Coded aperture

Coded apertures, such as grids or patterns of materials opaque to high-energy radiation like X-rays and gamma rays, create a "shadow" that allows the mathematical reconstruction of the original radiation source. Unlike lenses or mirrors, which cannot focus these wavelengths, coded apertures use computational methods to form images. The pinhole camera is a simple version but has low throughput due to its small aperture. More advanced masks with multiple holes improve this limitation.

The technique became essential in X-ray astronomy after the identification of the first cosmic X-ray source in 1965 using a modulation collimator. Coded apertures are used in various fields, including tomography. In complex systems, overlapping images from multiple apertures require algorithms for reconstruction, offering fault tolerance but higher background noise compared to focusing optics.

Well-known mask types include Fresnel Zone Plates and redundant arrays like URA and HURA, each with distinct resolution and computational properties. Space telescopes using coded apertures have been operational since 1985, with examples such as the Rossi X-ray Timing Explorer and Swift’s BAT. Future missions, including SVOM’s ECLAIRs, continue this tradition, while others combine masks with rotational modulation.

This method represents an early form of computational photography, linked to astronomical interferometry, and has been advanced by various researchers over the years.