Physics – Optics
Scientific paper
Oct 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995xmm..pres...21.&link_type=abstract
XMM Press Release INFO 21-1995
Physics
Optics
Scientific paper
As Roger Bonnet, director of ESA's science programme, explains: "The ISO launch will be the culmination of twelve years of intensive effort to build tile mast powerful and precise infrared space telescope to dote, We fully expect astronomers in Europe. and all over the wand, to be rewarded with unprecedented insights into the particularly rich and fertile sources of the universe hidden in the infrared." For the mission looks set to mark a new step forward for astronomy. To date, infrared astronomy has always been hampered by two major difficulties.
The first is that infrared radiation is thermal, calorific. This means that any body which is not absolutely cold naturally emits this type of "heat" or "hidden" radiation, Consequently, at the corresponding wavelengths, all telescopes, detectors and the Earth's atmosphere appear to shine spontaneously, Infrared observation is therefore tantamount to attempting to observe the stars in broad daylight using a telescope a million times more luminous than the stars themselves.
Moreover, the atmosphere is partly or totally opaque to infrared light. Hence the great attraction of conducting such observations from space, above the Earth's atmospheric layers. using a very low-temperature telescope.
The first - and, so far, truly - Satellite actually to have done so is the Infrared Astronomical Satellite (IRAS) - a joint UK/Dutch/US mission - which surveyed and catalogued the sky far ten months in 1983. A corner of the curtain was thus lifted, The scene was set. Now, the task is to observe each character, each object.
And here, infrared astronomy is hampered by a second major difficulty: the very detection of light. For while infrared radiation was discovered back in 1800 (in the solar spectrum). infrared-sensitive photoelectric detectors did not materialise until the Second World War, IRAS itself had to make do without the multi-dimensional set-ups that would have allowed it to record true images. It had to build up its images dot by dot, line by line.
One of ISO's major contributions will be the first in-orbit use of just such arrays of detectors, which can be compared to the CCD optics of video cameras now available to the general public. ISO will therefore be able to record very long photographic exposures and provide very detailed high- definition images. It will be able to observe a given object for up to ten hours continuously, exceeding IRAS performance with a thousand-fold increase in sensitivity and resolution ten times higher.
As the project managers maintain, "with its sensitivity to thermal radiation and its sharp images, the space, telescope should be able to detect an ice-cold object the size of human being 100 km away "
Of course, the technical development and technology for so innovative and powerful an instrument as this have thrown up enormous challenges. Hans Steinz, ESA project manager, explains; "This is the first satellite of its kind to be built in Europe and the work has drown an the efforts of no fewer than thirty-five highly specialised firms." The prime challenge was to develop the cryogenic systems, which now make ISO akin to a giant thermos flask filled with 2100 litres of superfluid helium chilled to -271°C (1.8° above absolute zero).
Another challenge was the telescope's ultralight gold-coated quartz mirror. The finish required for this centrepiece of the observatory is such that, if its 60 cm diameter were artificially expanded to the size of the Earth's diameter, the residual "bumps" and "dips" of the reflecting surface would be no more than one metre up or down, the size of a child. All these problems have left the development project two years behind schedule - which is not in fact bad going, given its scope and complexity.
Now, however, all the problems have been overcome. The satellites capabilities correspond fully to expectations and since late June ISO has been in Kourou at the Guiana Space Centre, where it is undergoing all the necessary pre-launch tests.
The ISO spacecraft takes the form of a white cylinder 3.5 metres in diameter and 5.3 metres long. Inside are the actual telescope, the four scientific instruments, the electronics, power and radio communication systems. On the side, wide solar panels supply the 600 watts of electrical power and at the same time serve as a baffle shielding the satellite from solar radiation.
The spacecraft is fully covered with highly effective thermal insolation. if - purely hypothetically - its cryostat were filled with boiling water instead of liquid helium, it would take six years for ISO to drop to ambient temperature. This insulation is vital for minimising the space telescope's sensitivity to thermal disturbance caused by solar and terrestrial radiation.
The four focal-plane instruments mounted behind the telescope's mirror represent the scientific heart of the satellite. This set of instruments, built by the principal investigators and industrial consortia in ESA Member States, will provide measurements of unprecedented sensitivity and precision over a very wide wavelength range: 2.5 to 250 microns, of which the 120 to 250 micron region is as yet entirely unexplored.
The Isophot photometer, built under the supervision of the Max Planck Institute, Germany, is the instrument providing the most far-ranging wavelength coverage. When operating at very long wavelengths, a region undetected to date, it should reveal extremely cold stars and cosmic matter. Isophot measures the intensity and spatial distribution of light and the spectral components in the near infrared.
The Isocam camera, responsibility for which lies with the Service d'Astrophysique de Saclay, France, will take photographs in the 2.5 to 17 micron range with two arrays, each of 1024 individual infrared detectors (32 x 32 pixel). Some twenty filters select the infrared spectral range of the observations ("colours" ) and four optical lenses allow various magnification factors (from 1 to 8). Depending on the lens chosen, the finest detail observable will be from 1.5 to 12 arcsec per pixel, i.e. 1/1200 to 1/150 times the full diameter of the moon.
The SWS and LWS spectrometers, built under the supervision of the Groningen Laboratory for Space Research (Netherlands) and the Queen Mary and Westfield College, London (GB), respectively, form a single unit that will analyse the composition of the radiation spectrum from 2.5 to 200 microns. They will thereby provide access to an extraordinarily extensive range of physical phenomena which can then be studied with very great sharpness and precision (resolving power: 50 to 30 000).
At Kourou final preparations are currently underway: electromechanical checks, gyroscope alignment, fitting of the thermal protection, and helium and fuel filling. ISO will then be placed in the fairing of its Ariane 44P launcher in preparation for the countdown.
The launch is scheduled for 8 November 1995 and could in fact take place as late as 21 February 1996. ISO will be lifted by Ariane into a very elongated 24-hour orbit with a perigee of 1000 km and an apogee of 70 000 km. This elliptical trajectory will yield very high-quality scientific data for the 16 hours a day that the satellite is outside the Van Allen belts, whose radiation produces stray signals that hinder measurement-taking.
Ground control and radio tracking operations will be carried by a hundred or so experts working in shifts round the clock at ESA's Villafranca station near Madrid. Reflecting the project's international dimension, Japanese scientists will be involved in these operations. The NASA station at Goldstone, California, will relay communications when the satellite is out of Europe's view. Full coverage will thereby be provided with real-time links throughout the 18-month mission.
The Villafranca station will receive 170 million bytes of data daily, and ultimately 90 billion units of information over the total duration of the mission - enough to fill 10 million typed pages of A4 paper which if placed end-to-end could cover the 7000 km between Paris and Miami.
Observing time is already oversubscribe
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