International Ultraviolet Explorer
International Ultraviolet Explorer was the first space observatory primarily designed to take ultraviolet electromagnetic spectrum. The satellite was a collaborative project between NASA, the United Kingdom's Science and Engineering Research Council and the European Space Agency, formerly European Space Research Organisation. The mission was first proposed in early 1964, by a group of scientists in the United Kingdom, and was launched on 26 January 1978, 17:36:00 UTC aboard a NASA Thor-Delta 2914 launch vehicle. The mission lifetime was initially set for 3 years, but in the end, it lasted 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 ground stations in the United States and Spain. 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 SN 1987A 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.
History
Motivation
The human eye can perceive light with wavelengths between roughly 350 and 700 nanometres. Ultraviolet light has wavelengths between roughly 10 nm and 350 nm. UV light can be harmful to human beings and is strongly absorbed by the ozone layer. This makes it impossible to observe UV emission from astronomical objects from the ground. Many types of objects emit copious quantities of UV radiation, though: the hottest and most massive stars in the universe can have surface temperatures high enough that the vast majority of their light is emitted in the UV. Active Galactic Nuclei, accretion disks, and supernovae all emit UV radiation strongly, and many chemical elements have strong absorption lines in the UV so that UV absorption by the interstellar medium provides a powerful tool for studying its composition.Ultraviolet astronomy was impossible before the Space Age, and some of the first space telescopes were UV telescopes designed to observe this previously inaccessible region of the electromagnetic spectrum. One particular success was the second Orbiting Astronomical Observatory, which had a number of UV telescopes on board. It was launched in 1968 and took the first UV observations of 1200 objects, mostly stars. The success of OAO-2 motivated astronomers to consider larger missions.
Conception
The orbiting ultraviolet satellite which ultimately became the IUE mission was first proposed in 1964 by British astronomer Robert Wilson. The European Space Research Organisation was planning a Large Astronomical Satellite, and had sought proposals from the astronomical community for its aims and design. Wilson headed a British team which proposed an ultraviolet spectrograph, and their design was recommended for acceptance in 1966.However, management problems and cost overruns led to the cancellation of the LAS program in 1968. Wilson's team scaled down their plans and submitted a more modest proposal to ESRO, but this was not selected as the Cosmic Ray satellite was given precedence. Rather than give up on the idea of an orbiting UV telescope, they instead sent their plans to NASA astronomer Leo Goldberg, and in 1973 the plans were approved. The proposed telescope was renamed the International Ultraviolet Explorer.
Design and aims
The telescope was designed from the start to be operated in real-time, rather than by remote control. This required that it would be launched into a geosynchronous orbit – that is, one with a period equal to one sidereal day of 23 h 56 m. A satellite in such an orbit remains visible from a given point on the Earth's surface for many hours at a time, and can thus transmit to a single ground station for a long period of time. Most space observatories in Earth orbit, such as the Hubble Space Telescope, are in a low Earth orbit in which they spend most of their time operating autonomously because only a small fraction of the Earth's surface can see them at a given time. Hubble, for example, orbits the Earth at an altitude of approximately, while a geosynchronous orbit has an average altitude of.As well as allowing continuous communications with ground stations, a geosynchronous orbit also allows a larger portion of the sky to be viewed continuously. Because the distance from Earth is greater, the Earth occupies a much smaller portion of the sky as seen from the satellite than it does from low Earth orbit.
A launch into a geosynchronous orbit requires much more energy for a given weight of payload than a launch into a low Earth orbit. This meant that the telescope had to be relatively small, with a primary mirror, and a total weight of. Hubble, in comparison, weighs 11.1 tonnes and has a mirror. The largest ground-based telescope, the Gran Telescopio Canarias, has a primary mirror across. A smaller mirror means less light-gathering power, and less spatial resolution, compared to a larger mirror.
The stated aims of the telescope at the start of the mission were:
- To obtain high-resolution spectra of stars of all spectral types to determine their physical characteristics;
- To study gas streams in and around binary star system;
- To observe faint stars, galaxies and quasars at low resolution, interpreting these spectra by reference to high-resolution spectra;
- To observe the spectra of planets and comets;
- To make repeated observations of objects with variable spectrum;
- To study the modification of starlight caused by interstellar dust and gas.
Construction and engineering
According to the agreement setting up the project the observing time would be divided between the contributing agencies with 2/3 to NASA, 1/6 to ESA and 1/6 to the UK's SERC.
Mirror
The telescope mirror was a reflector of the Ritchey–Chrétien telescope type, which has hyperbolic primary and secondary mirrors. The primary was across. The telescope was designed to give high-quality images over a 16 arcminute field of view. The primary mirror was made of beryllium, and the secondary of fused silica – materials chosen for their light weight, moderate cost, and optical quality.Instruments
The instrumentation on board consisted of the Fine Error Sensors, 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, 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 arcsecond for blind offsets
The FES acquisition images were the telescope's only imaging capability; for UV observations, it only recorded spectrum. 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 0.60-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 arcseconds; the smaller aperture was a circle about 3 arcseconds in diameter. The quality of the telescope optics was such that point sources appeared about 3 arcseconds across, so the 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 was 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 the photoelectric 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.