High Resolution Spectroscopy

On most instruments in the past, spectroscopy was performed with the same instruments as used for photometry. Imaging proportional counters have an energy resolution of about 400eV at 1 keV and CCDs less than 100eV. While the resolution slightly degrades with increasing energy, the resolving power ΔE/E of those instruments severely degrades toward lower energies. However, the X-ray spectroscopic data provide the most direct insight into the physical phenomena, which drive the X-ray energy release. Furthermore, most of the spectral features, both in emission and absorption occur at energies up to a few kiloelectronvolt, requiring a much better spectral resolution than provided by detectors alone. At low energies this can be only accomplished by means of dispersive elements, i.e., diffractive elements in the beam path of a telescope. Diffraction spectrometers as transmission or reflection gratings have an almost constant resolution Δλ, their resolving power Δλ/λ degrades, in contrast to detectors, with decreasing wavelength or increasing energy, respectively. This gives a “break even point” between both instrumentations (Fig. 8.1) at intermediate energies, which is now shifting more and more toward lower energies with the development of better energy resolving X-ray cameras, like bolometers and tunnel junction detectors.