Limiting sensitivities of coded-aperture telescopes for gamma-ray astronomy: Balloon-Borne fixed-mask systems

Abstract

The limiting sensitivities of coded-aperture imaging telescopes employing fixed masks are derived for continuum and line emission from cosmic point sources. The sensitivities are calculated for a single-source observation and do not take into consideration the many advantages offered by a multiplex system; for instance, low susceptibility to secular background changes and the ability to observe more than one source during an observation period. For the nuclear transition energy region, it is shown that the utilization of a coded-aperture mask by a particular detection system does not significantly degrade its performance relative to conventional, sequential scanning instruments. It is further shown that for short source observation times (e.g., typical of those obtained from stratospheric balloons), the coded-aperture imaging technique can be particularly advantageous. The effects of a non-uniform instrumental background on the imaging process are discussed and a correction procedure suggested. It is found that by careful planning of the observing program coupled with a stable instrument design, image degradation due to background non-uniformities can be made arbitrarily small and the resulting performance made to approach that predicted for an equivalent mask-antimask system.