R. Staubert and J. Triimper
The advancement of X-ray astronomy since its start about half a century ago has been strongly dependent on the development of instruments and observational techniques. Since the earth's atmosphere is opaque for X- and gamma-rays this field could only develop in parallel to space technology providing the necessary carriers, which can place X-ray astronomy telescopes and detectors near or beyond the boundaries of our atmosphere. In the early days, in the sixties and seventies, stratospheric balloons and rockets played an important role, albeit with severe limitations on altitude (^40 km, leaving still substantial absorption) and on observing time of a few minutes, respectively. Today, satellites are available allowing X-ray astronomy missions to last for a decade or longer. The principle mode of measurement in X-ray astronomy is to detect individual photons with the aim to determine the complete set of four properties: arrival direction (leading to images), the energy and the time of arrival of the photon, and its polarization angle. The first detectors were proportional counters and scintillation counters, originally developed for detecting charged particles in nuclear physics research. They had effective areas of a few hundred square centimeters and were usually equipped with mechanical collimators providing some indirect imaging capability through the restriction of the field of view (typically to a few square degrees) and the possibility for scanning observations. An important challenge for these detectors was the reduction of the background radiation, both from photons of the diffuse X-ray sky background and from charged particles of the ever present cosmic rays. This was achieved by narrow colimators and the invention of various techniques of anticoincidence and veto schemes, as perfected for example in multiwire proportional counters. The first X-ray satellite Uhuru, launched in December 1970, carried collimated gas proportional counters and was scanning the entire X-ray sky. The detection of ^400 X-ray sources marked a quantum leap in X-ray astronomy. The so called "gas scintillation proportional counter," combined the two physical detector principles and gave an improved energy resolution, but had limited application and scientific impact.
The next major step was the introduction of focusing and imaging X-ray optics, the Wolter telescope, together with imaging detectors in the focal plane providing two-dimensional X-ray images. The first satellite mission, the Einstein Observatory, carrying such a telescope with the imaging proportional counter (IPC) and the high resolution imager (HRI) as focal plane detectors allowed a break through in two areas: extended objects could directly be imaged, and for all sources the sensitivity was greatly improved through the focusing and the corresponding background reduction.
ROSAT performed the first all sky survey with an imaging telescope. Using a greatly improved telescope and detector technology, it provided a large step in the observational capabilities, both in the number of detected X-ray sources
125.000), and through the large number of pointings throughout the remaining 8 years of the mission. Today the standard focal plane detector is based on actively cooled pixelized solid state detectors (CCDs), which provide a higher energy resolution and wider energy range than proportional counters. The use of CCDs was pioneered by ASCA and further perfected on Chandra and XMM-Newton.
In parallel to imaging telescopes, high resolution grating spectrometers were developed, first used in the Einstein Observatory and today with great success in the Chandra and XMM-Newton missions. Intensive work has also gone into the development of very deeply cooled bolometers, which have a great potential because of their very high spectral resolution and large throughput. Unfortunately, the first bolometer flown on a satellite exploded with ASTRO-E, and the second attempt on Suzaku failed because the cryogenic coolant was lost before the observations commenced. At higher photon energies (>10keV), focusing becomes difficult and the current technique is imaging by spatial aperture modulation, the so called "coded mask" technique, first used in the Mir-KVANT mission and now on INTEGRAL. Efforts are underway to develop also focusing telescopes for hard X-rays and even gamma-rays by employing multilayer-coded reflecting surfaces or making use of Bragg reflection on crystals. Polarimetry is still in a rudimentary state. Imaging, high resolution spectroscopy, and high time resolution measurements have reached a high level of sophistication with a corresponding wealth of scientific results, but there is still a wide open field for further advances.
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