|This article needs additional citations for verification. (July 2007)|
Coded Apertures or Coded-Aperture Masks are grids, gratings, or other patterns of materials opaque to various wavelengths of light. The wavelengths are usually high-energy radiation such as X-rays and gamma rays. By blocking and unblocking light in a known pattern, a coded "shadow" is cast upon a plane of detectors. Using computer algorithms, properties of the original light source can be deduced from the shadow on the detectors. Coded apertures are used in X- and gamma rays because their high energies pass through normal lenses and mirrors.
Image formation, normally done at optical wavelengths by lenses and mirrors, must be done for non-focusable wavelengths via image modulation. The pinhole camera is the most basic form of such a modulation imager, but a single aperture mask can contain many holes, in one of several particular patterns, to improve the throughput for hard X-rays and γ-rays, for example. Multiple masks, at varying distances from a detector, add flexibility to this tool. Specifically the modulation collimator, invented by Minoru Oda, was used to identify the first cosmic X-ray source and thereby to launch the new field of X-ray astronomy in 1965. Many other applications in other fields, such as tomography, have since appeared.
In a coded aperture more complicated than a pinhole camera, images from multiple apertures will overlap at the plate or detector array. It is thus necessary to use a computational algorithm (which depends on the precise configuration of the aperture arrays) to reconstruct the original image. In this way a sharp image can be achieved without a lens. The image is formed from the whole array of sensors and is therefore tolerant to faults in individual sensors; on the other hand it accepts more background radiation than a focusing-optics imager (e.g., a refracting or reflecting telescope), and therefore is normally not favored at wavelengths where these techniques can be applied.
Different mask patterns exhibit different image resolutions, sensitivities and background-noise rejection, and computational simplicities and ambiguities, aside from their relative ease of construction.
- FZP = Fresnel Zone Plate
- ORA = Optimized RAndom pattern
- URA = Uniformly Redundant Array
- HURA = Hexagonal URA1
- MURA = Modified Uniformly Redundant Array
- Rossi X-ray Timing Explorer (RXTE) - ASM
- BeppoSAX - Wide Field Camera (1996-2002)
- INTEGRAL - IBIS and SPI
- Swift - BAT (2004- )
- Ultra-Fast Flash Observatory - Pathfinder mission UFFO-P (planned launch in mid-2013) and UFFO-100 (its next generation) 2
- Astrosat - CZTI (Possible launch in 2013)
- In addition, the SAS-3 and RHESSI missions detect radiation based on a combination of masks and rotational modulation
- Pinhole camera
- Rotational Modulation Collimator
- Computational photography
- Tomographic reconstruction and X-ray computed tomography