ESA's Infrared Space Observatory (ISO) has provided astronomers with a unique facility of unprecedented sensitivity for detailed exploration of the Universe. It operated at wavelengths from 2.5 to 240 microns, which means that ISO was not able to detect visible light - the kind of light a human eye can see - but infrared light. ISO could therefore see cool and opaque astronomical objects that remain hidden from non-infrared or ground-based telescopes. ISO has been the only telescope covering such a broad range of the infrared wave band.
The primary source of infrared radiation is heat or thermal radiation.
Even objects that we think of as being very cold, such as an ice cube,
emit infrared radiation. When an object is not quite hot enough to radiate
visible light, it will emit most of its energy in the infrared. Thus ISO
felt the 'heat' instead of the 'brightness' of the objects.
ISO's highly-elliptical orbit had a perigee at around 1.000 kms, an apogee at 70.500 kms and a period of almost 24 hours. The lowest parts of the orbit lay inside the Earth's van Allen belts of trapped electrons and protons. Inside these regions, ISO's detectors were scientifically unusable due to radiation impacts. ISO spent almost 17 hours per day outside the radiation belts and during this time all detectors were operated. There was no on-board data storage. Therefore, for scientific use ISO needed to be in continuous contact with a ground station: either the primary ground station and antenna at VILSPA, Madrid, Spain or the secondary antenna at Goldstone, USA.
ISO was launched by an Ariane 4 rocket on the 16th of November 1995. The planned in-orbit lifetime for ISO was 18 months, but thanks to meticulous engineering and some good fortune, the satellite's working life has stretched up to May 1998.
The ISO satellite consists essentially of a large cryostat containing about 2300 litres of superfluid helium, the payload module, the scientific instruments, and the optical baffles. They are kept at temperatures just a few degrees above the absolute zero, minus 273 degrees C. (otherwise, the instruments' emission would swamp the detection of the target emission).
The telescope has a 60-cm diameter primary mirror. A pointing accuracy of around a second of arc is provided by a three-axis stabilisation system consisting of reaction wheels, gyros and optical sensors.
The cold focal-plane units (FPUs) of the scientific instruments are mounted behind the telescope's primary mirror. They are connected to 'warm' instrument electronics boxes on the spacecraft platform.
The overall dimensions of ISO are:
The satellite was built by an industrial consortium headed by Aerospatiale,
France.
ISO camera, one of ISO's four instruments, consists of two similar optical channels which operate in two spectral regions with two different arrays of infrared detectors. In the short-wavelength (SW) optical channel, an InSb array operates in the 2.5 - 5.5 micron wavelength range. In the long-wavelength (LW) channel, a Si:Ga array covers the 4 - 17 micron band. Upon entering the camera, the optical beam, deflected by a pyramidal mirror, first encounters the 'entrance wheel'. This wheel carries a set of three polarising grids spaced at angles of 120 degrees, allowing polarisation measurements to be made in either channel.
Next, the beam encounters the 'selection wheel', which allows one of the two optical channels to be chosen by means of two Fabry mirrors. Illuminators mounted on this wheel are also used for the in-orbit calibration of the detectors. On the following two 'filter wheels', one for the long-wavelength and the other for the short-wavelength section, a total of 26 filters are mounted. These filters, including three Circular Variable Filters (CVFs), define the infrared spectral range of the observations. Finally, in each channel, a so-called 'lens wheel', positioned in front of the array, carries four lenses with different magnification factors. Choices of 1.5, 3, 6 and 12 arcsec per pixel plate scales are possible.
ISO's Infrared Camera (ISOCAM)
Each wheel, made from titanium, is driven by a superconductive stepper
motor (which eradicates Joule losses) in order to limit heat dissipation
inside the unit and thereby minimise temperature fluctuations. Vespel,
a composite polymeric material, has been chosen for the motor pinion, both
for its good elastic properties and satisfactory mechanical behaviour at
low temperatures, and for its low coefficient of friction.
ISOCAM was developed by a consortium of French, British, Swedish
and Italian institutes headed by C. Cesarsky of SAp, CEA-Saclay, France.
The Infrared Photo-polarimeter (ISOPHOT), one of ISO's four instruments, is designed to work between 2.5 and 240 microns. The 144 detector elements used in the instrument are divided into four arrays and three single detectors, mounted in three different subsystems. These subsystems, each of which is employed in a different photometric mode, are:
ISOPHOT was developed by a consortium of German, Spanish, Irish,
Finnish, British and Danish institutes headed by D. Lemke of MPIA, Heidelberg,
Germany.
The SWS, one of ISO's four instruments, is a spectrometer designed to cover the 2.4 to 45 micron band, with a spectral resolution (the capability to distinguish different monochromatic components) of between 1000 and 2000 using gratings; this can be raised to between 23 000 and 35 000 in the 12 - 44 micron range by use of "Fabry-Pérots". A variety of detectors have been chosen to cover the short-wavelength band, ranging from InSb (2.4 - 4.0 microns) to Si:Ga (4.0 - 13 microns), from Si:As (12 - 29 microns) to Ge:Be (28 - 45 microns), and for the "Fabry-Pérot", Si:P (12 - 26 microns) to Ge:Be (26 - 44 microns). The detectors consist of four 12-element arrays and two detector pairs for the "Fabry-Pérot".
SWS instrument was developed by a consortium of Dutch, German, Belgian
and US institutes headed by Th. de Graauw of SRON, Groningen, The Netherlands.
The Long-Wave Spectrometer (LWS), one of ISO's four instruments, is a spectrometer that operates in the infrared band between 43 and 196.8 microns in the grating mode and 47 to 196.8 microns in Fabry-Pérot mode. The resolving powers vary between 150 and 350 in grating mode and between 7000 and 10 000 in Fabry-Pérot mode. Three types of photo-conductive detectors have been used:
Although the three types of detectors are mounted in a single array, the stressed and unstressed detectors operate at different temperatures and are only weakly thermally coupled.
Part of the Long-Wavelength Spectrometer:
The Fabry-Pérot Exchange Wheel
LWS instrument was developed by a consortium of UK, French, Canadian,
Italian and US institutes headed by P. E. Clegg of Queen Mary and Westfield
College, London, United-Kingdom.
Spectroscopic and photometric capabilities
of every ISO instrument
Each of the four instruments receives a 3 arcmin field of view (drawn
to scale in the central part of the figure). The outer ring shows, in expanded
scale, more details of the detector fields of view as projected onto the
sky.
The electromagnetic spectrum includes gamma rays, X-rays, ultraviolet, visible, infrared, microwaves, and radio waves. The only difference between these different types of radiation is their wavelength or frequency. Wavelength increases, and frequency (as well as energy and temperature) decreases, from gamma rays to radio waves. In addition to visible light, radio, some infrared and a very small amount of ultraviolet radiation also reaches the Earth's surface from space. The atmosphere blocks out the rest, hence the need for an infrared space telescope like Europe's Infrared Space Observatory (ISO).
Infrared radiation lies between the visible and microwave portions of the electromagnetic spectrum. Thus infrared waves have wavelengths longer than visible and shorter than microwaves, and have frequencies which are lower than visible and higher than microwaves. Near infrared refers to the part of the infrared spectrum that is closest to visible light and far infrared refers to the part that is closer to the microwave region.
The primary source of infrared radiation is heat or thermal radiation. Even objects that we think of as being very cold, such as an ice cube, emit infrared. When an object is not quite hot enough to radiate visible light, it will emit most of its energy in the infrared. For example, hot charcoal may not give off light but it does emit infrared radiation which we feel as heat. The warmer the object, the more infrared radiation it emits. Humans, at normal body temperature, radiate most strongly in the infrared at a wavelength of about 10 microns (A micron is one millionth of a meter).
For this reason ISO, which operates at wavelengths from 2.5 to 240 microns, can see cold and opaque astronomical objects that remain hidden for non-infrared or ground-based telescopes. ISO has been, so far, the only telescope that covers the whole infrared range of the spectrum.
The gold-coated truncated-cone sun shade reflects direct illumination from the Earth back to space, in order to avoid stray light falling on the instruments.
The ISO telescope has a primary mirror with an overall diameter of 640 mm, a secondary mirror with a diameter of 87.6 mm, and a four-faced pyramidal mirror that distributes the light collected to the four instruments. The lightweight mirrors are made of fused silica and are gold-coated to give them good reflection characteristics in the infrared. The primary mirror is circumferentially mounted onto the optical support structure via three fixation devices, each consisting of an invar pad fixed to the mirror and crossing blades that provide the required degrees of freedom. The secondary mirror is mounted on a tripod. Both mirrors are cooled by copper straps connecting their rear faces to the helium-cooled optical support structure.
ISO Telescope primary mirror before
application of reflective gold coating.
The first requisite for ISO is to provide a telescope, including baffles, that is kept very cold. The focal-plane units (FPUs) of the scientific instruments and the infrared detectors inside those units must also be maintained at temperatures close to absolute zero (minus 273 degrees C.)
The solution adopted for ISO is to enclose the telescope in a cryostat. The main element is a toroidal tank containing 2286 litres of super-fluid helium (HeII) at temperatures below minus 271 degrees C. The tank is insulated from external heat inputs by three vapour-cooled radiation shields (VCS) equipped with multi-layer insulation (MLI).
