Focusing X-ray telescopes have been the most important factor in X-ray astronomy’s
ascent to equality with optical and radio astronomy. They are the prime tool for studying
thermal emission from very high temperature regions, non-thermal synchrotron radiation
from very high energy particles in magnetic fields and inverse Compton scattering of
lower energy photons into the X-ray band. Four missions with focusing grazing incidence
X-ray telescopes based upon the Wolter 1 geometry are currently operating in space
within the 0.2 to 10 keV band. Two observatory class missions have been operating since
1999 with both imaging capability and high resolution dispersive spectrometers. They are
NASA’s Chandra X-ray Observatory, which has an angular resolution of 0.5 arc seconds
and an area of 0.1 m2 and ESA’s XMM-Newton which has 3 co-aligned telescopes with a
combined effective area of 0.43 m2 and a resolution of 15 arc seconds. The two others are
Japan’s Suzaku with lower spatial resolution and non-dispersive spectroscopy and the
XRT of Swift which observes and precisely positions the X-ray afterglows of gamma-ray
bursts. New missions include focusing telescopes with much broader bandwidth and
telescopes that will perform a new sky survey. NASA, ESA, and Japan’s space agency
are collaborating in developing an observatory with very large effective area for very
high energy resolution dispersive and non-dispersive spectroscopy. New technologies are
required to improve upon the angular resolution of Chandra. Adaptive optics should
provide modest improvement. However, orders of magnitude improvement can be
achieved only by employing physical optics. Transmitting diffractive-refractive lenses are
capable theoretically of achieving sub-milli arc second resolution. X-ray interferometry
could in theory achieve 0.1 micro arc second resolution, which is sufficient to image the
event horizon of super massive black holes at the center of nearby active galaxies.
However, the physical optics systems have focal lengths in the range 103 to 104 km and
cannot be realized until the technology for accurately positioned long distance formation
flying between optics and detector is developed.
Low-Temperature and High-Energy-Resolution Laser Photoemission Spectroscopy
