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From the description of the Compton telescope given previously (Schonfelder et al., this volume), one can see that the accuracy with which one determines the position of a cosmic gamma-ray source depends not only on the measurements of the energy deposited in the upper (D1) and lower (D2) detectors, but also on how accurately one estimates the (X, Y, Z) positions of each gamma-ray or neutron interaction (an event). If nothing were known about the position of each event except in which module it occured, it would increase the uncertainty in the position of a source by on the order of 10°. Within each COMPTEL module, one extracts position information from comparisons of relative intensities of signals in the photomultipier tubes. This technique was introduced in the 1950’s for medical imaging by Anger (1958), and later was adapted to astrophysical applications (Zych et al. 1983; Schonfelder et al. 1984; Stacy 1985).

The imaging Compton telescope COMPTEL on the Gamma Ray Observatory has unusual spectroscopic capabilities for measuring solar γ-ray and neutron emissions. Flares can be observed above the 800 keV γ-ray threshold of the telescope. The telescope energy range extends to 30 MeV with high time resolution burst spectra available from 0.1 to 10 MeV. Strong Compton tail suppression facilitates improved spectral analysis of solar flare γ-ray emissions. In addition, the high signal-to-noise ratio for neutron detection and measurement provides new neutron spectroscopic capabilities. For example, a flare similar to that of 1982 June 3 will yield spectroscopic data on > 1500 individual neutrons, enough to construct an unambiguous spectrum in the energy range of 20 to 150 MeV. Details of the instrument response to solar γ-rays and neutrons are presented.

The COMVIEL telescope onboard the NASA Gamma Ray Observatory is an instrument with imaging properties quite different from telescopes at longer wavelengths: Focusing of the incoming radiation is impossible due to the penetrating power of the gamma ray photons, the photons rather interact with the electrons and nuclei of the detector material in a particle-like manner. This affects the way the response of the instrument is described, determined, and used. The instrument consists of 2 planes of detector modules, which measure a photon via two consecutive interactions of probabilistic nature: a Compton scatter in the upper detector plane (’D1’), and an absorption in the lower detector plane (’D2’) (see figure 1). The active scintillation detectors are surrounded by plastic anticoincidence detector domes to reject events originating from charged particles. Background events are suppressed by measurement of the pulse shape of the scintillation light flash in the upper detectors, and by measurement of the time of flight of the scattered photon from upper to lower detector plane.