Coded aperture imaging in low energy gamma ray astronomy

The applicability of the coded aperture technique of producing images in the low energy gamma-ray regime is investigated with particular reference to the development of the ZEBRA balloon-borne telescope. This instrument operates in the energy range from 200 keV to 10 MeV, an area of the electromagnetic spectrum of great astrophysical importance, and it has been designed so as to produce images of the celestial sphere with a resolution of 1o.11 x 1o.15 within a fully coded field of view of 6o.84 x 8o.83. The point source location accuracy is at the level of a few arcminutes. Possible sources of imperfection in the images produced by the ZEBRA telescope have been addressed and methods with which to eliminate or reduce their effects have been investigated. In particular, a fast algorithm by means of which vignetted sources may be identified and their true positions and intensities calculated has been evaluated. It is shown to be particularly suited for use with the next generation of gamma-ray instruments where the sensitivities will be such that few sources will lie in one image. The performance of the telescope is presented in terms of sensitivity to both continuum flux and line emission, calculated with a detailed estimation of the expected background. The imaging capabilities have been investigated by means of both experimental tests with partial and complete detection planes, and with Monte-Carlo computer simulations of typical observations which would be performed during the flight of the instrument. (DX90528)