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Electromagnetic radiation in the X-ray band – nanometer and shorter wavelengths, cannot be focused as easily as for optical, radio and other lower energy bands. It is possible to use the grazing angle incidence technique to focus these higher energy photons, as in the case of the Soft X-ray Telescope on ASTROSAT, but this focussing technique can be used only for fields narrower than ~ 1°. Coded mask imaging is one possible way of performing wide field imaging with photons of energy greater than a few keV. It comprises of utilising the shadows of a multiple pinhole mask plate cast on the detector, with the shift in the shadows encoding the location of the source in the sky. For the literature on the coded mask concepts one can refer to the webpage on GSFC, NASA site Coded Aperture Imaging in High-Energy Astronomy by Jean in 't Zand
Two of the four X-ray instruments aboard ASTROSAT are based on the coded mask imaging concept: 
The SSM uses a one-dimensional imaging system with a position sensitive proportional counter as the detector. The CZTI comprises of a two dimensional mask plate mounted on top of a pixellated CZT detector array. The coded mask plates for both the SSM and the CZTI have been designed based on pseudo-noise Hadamard set Uniformly Redundant Arrays.