer

2.5–17 ␮m 2.5–240 ␮m 2.5–45 ␮m 43–197 ␮m

right to the end of the mission. That is very satisfying for all the technicians who had spent years developing and testing low friction and vibration proof bearings, etc., which were finally successful in spite of many setbacks during prelaunch tests. For the first time ever in a space mission two of the instruments (LWS and PHT) incorporated stressed Ge:Ga detectors for research in the spectral band beyond the IRAS 100 ␮m range. Using these small gallium doped germanium crystals (Fig. 2), which were stretched almost to breaking point, infrared radiation can be detected as far as 240 ␮m. These sensors behaved most stably and predictably even under cosmic radiation and have provided new knowledge about the coldest matter ( 苲 12 K) in the universe. This in turn facilitates planning for the space observatories SIRTF (NASA) and FIRST (ESA), in which larger Ge:Ga cameras with stressed sensors will be used. The list of new technological developments featured on ISO is even longer: far infrared filters with high transmission and without shortwave leaks, polarisers for 200 ␮m (H.-P. Gemu¨nd, E. Kreysa, MPI fu¨r Radioastronomie, Bonn), black colours for 200 ␮m (Herberts), preamplifiers and multiplexers for operation at T 苲 2 K (IMEC) and so forth were developed in Germany and Europe.

4. Scientific return of the mission 3. New instruments—all fully operating Powerful instruments cannot do without drives and moveable parts. Filters have to be changed, grids moved and beams redirected. Before the mission the risk of failure of such mechanisms in the cryovacuum was regarded as high. Power consumption had to be kept down to the milliwatt level, in order not to deplete the helium supply by heat dissipation. The four instruments (Table 1) from various European countries—2 spectrometers (NL, UK), a camera (F) and a photopolarimeter (D)—were well equipped with cryo-mechanisms, and this is a novelty in a cooled space observatory [1,2]; for a description of all the other instruments and ISO see Astron. Astrophys., 315, 1996). All the equipment was fully functioning

The excellent performance of all the instruments, the satellite and ESA’s ground observatory in Villafranca near Madrid have resulted in a huge scientific data base. This will be evaluated during the postoperational phase (lasting until the end of 2001) and in the following decade by ‘observing’ in the ISO data archive. Here a few examples of results will be selected, in order to demonstrate the astrophysical areas and questions which are addressed. 4.1. Dust in the universe As interstellar dust particles with a diameter of less than 1/1000 mm are roughly the size of the wavelength’s

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Fig. 2. The four pixel camera ISOPHOT C200 in connection with scanning by the satellite allows imaging of very cold celestial objects. By stressing a 1 mm3 Ge:Ga crystal by a screw the sensitivity range can be extended from 110 to 240 ␮m. The signals are amplified and multiplexed by a cold CMOS electronics (Battelle, IMEC). This camera is also used for the serendipity sky survey (see Fig. 9).

visible light, this radiation is greatly weakened by intense interaction. Large parts of the universe therefore remain hidden behind thick interstellar clouds. The much longer infrared wavelengths penetrate the dust almost unimpaired. Although dust represents only one thousandth of the matter in the cosmos, in the far infrared it dominates the appearance of the sky. As a result of its wide distribution it absorbs the shortwave radiation of the hot stars and is thereby heated. Depending on the distance from the star and the size of the particles (smaller ones get hotter!), the dust reaches a temperature of T 苲 10–300 K. Further temperature increase of the particles is limited by radiating off infrared radiation. Thus no energy is lost: the luminosity of invisible young stars with thick circumstellar shells or the black holes surrounded by thick dust rings appears on the much larger surface of the surrounding cloud as infrared radiation. The dust transforms the ultraviolet of the hot core into the infrared of the packaging, which can be completely detected by ISO. 4.1.1. Cirrus clouds—with holes IRAS already showed us a patchy sky at 100 ␮m. All over the Milky Way large and small, thick and thin interstellar dust clouds glow (Fig. 3). Only red giant stars and galaxies show up as dots. This galactic cirrus is an obstacle to the research of weak sources in the more distant universe. Since for cost reasons the main mirrors of the cooled telescopes were small—60 cm on both

IRAS and ISO, the aperture size at wavelengths of 100 ␮m must be as large as 1.5 arc min, in order to catch the diffraction image of a point source. The signal of the cirrus foreground in such a large aperture often surpasses the weak point sources and, moreover, it is spatially variable: the sensitivity of the telescope is ‘cirrus limited’. The prelaunch predictions for ISO’s 200 ␮m measurements were somewhat pessimistic: the aperture had to be further enlarged and the radiation of the interstellar dust with temperatures of 苲 17 K reaches its radiation maximum at that wavelength. With ISOPHOT both the cirrus structures near 100 ␮m were investigated with considerably higher spatial resolution (i.e. to higher spatial frequencies) than ever before, and for the first time the 175 ␮m cirrus was also studied [3]. The ISOPHOT maps at 175 ␮m show two features: on the one hand there are ‘holes’ in the cirrus, through which the view into the distant universe is unhindered. On the other hand the signal difference in the cirrus emission of two neighbouring sky regions (cirrus fluctuation noise) decreases with decreasing aperture size. In this way one can extrapolate the cirrus noise to much smaller apertures (sensor pixels) for future large infrared telescopes (FIRST 3.5 m). The cirrus fluctuations will then be limited to a few mJy over most of the sky. At 苲 20 mJy, however, the fluctuations of extragalactic sources could cause confusion. On ISO it turned out to be advantageous to map the environment of a faint source, in order to distinguish it from cirrus knots. The

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galaxy cluster (Fig. 4, below right) with the larger Coma cluster. The interstellar matter from the arriving galaxies is swept out by the hot intergalactic plasma and the dust soon ‘evaporates’. For this reason a dust/gas mass ratio of only 1:10 000 was determined, which deviates considerably from the usual 1:100 found in the interstellar matter of the Milky Way. Five more galaxy clusters were measured with ISOPHOT: from very young ones with chaotic x-ray pictures to old stable clusters with almost concentric x-ray contours. The dust characteristics still to be determined [4], especially the mass, will give us information about the history of the development of these large building blocks of the cosmos. 4.2. Andromeda—puzzling dust

Fig. 3. Much of the infrared sky is bright due to thermal emission of interstellar dust. ISO has shown the clumpiness of these ‘cirrus’ structures at 90–100 ␮m in greater detail. For the first time the similarity of the cirrus structures seen at 90–100 and 170–180 ␮m has been demonstrated [3].

The optical shape of the Andromeda galaxy M31 is often described as a classic Sb spiral. Its brightness increases considerably with its concentration of stars towards its centre. Parts of spiral arms are visible near the edges. IRAS pictures at 100 ␮m had indicated the concentration of cold dust in a ring of neutral hydrogen and molecular clouds with a radius of 10 kpc. The ISOPHOT mapping at 175 ␮m (Fig. 5), which was able to identify much colder dust, shows a similar deficit in the central region. Although the total mass of the 苲 17 K cold dust discovered by ISO is ten times higher than previously estimated, the dust luminosity inside the 10

much larger cameras in the telescopes of the future will provide the necessary source environment maps even faster. 4.1.2. Intergalactic dust As the density in the space between the galaxies further decreases by several orders of magnitudes compared with the ultra high vacuum in the interstellar regions, intergalactic dust emission has not yet been discovered. Its infrared radiation should reveal it as soon as it can be separated from the overlaying stronger cirrus signal of the Milky Way dust. ISOPHOT measurements at 120 and 185 ␮m in the Coma galaxy cluster have been successful for the first time [4]. The separation of the foreground components was possibly due to their differing colour temperatures. The intergalactic dust is several degrees warmer than the 17 K dust foreground of the Milky Way. Dust in the Coma cluster, however, is continuously destroyed in its hot plasma gas (T 苲 100 Mio degrees) within several million years. The fact that it could nevertheless be identified is an indication of its continuous resupply during the merging of the smaller

Fig. 4. The COMA cluster of galaxies is embedded in a 100 Mio K hot plasma gas (contours by ROSAT). Emission of 30 K dust mixed with this gas was detected by ISOPHOT (signal in the upper figure) along the 2 scan lines indicated in the map [11].

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(insert in Fig. 5) Instead one sees the clearly delineated 10 kpc ring and, less clearly, two further rings. This pattern may result from the above mentioned interaction with another galaxy. The nucleus of M31 is relatively weak at 175 ␮m, because at T 苲 34 K it is too ‘warm’. Compared to our Milky Way the Andromeda galaxy has a larger mass and is brighter in the optical. On the other hand, in the infrared it only achieves 20% of the Milky Way’s luminosity, in spite of the cold dust masses discovered by ISOPHOT, which amount to 50% of the equivalent value in our stellar system. 4.3. Quasars

Fig. 5. The Andromeda galaxy M31 as seen by ISOPHOT at a wavelength of 175 ␮m. A classical spiral structure is not obvious, but rather concentric rings of cold ( 苲 16 K) dust indicating regions with higher star formation rates are recognizable. The optically bright nucleus is rather faint in the infrared, because it is fairly warm ( 苲 30 K). The insert shows the galaxy transformed to a face-on view. [6].

kpc ring scarcely increases towards the centre, contrary to the optical luminosity of stars. What could the reason be for this strange phenomenon? Has the interstellar gas on larger (and therefore colder) dust nucleii been frozen off? Or was the interstellar matter reduced in the centre by some disruption—M31 may have 2 nucleii resulting from an interaction with another galaxy? Or is there too little ultraviolet radiation of hot stars for the dust to heat up? This is suggested by spectra made by ISOCAM in the mid infrared [5]. Instead of the usual series of lines of the graphit-like polycyclic aromatic hydrocarbon (PAH) at 6.2, 7.7, 8.6 and 11.3 ␮m, only the last named line can be observed. PAH grains originate from the HAC grains (amorphous hydrocarbon) as a result of ‘graphitisation’ in the high energy UV light. HACs are believed to be produced in the envelopes of carbon stars. The ultraviolet radiation field, which is contributed to largely by young, hot, short lived stars, is obviously so weak in M31, due to an unusually low star formation rate, that the PAHs which are so abundant in the Milky Way have not materialized in Andromeda. If the galaxy as viewed almost completely edge on ( 苲 12°) is turned on the computer to a face-on view, then the supposed spiral arm pattern is not recognisable

These objects as well as other active galaxies can have luminosities of up to 4 orders of magnitude higher than the Milky Way galaxy. The central engine is supposed to be a black hole onto which matter is continuously accreted and transformed into radiation in an accretion disk surrounding the black hole. This core is inside a thick dust torus heated up by the very hot accretion disk and reradiates in the infrared. ‘Unification’ models suggest that most active galaxies, including quasars, have the same internal building plan; their different appearance depends only on the viewing angle of the observer. Fig. 6 shows the radio image of powerful jets emitted into polar directions of the central engine. ISOPHOT observations of several quasars, some for the first time at infrared wavelength, support the ‘unified models’. The knowledge gap in the infrared between optical and radio wavelengths is bridged by a flat link in pole-on seen quasars: here the infrared is synchrotron emission produced by high energetic charged particles accelerated in the magnetic field of the jet. Polarization, a characteristic of synchrotron radiation, was measured for the first time in the far infrared, thanks to movable filters in the cryogenically cooled instruments. Side-on seen quasars, on the other hand, exhibit ‘dust bumps’ in the infrared, that is thermal radiation of the heated torus. Remarkably, the spectrum of the quasar 3C 48 is very similar to the (active) radio galaxy Cyg A [6]. 4.4. View to the early universe Several decades ago Lockman and Marano already noticed in the course of their radioastronomical measurements regions with diameters of only a few degrees, which revealed a very low concentration of neutral hydrogen (HI). As gas and dust are thoroughly mixed, the fields named after them are mostly free of dust and show no or low infrared cirrus emission. These windows allow a view to the distant (and young) universe, unhampered by the interstellar dust foreground. J.-L. Puget (Paris) together with an international group of co-investigators has used the ISOPHOT 200 ␮m camera to search for young stars in these fields. 24 objects

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Fig. 6. Depending on the viewing angle onto a quasar or another active galaxy the infrared spectra are different. A pole-on view shows a Dopplerboosted synchrotron spectrum due to powerful jets, a side-on view shows dust emission from the suspected torus surrounding the central engine [12].

have been discovered in an area of 1.5 square degrees in the Marano region in the southern sky and are interpreted as strongly red shifted galaxies (Fig. 7 shows a subfield). These galaxies with a typical brightness of 苲 100 mJy at 175 ␮m contribute 12% to the sky’s far infrared surface brightness; the rest is from weaker galaxies. A surprizingly large extragalactic sky brightness of 苲 1.5 MJy/sr was recently announced by the COBE satellite scientists after 7 years of data reduction. With this satellite’s wide field (0.7°) view they were able to determine the integrated luminosity, whereas ISO’s 2 arc min resol-

ution allowed the contribution of individual galaxies to be determined. These results mean that dust produced by the first generation of stars already existed in large quantities in the early universe. Star formation in the young galaxies (13 billion years ago) was much more frequent. The identification of the galaxies now has to be followed up by observations of their spectra at shorter wavelengths with the aim of establishing their exact distances and ages. A similar programme led by K. Mattila (Helsinki) on extragalactic background radiation is expected to make a further considerable contribution to this subject. It involves faint source countings as above and in addition the absolute measurements of the integrated extragalactic background radiation. The combination of several far infrared wavelength bands will allow other foreground components such as the galactic cirrus and the thermal emission of interplanetary dust to be disentangled. 4.5. Powerful spectroscopy

Fig. 7. The Marano field at the southern sky has low galactic cirrus emission and is therefore a window to the more distant universe. J.L. Puget (IAS, Paris-Orsay) and his collaborators have mapped this area with the ISOPHOT C200 camera at a wavelength of 175 ␮m. Two dozen sources (arrow) found are interpreted as very young galaxies with a high rate of star formation. ISO’s high spatial resolution allows the identification of these sources, while COBE determined the integrated extragalactic background radiation.

For the first time medium and high resolution spectrometers were used throughout the infrared. Two examples serve to stand for the wealth of results obtained. The question of what powers ultraluminous galaxies with energy outputs of more than 2 orders of magnitude higher than that of our Milky Way was addressed by a team from MPE Garching [7,8]. High luminosity could be produced in principle by mass accretion onto a black

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hole in the nucleus of the galaxy, or by a burst of star formation. With optical observations the question cannot be answered because usually the nucleus is invisible, hidden by absorbing dust. Searching for infrared lines with SWS and ISOPHOT-S, however, allows distinction: Several of these active galaxies exhibit fine structure lines of highly excited species such as O IV, Ne V, etc., which require photons of several hundred eV in order to ionizise these atoms to these levels. Even the hottest young stars with temperatures of 苲 50 000 K cannot deliver such energetic photons. Therefore a black hole surrounded by a hot accretion disk has to be assumed where these strong fine structure lines are seen. The MPE team also found an interesting anti-correlation of the highly excited atomic lines with the presence of dust features assigned to small polycyclic aromatic hydrocarbon particles (PAHs). Ultraluminous galaxies which are dominantly powered by a burst of star formation exhibit strong PAH features (for instance the prominent line at 7.7 ␮m) and only weak or no lines of highly excited atoms (see Fig. 8). The discovery of water in many places in the universe is another achievement of the ISO spectrometers LWS and SWS. Cosmic water lines cannot be detected from ground or aircraft altitudes because of strong terrestrial waterlines. Water was found in the atmospheres of the outer planets as well as on Saturn’s moon Titan. It freezes out in the atmosphere and it is continuously resupplied by interplanetary dust and ice from the cometary cloud in the outer solar system [9]. Large amounts of water are produced in the process of star formation. Shock waves driven by powerful jets and wind streaming

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away from these young objects and colliding with the surrounding interstellar gas deliver the energy to combine free oxygen and the abundant hydrogen to water [10]. 50 earth oceans can be filled every day by each of these objects, located for instance in the Orion nebula. The study of waterlines is of astrophysical interest, because they are important cooling lines and may be a key to the oxygen depletion in the interstellar medium.

5. Serendipity survey The time it takes to slew from one object of pointed observation to the next is usually wasted, both on ground based and space telescopes. Long before the launch plans were made for ISO to take measurements during this time as well. The sky slews cannot be predicted, the satellite follows curved tracks from one object to the next, avoiding the large ‘forbidden’ sky regions around the earth and the sun. The speed—at a maximum of 7° per minute—is constant for most of a long slew. It seemed sensible to register the signals of the 200 ␮m camera during the slews. The relevant direction of the telescope can be derived from the signals from the gyroorientation system on board. This serendipity survey was very successful. 500 hours of what would otherwise have been lost time was gained for observation in the wavelength range of 175 ␮m, which is still almost unexplored so far. The data thus obtained consists of a total of 150 000° strip maps of 3 arcmin width. In the sky regions most frequently chosen by the hundreds of ISO observers (e.g. star for-

Fig. 8. Infrared spectroscopy allows the determination of the power source of galaxies which are inaccessible at visible wavelengths due to dust absorption. Active galactic nuclei (AGN, crossed rectangles) show lines of highly excited atoms and no PAH dust features. Starburst galaxies (triangles) exhibit strong PAH lines at 7.7 ␮m and no lines of highly excited atoms. Ultraluminous galaxies (filled circles) can be powered by an active nucleus and/or a starburst [7].

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mation regions in Ophiuchos, the Magellanic Clouds) the strips are very close together (Fig. 9). On the other hand the total sky map with all the strips has large gaps: regions which were inaccessible (because they were always in the forbidden direction of the earth) for the whole mission. The thorough evaluation of a test field around the frequently observed ecliptic North Pole is very promising for the results of the whole survey [4]. One galaxy on average is found on every 40° slew length—nearly 3000 are to be expected. As many of them are ‘grazed’ with only one or two pixels of the camera, reliable data on brightness is expected for about 1000 of them, which were detected in at least three pixels with a signal-tonoise ratio > 3. On the assumption that the galaxies found by IRAS at 100 ␮m are also bright at 175 ␮m, the reliability of the ISO detections can be calculated: the serendipity survey can provide almost complete data on the objects down to 2 Jy brightness. Using all this data, studies will soon be possible on the distribution of energy in the spectra, the luminosity, the dust content, the energy sources and so on. The test field already has examples: at 175 ␮m unusually bright objects may be interacting (merging) pairs of galaxies (Fig. 10) or they may turn out to be cold dust knots in the galactic cirrus. Apart from the discovery of mainly extragalactic point sources, the comparison of the new 175 ␮m strip maps with the relevant 100 ␮m strips from IRAS can provide colour temperature maps of the sky. In this way the coldest regions in the Milky Way, probably prestellar nucleii in molecular clouds with temperatures of ⫺ 260°C, are to be identified. This work is being done in the MPIA ISOPHOT Data Centre, with the support of working groups at ESA, Villafranca, at Imperial College,

Fig. 10. Detected by a strong 175 ␮m signal in the Serendipity survey the object turned up as a pair of interacting (merging?) galaxies. The interaction probably triggered a burst of star formation, heating up the interstellar dust in these galaxies [4].

London, at the Astrophysikalisches Institut Potsdam, and at NASA’s Infrared Data Centre in Pasadena.

6. Over 100% efficiency? The serendipity survey, which will set a pattern for future satellite missions, already led to a 95% efficiency in the use of the theoretically available observing time.

Fig. 9. Sky coverage by the serendipity slews with ISOPHOT’s C200 camera. About 15% of the sphere have been investigated at a wavelength of 175 ␮m indicating the coldest spots ( 苲 12 K) in the Milky Way and several thousand galaxies.

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The excellent functioning of all systems and instruments during the mission gave rise to two further ideas, which were also taken up successfully. After the final evaporation of the helium astronomical measurements were no longer possible—with one exception: in the shortwave part of SWS the spectra of 300 bright stars with a spectral resolution of ␭/⌬␭ ⬇ 2000 were measured in spite of the steadily rising temperature on ISO. These measurements are being used for a spectral atlas of a wavelength region which is inaccessible from ground. In the band observed (2.4–4.0 ␮m) cool stars reveal important molecular bands, whereas hot stars show helium and hydrogen lines of the Brackett and Pound series, which are used for research on the outer layers and stellar winds. In parallel ESA investigated subsystems of the satellite, which were all fully functional at the end of the mission. There was an intentional test of emergency procedures after switch-off of crucial subsystems such as gyros or optical attitude sensors. These simulated ‘failures’ could help future missions based on similar satellite service modules to be saved in case of an emergency. In addition the flight spare units (‘cold redundance’) for the star sensors and the on board computer were tested successfully. Acknowledgements ISO is an ESA project with instruments funded by ESA member states, especially the PI countries France, Germany, the Netherlands and the United Kingdom, with participation from ISAS and NASA. The ISOPHOT instrument was funded by DARA (now DLR), the MaxPlanck Society and the ISOPHOT CoI institutes in particular in Denmark, Germany, the United Kingdom and Spain. The ISOPHOT flight hardware was manufactured by Dornier, Zeiss and Battelle. I am indebted to colleagues from the three other instrument consortia for a fruitful collaboration, as well as to ESA for the successful preparation and operation of the mission. Compliments go to the European Space industry for their excellent work, in particular to the prime contractor Aerospatiale and the payload contractor Daimler-Benz Aerospace.

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