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A Brief History of X-ray Telescopes

The first usage of X-ray telescopes for astronomy was for observing the Sun, the only X-ray source in the sky producing an abundance of signal. Because the Sun is so bright in X-rays, the focusing element can be small, and photographic film can be used as a detection medium. The first X-ray picture of the Sun employing a rocket-borne telescope was taken by John V. Lindsay of the NASA Goddard Space Flight Center and collaborators in 1963. The first orbiting X-ray telescope flew on Skylab in the early 1970's and recorded over 35,000 full-disk images of the Sun over a 9-month period.

The utilization of X-ray mirrors for extra-solar X-ray astronomy had to await the development of electronic detectors with both high quantum efficiency and the ability to determine the location of the arrival of an X-ray photon in two dimensions. The first such detectors were the Imaging Proportional Counter and the microchannel plate detector. Subsequently, more sensitive detectors including CCD spectrometers and imaging gas scintillation proportional counters have been employed.

As was true with the solar observations, the initial observations employing X-ray imaging systems utilized sounding rockets. The first successful X-ray image of an extra-solar object was obtained by Paul Gorenstein of the Smithsonian Astrophysical Observatory and collaborators in 1975. They used a Kirkpatrick-Baez mirror coupled with an imaging proportional counter to obtain an image of the Virgo cluster of galaxies (and subsequently two other clusters). The inaugural use of Wolter optics for extra-solar astronomy was performed by Saul Rappaport of MIT and his collaborators in 1977. Over the course of two rocket flights, they obtained the first true images of supernova remnants. Figure 1 shows their image of the Cygnus Loop supernova remnant.

Cygnus Loop X-ray Supernova Remnant
Figure 1 - The Cygnus Loop Supernova Remnant in X-rays

Major advances in imaging, and in X-ray astronomy in general, came with the launch of the first orbiting X-ray telescope, the Einstein Observatory, in 1978. Einstein demonstrated the ubiquity of X-ray emission from all classes of objects, and revealed for the first time the structure of extended objects such as nearby galaxies and supernova remnants. Many other imagers have subsequently been placed into orbit. Among the missions that have carried imagers are EXOSAT, ROSAT, and ASCA. In Figures 2 & 3, we show the ROSAT HRI and PSPC images of the Cygnus Loop. The incredible detail in these pictures demonstrates the power of orbiting X-ray imagers.

Cygnus Loop X-ray Supernova Remnant (ROSAT HRI)
Figure 2 - The Cygnus Loop Supernova Remnant as seen by the ROSAT High Resolution Imager (HRI) instrument.
Cygnus Loop X-ray Supernova Remnant (ROSAT PSPC)
Figure 3 - The Cygnus Loop Supernova Remnant as seen by the ROSAT Position Sensitive Proportional Counter (PSPC) instrument.

Contributed by Rob Petre of the Laboratory for High-Energy Astrophysics, GSFC

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