Overview

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.