High-energy astronomy, specifically X- and gamma-ray astronomy are young sciences, they are children of the space age – born only 50 years ago. We need to go to space to get a good view of the high-energy sky. Contrary to optical astronomy which cover only a narrow span of wavelengths the high energy astronomy spans many decades of energy. It is therefore no surprise that the observational challenges vary strongly between observations of low energy X-rays and of high energy gamma-rays. The differences are clearly illustrated when we compare the million known sources of keV X-rays, with the scant thirty sources known to emit at 1 MeV. At still higher energies life gets a bit easier, and the number of known sources raises again. The reasons for this huge discrepancy will be discussed as well as the efforts to penetrate into the difficult energy band around 1 MeV.
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