= X-ray astronomy
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<X-ray astronomy> infers the properties of celestial sources from their <X-rays>. It requires instruments above the strongly absorbing layers of <Earth>'s <terrestrial atmosphere>. <Riccardo Giacconi>'s 2002 Nobel lecture connects the early rocket detections to later <grazing-incidence X-ray optics>, which collect <photons> over a large entrance area onto a small detector.<The Dawn of X-Ray Astronomy by Riccardo Giacconi>[The Dawn of X-Ray Astronomy, introduction and section 4, pp. 123–124]{ref}
The measurement is an <inverse problem>: a count rate mixes incident <photon> arrival rate, detector response, and background. A <point-like astronomical source> produces a direction-dependent excess as the instrument scans; <diffuse X-ray background> contributes over many directions. Neither a count excess nor a narrow feature by itself determines the emission mechanism. The useful progression is from detecting an excess, to locating it, to measuring its <spectrum> and <light curve>.
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