Simulations of a 2 × 1.5D coded aperture camera for X-ray astronomy

¹ SRON Space Research Organization Netherlands, Niels Bohrweg 4, 2333 CA Leiden, The Netherlands
² INAF-IAPS Roma, via Fosso del Cavaliere 100, 00133 Rome, Italy
³ Institute of Space Sciences (ICE-CSIC), Campus UAB, 08193 Cerdanyola del Vallès (Barcelona), Spain
⁴ Institut d’Estudis Espacials de Catalunya (IEEC), Barcelona, Spain

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 10 February 2026
Accepted: 16 March 2026

Abstract

The concept of two perpendicular, 1D coded aperture cameras, necessitated by the imaging capability of the detector, is applied in the design of the Wide Field Monitor (WFM). The goal of this instrument is to monitor the variable X-ray sky for transient activity. Each camera features a fine angular resolution in one direction (typically 5 arcmin) and a coarse one in the other (5 degrees). The coarse perpendicular resolution makes the camera “1.5D”. The WFM has been studied for a number of space-bound X-ray observatory concepts: the Large Observatory For Timing (LOFT), the enhanced X-ray and Timing Polarimetry mission (eXTP), the Spectroscopic Time-Resolving Observatory for Broadband X-rays (Strobe-X), the Astrophysics of Relativistic Compact Object mission (ARCO), and now the Lunar Electromagnetic Monitor in X-rays (LEM-X). We report a study of two decoding algorithms for this instrument and its imaging performance. Detector responses to the X-ray sky are simulated, including the signal processing. The decoding algorithms are the iterative removal of sources (IROS), in combination with cross-correlation, and the maximum-likelihood method (MLM). IROS is most suited to the determination of the point-source configuration of the observed sky and MLM for the optimum determination of the source fluxes. The simulation results show that despite the 1.5D imaging of each camera, the reconstruction of scientific data is as if each camera pair were replaced by a single 2D camera with the same angular-resolution specification and a detector size equal to that of the two in the pair, except that source confusion is somewhat higher in the former. Thus, the WFM is a high-performance monitoring instrument with straightforward and proven technology that enables the identification of new cosmic X-ray sources, like X-ray novae, gamma-ray bursts, electromagnetic counterparts to gravitational-wave events, and interesting behavior of persistent X-ray sources, like accretion-disk state changes.