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 400 eV at 1 keV and CCDs less than 100 eV. While the resolution slightly degrades with increasing energy, the resolving power AE/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 AX, their resolving power AX/X 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.
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