On the properties of images from coded-mask telescopes Free

We consider coded-mask telescopes with cyclic mask patterns that are ‘optimum’, i.e. patterns that are based on uniformly redundant arrays (URAs), which can be derived using cyclic difference sets. The properties of images obtained with different reconstruction arrays are compared to find the best options. In the usual case where the detector background is imperfectly known, it is easily shown that, although the elements of an image reconstructed by any of the usual algorithms are nearly independent of each other and of the detector background, they cannot be perfectly so. The correlation between image elements that results is evaluated for various reconstruction algorithms and the circumstances in which it may be important are discussed. Although the effect is small when point sources are considered, it may be significant where sources of emission cover an appreciable fraction of the field of view. Masks based on ‘modified URAs’ lead to slightly worse interdependence of image pixels.