The International Ultraviolet Explorer (IUE) was an astronomical observatory satellite primarily designed to take ultraviolet spectra. The satellite was a collaborative project between NASA, the UK Science Research Council and the European Space Agency (ESA). The mission was first proposed in early 1964, by a group of scientists in the United Kingdom, and was launched on January 26, 1978 aboard a NASA Delta rocket. The mission lifetime was initially set for 3 years, but in the end it lasted almost 18 years, with the satellite being shut down in 1996. The switch-off occurred for financial reasons, while the telescope was still functioning at near original efficiency.
It was the first space observatory to be operated in real time by astronomers who visited the groundstations in the United States and Europe. Astronomers made over 104,000 observations using the IUE, of objects ranging from solar system bodies to distant quasars. Among the significant scientific results from IUE data were the first large scale studies of stellar winds, accurate measurements of the way interstellar dust absorbs light, and measurements of the supernova SN1987a which showed that it defied stellar evolution theories as they then stood. When the mission ended, it was considered the most successful astronomical satellite ever.
The instrumentation on board consisted of the Fine Error Sensors (FES), which were used for pointing and guiding the telescope, a high resolution and a low resolution spectrograph, and four detectors.
There were two Fine Error Sensors (FES), and their first purpose was to image the field of view of the telescope in visible light. They could detect stars down to 14th magnitude, about 1500 times fainter than can be seen with the naked eye from Earth. The image was transmitted to the ground station, where the observer would verify that the telescope was pointing at the correct field, and then acquire the exact object to be observed. If the object to be observed was fainter than 14th magnitude, the observer would point the telescope at a star that could be seen, and then apply "blind" offsets, determined from the coordinates of the objects. The accuracy of the pointing was generally better than 2 arcseconds for blind offsets.
The FES acquisition images were the telescope's only imaging capability; for UV observations, it only recorded spectra. For this, it was equipped with two spectrographs. They were called the Short Wavelength Spectrograph and the Long Wavelength Spectrograph, and covered wavelength ranges of 115 to 200 nanometres and 185 to 330 nm respectively. Each spectrograph had both high and low resolution modes, with spectral resolutions of 0.02 and 0.6 nm respectively.
The spectrographs could be used with either of two apertures. The larger aperture was a slot with a field of view roughly 10 × 20 arcsec; the smaller aperture was a circle about 3 arcsec in diameter. The quality of the telescope optics was such that point sources appeared about 3 arcsec across, so use of the smaller aperture required very accurate pointing, and it did not necessarily capture all of the light from the object. The larger aperture was therefore most commonly used, and the smaller aperture only used when the larger field of view would have contained unwanted emission from other objects.
There were two cameras for each spectrograph, one designated the primary and the second being redundant in case of failure of the first. The cameras were named LWP, LWR, SWP and SWR where P stands for prime, R for redundant and LW/SW for long/short wavelength. The cameras were television cameras, sensitive only to visible light, and light gathered by the telescope and spectrographs first fell on a UV-to-visible converter. This was a caesium-tellurium cathode, which was inert when exposed to visible light, but which gave off electrons when struck by UV photons due to thephotoelectric effect. The electrons were then detected by the TV cameras. The signal could be integrated for up to many hours, before being transmitted to Earth at the end of the exposure.
Use of the telescope was divided between NASA, ESA and SERC in approximate proportion to their relative contributions to the satellite construction: two thirds of the time was available to NASA, and one sixth each to ESA and SERC. Telescope time was obtained by submitting proposals, which were reviewed annually. Each of the three agencies considered applications separately for its allocated observing time. Astronomers of any nationality could apply for telescope time, choosing whichever agency they preferred to apply to.
If an astronomer was awarded time, then when their observations were scheduled, they would travel to the ground stations which operated the satellite, so that they could see and evaluate their data as it was taken. This mode of operation was very different to most space facilities, for which data is taken with no real time input from the astronomer concerned, and instead resembled the use of ground-based telescopes.
For most of its lifetime, the telescope was operated in three eight-hour shifts each day, two from the US ground station at the Goddard Space Flight Center in Maryland, and one from the ESA ground station at Villanueva de la Cañada near Madrid. Because of its elliptical orbit, the spacecraft spent part of each day in the Van Allen belts, during which time science observations suffered from higher background noise. This time occurred during the second US shift each day, and was generally used for calibration observations and spacecraft 'housekeeping', as well as for science observations that could be done with short exposure times.
The twice-daily transatlantic handovers required telephone contact between Spain and the US to coordinate the switch. Observations were not coordinated between the stations, so that the astronomers taking over after the handover would not know where the telescope would be pointing when their shift started. This sometimes meant that observing shifts started with a lengthy pointing manoeuvre, but allowed maximum flexibility in scheduling of observing blocks.
Data was transmitted to Earth in real time at the end of each science observation. The camera read-out formed an image of 768×768 pixels, and the analogue-to-digital converter resulted in adynamic range of 8 bits. The data was then transmitted to Earth via one of six transmitters on the spacecraft; four were S-band transmitters, placed at points around the spacecraft such that no matter what its attitude, one could transmit to the ground, and two were VHF transmitters, which could sustain a lower bandwidth, but consumed less power, and also transmitted in all directions. The VHF transmitters were used when the spacecraft was in the Earth's shadow and thus reliant on battery power instead of solar power.
In normal operations, observers could hold the telescope in position and wait approximately 20 minutes for the data to be transmitted, if they wanted the option of repeating the observation, or they could slew to the next target and then start the data transmission to Earth while observing the next target.
The data transmitted were used for "quick look" purposes only, and full calibration was carried out by IUE staff later. Astronomers were then sent their data on magnetic tape by post, about a week after processing. From the date of the observation, the observers had a six month proprietary period during which only they had access to the data. After six months, it became public.
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