1994A&A...288..665I

We consider issues that concern the mathematical description of coded aperture patterns. Primarily this involves the relation between the open fraction of such patterns and the signal-to-noise ratio of imaged point sources. A refinement of the corresponding theory is introduced, taking into account the spatial response of the coded aperture camera. From this we predict that patterns with an open fraction of less than 0.5 can enhance the performance of coded aperture cameras to bright point sources, as opposed to what was previously thought. As an application of the refined theory, we tested candidate open fractions in the instrumental configuration of two identical, wide field, coded aperture X-ray cameras (1.8-30 keV), that will be part of the X-ray satellite SAX (to be launched in late 1995). These tests consisted of numerical simulations of several types of observations, and show that open fractions between 0.25 and 0.33 are to be favored for the SAX cameras. The improvement in signal-to-noise ratio with respect to the commonly used open fraction of 0.5 is up to 30%. Whenever telemetry limits the data coverage, this profit may well be larger. We also address additional aperture constraints as applied to the SAX cameras, such as the aperture geometry and pattern. From this analysis we propose a new type of coded aperture pattern for the SAX cameras with an open fraction equal to 0.33, which possesses near-ideal intrinsic mathematical properties.