The History Behind EUVE

Extreme Ultraviolet Astronomy is still in its infancy even though scientists obtained evidence nearly 20 years ago that sources beyond the Solar System could be detected. Before embarking on a mission as ambitious as the Extreme Ultraviolet Explorer, scientists had to be sure they could observe a reasonable number of diverse, scientifically interesting objects at extreme ultraviolet wavelengths through the interstellar medium. They also needed assurances that the technology existed to conduct such a mission.

Understanding the Interstellar Medium

Early thinking on the distribution of interstellar gas was strongly influenced by studies carried out by ground-based radio telescopes during the 1950s. These observations showed that the spiral-shaped Galaxy was pervaded by diffuse clouds of hydrogen, helium and less abundant gases. Because hydrogen efficiently absorbs extreme ultraviolet radiation, scientists concluded that the interstellar medium would block this radiation from virtually any object outside the Solar System. Their views were supported by early sounding rocket experiments, which also failed to detect any sources.

Sugestions that the scientists were wrong began to emerge from observations by NASA's Copernicus satellite (Orbiting Astronomical Observatory-3), launched in 1972. The mission showed that the interstellar medium -- once thought to be an evenly distributed, absorbing fog -- was pervaded by hot, low density regions shaped like bubbles and tunnels, like an ant's nest or rabbit warren beneath apparently solid ground.

However, it was not until 1975 that a crucial Berkeley-developed experiment conducted during the Apollo-Soyuz mission completely disproved this view. The NASA crew repeatedly oriented the spacecraft to point a relatively crude extreme ultraviolet telescope at 30 celestial targets. Five sources were detected, including HZ 43, an unusually hot white dwarf in the nearby constellation of Coma Berenices.

The 1975 discovery was a triple milestone in astrophysics. It identified HZ 43 as the hottest and most luminous white dwarf star then known. It proved that white dwarf stars, though faint in ordinary visible light, are powerful emitters of ultraviolet radiation. And it showed that the density of the local interstellar gas was low enough to allow significant numbers of scientifically intriguing objects to be observed and studied at these wavelengths.

Following the Apollo-Soyuz mission, scientists from around the world further studied nearby objects and discovered additional extreme ultraviolet sources using a variety of instruments flown on sounding rockets, NASA's Voyager spacecraft, and EXOSTAT, a European X-ray satellite. Together, these observations revealed a sufficient number of interesting extreme ultraviolet sources to warrant the Extreme Ultraviolet Explorer mission.

Technological Advances

The first extreme ultraviolet observations were of the Sun in 1959. However, astronomers had to wait more than 16 years for instrumentation to be developed with sufficient sensitivity and imaging quality to detect sources beyond the Solar System.

The extreme ultraviolet region of the spectrum is a particularly difficult one to observe. Light at these wavelengths is absorbed rather than reflected by conventional telescope mirrors. It also is absorbed by all but the thinnest materials. Therefore, components used in standard telescopes and spectrometers, such as lenses, filters, and transmission gratings, cannot be used for extreme ultraviolet studies. Similarly, ordinary visible light and ultraviolet light detectors cannot be used at these wavelengths because their protective covers absorbs the extreme ultraviolet light, preventing it from reaching the actual detection devices. And because extreme ultraviolet studies departed from typical astronomical investigations, standards for calibrating laboratory measurements had not been established.

Fortunately, the laws of physics provide instrument inventors with ways to overcome these seemingly insurmountable problems. First, it has long been known that mirrors will reflect X-ray and extreme ultraviolet radiation if the light strikes the mirrors at sufficiently steep angles. Scientists, therefore, developed special "grazing-incidence" mirrors in which the light collecting surface does not directly face the source, but is instead positioned almost parallel to the incoming radiation. This approach also works with diffraction gratings, which can be used at grazing-incidence angles to separate the incoming extreme ultraviolet radiation into its individual wavelengths.

Unfortunately, when incoming light strikes such mirrors or gratings, the slightest surface irregularity will change the direction of the reflecting light rays, scattering them like gold balls that hit irregularities on the green. As a result, grazing-incidence mirror and grating surfaces also must be made exceptionally smooth, a requirement that demands high-precision machining and careful polishing.

Being able to produce high-quality extreme ultraviolet images and spectra is of no value, however, unless a detector is available to record them. Fortunately, Berkeley scientists have developed detectors that do not require a protective cover and, therefore, can work in this spectral region. These detectors not only sense the position of each incoming photon, but also record the exact moment it is received. From this information, photographic quality images can be created.

With the scientific doubts removed and the calibration methods and instrumentation improved, the time was right for the Extreme Ultraviolet Explorer mission.