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The International Ultraviolet Explorer Satellite

While attending the FAS Convention at Herstmonceux on 08 October 1988, I attended the lecture by Dr David Stickland, of the Rutherford Appleton Laboratory (RAL), about the International Ultra-violet Explorer (IUE) satellite. If the number of people in the conference room at Herstmonceux Castle for the lecture was anything to go by, the IUE certainly has been the world's most popular instrument. The conference room is in the nature of a large hall, and it was packed so full that some people were standing. I estimate that I was about 480th out of some 500 people to get into the lecture. My coat became my seat on the carpet right by the speaker, so I had no excuse for missing anything, and mine must have been the most visible hand to him later raised to ask questions. The wall behind was my seat-back. Anthony (my son) chose a perch in the wall undoubtedly meant originally to house the bust of some bygone dignitary. We were soon transported away from the physical trappings of Earth, out into the ever-visible "skies" of geostationary orbit.

IUE uses a telescope with a 45 cm diameter beryllium mirror to make observations of the sky in the UV portion of the spectrum. It operates in an elliptical geostationary Earth orbit, outside the Earth's turbulent atmosphere, bad weather conditions and atmospheric glare. Other advantages of its location include freedom from distortion of the telescope mass by Earth's gravity, no trouble with the atmosphere attacking the optics, and no problems with wind-buffeting. One disadvantage of a telescope in geostationary orbit is the lack of an atmosphere to shelter the instrumentation from ultra-fast-moving particles.

IUE has been enormously successful. Its reception range of 110 nm to 300 nm encompasses a colossal amount of information available in the higher energy bands of the spectrum beyond visible light. Its location above the Earth's atmosphere enables it to undertake observations that are not possible with Earthbound equipment, as any telescope underneath the ozone layer and immersed in the atmosphere is shielded by atmospheric oxygen from most UV radiation of shorter wavelength than about 300 nm. For a given size of telescope, the shorter wavelengths of UV afford greater angular resolution than visible light.

It may seem amazing, but IUE has now been operational for twelve years. It is still functioning, although quite expectedly now with somewhat reduced capability. When one considers the environment in which it operates, it is hardly surprising that its solar cells have become a little degraded. But apparently the satellite is good for a few years yet if past performance is anything to go by. The fact that it started out with half a dozen stabilisers and now has only two or three still operational does not seem to have affected operation nearly as much as expected. It seems that the satellite's equipment complement was over-designed and that ground controllers exercised considerable ingenuity in maintaining its operation. David said that there are plans which will allow use of IUE with only one operational stabiliser - and tentative programmes for operation even when there are none...

IUE uses heaters around its beryllium mirror to maintain a constant temperature and retain the appropriate figure. If the temperature of the mirror was not controlled in this way, it might distort enough to impair results. Beryllium was chosen for its lightness, thermal conductivity and hardness. David hadn't all the info with him to answer my questions on this fully. (I know how he feels - preparation for every question requires a pantechnicon of information!) In operation (as of course in standby), the delicate sensors at the focus of the telescope have to be kept from pointing at the Sun or Moon and, if the telescope traverses a bright star like Capella, operators have to defocuss the optics as bright light would burn out the material of the sensors. Power to manouevre the telescope is limited and it is not possible to swing IUE around rapidly in the free suspension of space and then quickly fix it in a rock-steady orientation for several hours of observation. As a result, operation of the telescope is most efficient when observing targets are not at a great angular distance from one another; if the telescope has to be moved through a large angle between observing targets in concurrent observing programmes, up to an hour can be lost in completing repositioning.

Perhaps the most significant single piece of research accomplished by IUE was the identification of the UV source which blew up as supernova 1987A. Supernovae emit a lot of high energy UV light. The excellent resolution of IUE at UV wavelengths was able to resolve components of the progenitor star system that were almost inseparable in the longer wavelengths of visible and radio frequencies.

The most common use of the telescope is in ultraviolet stellar spectroscopy. The rays from a chosen star pass through a spectroscope and an image of the spectrum is beamed down to ground stations on the Earth for analysis. By comparing the spectrum of the star with spectral lines known from laboratory experiments on Earth it is possible to determine its composition.

Any serious astronomer, professional or amateur, can apply for time on IUE. If an astronomer has a research programme to undertake which involves study of the UV spectrum of stars or nebulae, including objects in the deep southern skies (yet another advantage of a telescope in geostationary orbit not restricted to view only objects above a certain declination), all that is necessary is to formulate a research programme and apply to RAL for time on IUE. Assuming that RAL accepts the proposal one then goes on to apply for accommodation in Spain to attend the IUE ground station in Madrid for the duration of the observing period. The normal period of time allotted on IUE is about a fortnight. The terms of the agreement regarding intellectual property are that results remain the exclusive property of the researcher for six months after the observations are taken, then they are published.

Not having the ability, due to other commitments, to vacate England for Spain to pursue research with IUE, I did not ask David who pays the cost of accommodation, fares or any other incidentals. But for anyone who may be able to go on this sort of half-work, half-play holiday, it seems a great idea! David said that one could arrange things so the day could start with breakfast at 09:30, spend a few hours on the observing programme, be ready for an evening out afterwards, followed by the requisite sleep in preparation for the next day, and so on...

Many thanks to David for a very absorbing lecture and slide show.

Roy Adams