Coded Mask Imagers when to use them – and when not

Coded mask imagers are among a class of high energy astronomical instruments in which spatial or temporal coding, or a combination of the two, allows ‘multiplexing’ of the detection process. In this way many ‘pixels’ of an image can be observed and measured simultaneously. In certain circumstances this gives an advantage in signal to noise ratio, compared with an alternative approach in which each pixel is examined sequentially. The signal-to-noise ratio will, however, never be as good as that possible with focussing optics. The possible types of multiplexed instrument are discussed in the context of gamma-ray line astrophysics and compared with instruments in which optics are used to concentrate the flux.