Telescope Concepts in Gamma-Ray Astronomy

This chapter outlines the general principles for the detection and characterisation of high-energy $\gamma$-ray photons in the energy range from MeV to GeV. Applications of these fundamental photon-matter interaction processes to the construction of instruments for $\gamma$-ray astronomy are described, including a short review of past and present realisations of telescopes. The constraints encountered in operating telescopes on high-altitude balloon and satellite platforms are described in the context of the strong instrumental background from cosmic rays as well as astrophysical sources. The basic telescope concepts start from the general collimator aperture in the MeV range over its improvements through coded-mask and Compton telescopes, to pair production telescopes in the GeV range. Other apertures as well as understanding the measurement principles of $\gamma$-ray astrophysics from simulations to calibrations are also provided.