Figure 1. Qualification model (QM) of the ISOCAM Focal-Plane Unit (FPU). The two detector arrays are not mounted on this model
Note: This article is an update of an article that originally appeared in ESA Bulletin No. 61 (February 1990).
The payload of ESA's Infrared Space Observatory (ISO) consists of four scientific instruments: a camera (ISOCAM), an imaging photo-polarimeter (ISOPHOT), a long-wavelength spectrometer (LWS), and a short-wavelength spectrometer (SWS). Each of these instruments was built by an international consortium of scientific institutes using national funding. ESA was responsible for their subsequent integration into the ISO spacecraft and will carry out the in-orbit operations.
ESA's Infrared Space Observatory (ISO) is an astronomical satellite that will provide astronomers with an unique facility of unprecedented sensitivity for detailed exploration of the Universe. Operating at wavelengths from 2.5 to 200 microns, it will be able to study objects in the solar system right out to the most distant extra-galactic sources.
The ISO satellite consists essentially of a large cryostat, the payload module, containing about 2300 litres of superfluid helium to maintain the Ritchey-Chrétien telescope, the scientific instruments, and the optical baffles at temperatures between 2 K and 8 K.
The telescope has a 60-cm diameter primary mirror, and is diffraction-limited at a wave-length of 5 microns. A pointing accuracy of a few arcseconds 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 main characteristics of the four instruments are summarised in Table 1.
The ISO instruments use photo-conductors made from indium-antimonide (InSb), silicon (Si) and germanium (Ge). The last two are doped with various materials to achieve particular sensitivities in different wavelength ranges of interest. To extend the long-wave-length coverage of the gallium-doped germanium detector, the detector crystal is 'stressed' by applying mechanical pressure using a clamp. Infrared radiation falling on the detector produces a proportional photo-electric current, which is integrated as the output signal.
For correct operation, the detectors have to be kept at well-defined stable temperatures (ranging from 1.8 to 10 K for different materials). The detectors for the longer wavelengths (Ge) require the lowest operating temperatures.
Detectors are configured either as single elements (up to 1x1x1 mm in size), linear arrays (max. 64 pixels), or two-dimensional arrays (max. 32x32 pixels), and are directly connected to the pre- amplifiers and multi-plexers within the FPUs.
Filters are used to select specific wavelength bands for each detector. Materials like germanium, silicon, calcium-fluoride, sapphire, and quartz are used as carrier substrates for multilayer interference filters.
The dichroic beam splitters, which combine the functions of beam separation and filtering, are made from crystals such as sapphire, strontium-fluoride or lithium-fluoride. For longer wavelengths, multilayer metal-mesh assemblies are used.
Polarisers allow different orientations of the electromagnetic field vectors of the infrared radiation to be distinguished.
Diffraction gratings (ruled on aluminium blanks, 8 to 100 lines/mm) disperse the wavelengths for spectroscopy, for which Fabry-Pérot etalons provide a very high resolution. Within an etalon, a resonant cavity between two partly reflecting, partly transmitting mirrors (metal meshes) is created. Only the 'resonant' wavelengths are transmitted. Tuning is performed by changing the distance between the metal meshes.
The ISOCAM consists of two similar optical channels which operate in two spectral regions with two different arrays of infrared detectors, each with 32x32 elements (Fig. 1).
In the short-wave (SW) optical channel, an InSb array operates in the 2.5 - 5.5 micron wavelength range. In the long-wave (LW) channel, a Si:Ga array covers the 4 - 17 micron band.
Figure 1. Qualification model (QM) of the ISOCAM Focal-Plane Unit (FPU). The two detector arrays are not mounted on this model
A schematic of the camera layout is shown in Figure 2. Upon entering the camera, the optical beam, deflected by a pyramidal mirror, first encounters the 'entrance wheel'. In addition to clear apertures, this wheel also carries a set of three polarising grids spaced at angles of 120 degrees. These grids allow polarisation measurements to be made in either channel.
Figure 2. Schematic of ISOCAM instrument's opto-mechanical design
Next, the beam encounters the 'selection wheel', which allows one of the two optical channels to be chosen by means of two Fabry mirrors. This wheel is also used for the in-orbit calibration of the detectors.
On the following two 'filter wheels', one for the long-wave and the other for the short-wave 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 for matching the fixed pixel size of the detectors to the desired pixel field of view (PFOV) on the sky or, in other words, the size of the window through which the detector will observe the sky. Choices of 1.5, 3, 6 and 12 arcsec per pixel are possible.
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.
Figure 3. Schematic of the ISOPHOT instrument's opto-mechanical design
In ISOPHOT-C, two two-dimensional arrays (3x3 Ge:Ga and stressed 2x2 Ge:Ga) are each used in combination with nine bandpass filters and three polarisers.
Finally, with ISOPHOT-S selected, by means of wheel 1, the 128 detector elements, divided into two 64-element Si:Ga arrays can be used as spectro-photometers, giving a spectral resolution of about 100.
Directly behind the entrance aperture, before the radiation is deflected into one of the three subsystems, the beam encounters the 'focal-plane chopper', which can be used for:
Driven by a magnetic coil, the tilting mirror of the chopper allows a beam throw ranging from 5 to 360 arcsec at a frequency of between 1/256 Hz and 16 Hz. A position sensor of the field-plate type enables the drive to generate any travel/time cycle within the limits specified above.
A variety of detectors have been chosen to cover the short-wave 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-Pérots, 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-Pérots.
* The 'spectral resolution' is the capability to distinguish different monochromatic components.
The instrument consists of two parallel sections, which work in two infrared sub-bands at 2.4 - 13 and 12 - 45 microns (Fig. 4).
Figure 4. Schematic of the SWS instrument's design
An optical input unit contains three different entrance apertures, which are used for different sub-bands. The aperture can be selected by means of shutter blades, repointing the telescope when necessary. Dichroic beamsplitters and transmission filters behind the shutter system define six different sub-bands, necessary to separate the different spectral orders of the gratings. The beamsplitters provide three transmission paths into the 2.4 - 13 micron section and three reflection paths into the 12 - 45 micron section.
Using collimating optics (two independent sets of toroidal and paraboloidal cylindrical mirrors), the beams are focussed onto two diffraction gratings, which disperse the radiation. Each grating has its own scanning mirror, allowing use of two sub-bands at the same time.
After reflection from the grating, each sub-band almost retraces its path before it is finally refocussed on the detector blocks by means of re-imaging optics. Further filtering is applied in the detector blocks.
Tunable Fabry-Pérot etalons allow a resolving power between 23 000 and 35 000 to be achieved in the 12 - 44 micron range by suitably deflecting the beam coming from one of the two gratings.
The grating drive employs a linear motor and has two flexural pivot hinges. The yoke, which carries the flat scanning mirror, is pushed by a coil in the field of a permanent samarium-cobalt magnet, against the counterforce of the flexural pivots. The full range of rotation is 12 degress, with a position reproducibility of 3 arcsec. The power con-sumption of this unit is less than 1 mW.
The LWS is a grating 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 will be 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.
The LWS has three main subsystems (Fig. 5):
Figure 5. Schematic of the optical components of the LWS
After having been deflected by the first five mirrors, which define the field of view and produce a parallel beam, the radiation passes through the FP exchange wheel. In addition to the two FP etalons, the latter also carries a clear aperture.
In the low-resolution mode (neither of the two Fabry-Pérot etalons selected), the beam illuminates the diffraction grating directly, via mirrors M5 and M6. The grating disperses the radiation, which the refocussing mirror M8 then brings to a focus at the ten-detector assembly. A resolving power of about 200 is achieved by this approach.
The high-resolving mode (up to 10 000) is activated when the FP wheel is rotated and one of the two etalons is selected. The short-wavelength FP is optimised for wavelengths from 47 to 110 microns; the long-wavelength FP covers the range 110 - 197 microns.
Each of the two FPs (Fig. 6) consists of three triangular-shaped plates carrying three electromagnet assemblies, which move the meshes of the etalons against the force of three leaf springs.
Figure 6. The two Fabry-Pérot etalons of the LWS mounted on the exchange wheel
The instrument electronics can be divided into:
With the exception of the detector read-out, which is part of the focal-plane unit (FPU) and is mounted inside the cryostat, the remainder of the electronics are mounted either on the spacecraft's equipment platform or on the cryostat itself (ISOCAM preamplifier).
Single-element detectors (LWS, SWS and ISOPHOT) are read-out by means of integrating amplifiers, located close to the detector itself, on the support structure. The detector signals, which are generally in the microvolt range, are amplified and bandwidth-limited before being converted to digital form (12-bit analogue-to-digital converter).
This involves various tasks, such as:
Bias voltages are variable within a certain range and can be commanded to achieve optimum detector performance. Voltages are as high as 100 V and need extremely good filtering as any noise would directly feed into the detector amplifier and thereby cause serious degradation.
As detector performance is temperature-dependent, this temperature must either be known exactly or controlled within narrow limits. For this purpose, the FPUs contain a variety of temperature sensors capable of measuring in the range up to 10 K.
Various motor types are used to rotate/move the FPU mechanisms such as the filter wheels and the grating. All of these motors have been carefully selected for minimum power dissipation.
Mechanisms are monitored using position encoders. The CAM instrument, for example, uses magneto-resistors for wheel origin and step counting. Hall sensors are used to determine the positions of the three PHT ratchet wheels. Servo loops are used for the gratings and etalons in the spectrometers.
All instruments will be completely controlled by one of two redundant 16-bit microprocessor assemblies.
Each assembly consists of a processor, memories and spacecraft interfaces. Address, data and control busses are generally separated, but each assembly can interface to any part of the analogue electronics.
In summary, the assemblies will:
Operational software is stored in read-only memory and can be called automatically or by ground command. Some of the operational software is loaded in RAM.
All instruments use redundant converters to generate secondary voltages from the 28 V spacecraft bus.
Instrument sensitivities dictate the use of opto-couplers and pulse transformers as digital interface circuits in order to separate the various grounding points. For analogue signals, differential stages are used.
A total mass of 90.5 kg is allocated to the instruments (36.1 kg to the FPUs). The total instrument power is 80 W, but only 10 mW are allowed to be dissipated per FPU for thermal reasons.
A more unusual 'resource' on ISO is the limited cross-section of the cryo-harness which connects the FPUs with the warm electronics. Stainless-steel and brass in two wire gauges have been selected, to minimise the heat loss along the harness.
With an average cryo-harness length of 5 m, this leads to typical operational resistance values of 500 ohm for stainless steel. Brass wires are only used where the higher wire resistance would lead to impractical voltages and/or high dissipative losses in the harness.
The scientific instruments that make up the ISO spacecraft's scientific payload constitute a complete, complementary and versatile package for infrared astronomy.
Having completed the design and development phase, the qualification models of the instruments were delivered to ESA for integration into a suitable cryostat and subsequently underwent a test and verification programme. Upon delivery of the flight models, those models were integrated into the ISO satellite. After the completion of the full test programme, ISO is now awaiting launch.
The authors wish to acknowledge the efforts of the ISO Principal Investigators (noted in Table 1) responsible for the four instruments making up the ISO spacecraft's scientific payload, and their groups, on whose work this article was based.
ESA Bulletin Nr. 84.