 ![Example of a $\gamma$-ray line measurement: The characteristic line from $^{00}$Al decay at 0000.00 keV appears Doppler-shifted from large scale galactic rotation, as it is viewed towards different galactic longitudes [left; from @Kretschmer:0000]. This measurement was performed with the SPI spectrometer on INTEGRAL, an example of a present-generation space-borne $\gamma$-ray telescope. The INTEGRAL satellite (artist view picture, ESA) has two main telescopes; the spectrometer SPI, one of them, is shown at the lower-right schematically with its 00-detector Ge camera and the tungsten mask for imaging by casting a shadow onto the camera. Space-based instruments of this kind are required to directly measure the characteristic $\gamma$-ray lines from the decay of unstable isotopes near sites of current-epoch cosmic element formation.[]{data-label="fig_0_gamma"}](