Background
The predecessor of XOSS, BOSS, proposed using large satellite for multiple occultations to improve resolution for closely spaced objects such as binary stars and to distinguish dim objects around bright sources such as planets around a star. The concept of occultation is not new. Natural occulters such as the Moon and the Earth have been used to improve resolution on both ground and space-based telescopes. For example, BEOM, the Burst and Transient Source Experiment (BATSE) Earth Occultation Monitoring system, has sucessfully filtered out background signals for observations of hard X-rays in the energy range of 20 keV to 2 MeV. With an artificial occulter, we can schedule the occultation whenever we desire and control the occulter to minimize angular velocity relative to the detector.
To make the best of the occultation method, the occulter is set up as a mask of uniformly redundant arrays (URA). In theory, a pinhole camera has the best resolution but it allows very little light through. To compromise between photon count and resolution, the URA mask is 50% transparent and 50% opaque to photons. As the mask moves across the detector.s field of view (FOV), we can determine the direction of the incoming photons by measuring how the mask pattern shifted. This is known as coded aperture imaging. The mask pattern encodes the image of the sky and a correlation method is used to decode the image. This method can also subtract out part of the background during reconstruction calculation to improve the signal, and is best when utilized with URA to maintain high resolution. This method works particularly well for X-rays because the size of the occulter is not constrained by diffraction loss. The diffraction of X-rays around the occulting satellite can essentially be neglected.
Coded Aperture Imaging
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