The optimisation of discrete pixel coded aperture telescopes

Abstract

Following reviews of the astrophysical goals for the INTEGRAL imager and the developments which have made such a mission possible, the fundamental characteristics of coded aperture telescopes based on pixellated detector arrays are investigated in detail. The point source capability of such a telescope is examined first, resulting in a relationship describing the sensitivity dependence on the detector pixel/mask element sampling ratio. To complement this work, the sensitivity of the coded mask technique is then examined with respect to extended sources. It is shown that mask patterns optimised for high angular resolution imaging are less sensitive to extended sources. The situation can be improved at the expense of the point source capability by reducing the resolution of the mask pattern. However, several techniques including the use of wavelet transforms and dual-scale mask configurations, are shown to be capable of either extracting source-scale information or improving the extended source sensitivity. Throughout these chapters it has been assumed that a coded mask observation leads directly to a shadowgram which can be deconvolved.

On the contrary, it is shown that for a pixellated detector array, deconvolution can proceed only after some form of incident pixel reconstruction (IPR) has been applied to account for multiple pixel interactions. The optimum IPR algorithm is derived for the INTEGRAL imager EIDOS and the effects of IPR on telescope performance are discussed. Although it is demonstrated that in certain situations correlation analysis is the optimum method of image reconstruction, the reliability of correlation comes into question under extreme observing conditions. Consequently, the final chapters focus on maintaining the correlation image quality through the use of pre and post-processing techniques. The penultimate chapter outlines the theoretical development of these techniques and the final chapter covers the results of their application to a prototype coded aperture imager based on EIDOS detector technology.

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