Optimizing wide-field coded aperture imaging: radial mask holes and scanning

Abstract

Imaging at hard X-ray energies (~10-600 keV) over very large fields of view (~60 o per telescope) is required to conduct a high sensitivity all-sky and all-time survey for black holes. The proposed Energetic X-ray Imaging Survey Telescope (EXIST) could achieve the high sensitivity required for the mission science objectives by scanning an array of widefield coded aperture telescopes with aperture mask holes radially aligned to minimize auto-collimation by the thick (~7mm) masks required for high energy imaging. Simulation results from a preliminary design study are reported which quantify the improvement in off-axis imaging sensitivity vs. the conventional case with mask holes all perpendicular to the mask. Such masks can be readily constructed from a stacked laminate of thin (1mm) laser-etched W sheets. An even more dramatic increase in coded aperture imaging sensitivity, and dynamic range, for a realistic telescope and imaging detector with typical systematic errors can be achieved by continuously scanning the field of view of the telescope over the source region to be imaged. Simulation results are reported for detectors with systematic errors 1-10%, randomly distributed but unknown in each detector pixel. For the simplified case of a 1-D coded aperture telescope scanning along its pattern, the systematics are removed identically. Results are also presented for the 2-D case with both 1-D and partial 2-D scanning which demonstrate the feasibility of a coded aperture scanning telescope with systematic errors achieving nearly Poisson-limited sensitivity for signal/background ratios S/B ~ 10 -4 , in constrast to limits typically ~10-100X worse that have been actually achieved by pointed or dithered coded aperture telescopes flown (or proposed) previously.

Sensitivity of coded mask telescopes

“…The finite thickness of the mask elements and/or that of a supporting grid, or the partial transparency of the structure may modify the offaxis response. Grindlay and Hong [45] have discussed approaches to some of the problems associated with such complications. …”

“…In a well designed instrument and with appropriate treatment of the data, performance close to the Poisson limit can nevertheless be achieved even for comparatively long observations -particularly if the telescope orientation is 'dithered' or scanned during the observation to reduce the systematic noise, as forms part of the INTEGRAL observing strategy [2], as is important for SWIFT/BAT survey work [44], and is planned for EX-IST [45,46].…”

Monte-Carlo simulations of the expected imaging performance of EXIST high-energy telescope

“…Either of them will have some effect on the imaging performance of the EXIST telescope. One possible approach to overcome the problems due to non-uniformities in the detector plane is scanning 11 . In this approach, the pointing direction of the coded aperture telescope is changed continuously (or in very small steps) such that the X-ray source of interest passes across the full field of view.…”

A Fieldable-Prototype, Large-Area, Gamma-Ray Imager for Orphan Source Search

“…In addition to the data collected for the source injection studies and the cross-country trip, we also examined the need for the mask/antimask approach. With the hypothesis that the generation of scanned images significantly reduces the likelihood of artifacts in the image [21], the primary reason to use the mask/antimask approach is to remove the offset from zero counts due to unmodulated background reported earlier [4]. However, a careful examination of how the images are formed allows one to correct for this in the data processing.…”