Uniformly Redundant Arrays (URA's) are two–dimensional binary arrays with constant sidelobes of their periodic autocorrelation function. They are widely agreed upon to be optimum mask patterns for coded aperture imaging, particularly in imaging systems with a cyclic coded mask. In this paper, a survey of all currently known construction methods for URA's is given and the sizes and open fractions of the arrays resulting from each construction method are pointed out. Alternatives to URA's for situations in which a URA does not exist, are discussed.

Uniformly Redundant Arrays

Reference structure tomography (RST) uses multidimensional modulations to encode mappings between radiating objects and measurements. RST may be used to image source-density distributions, estimate source parameters, or classify sources. The RST paradigm permits scan-free multidimensional imaging, data-efficient and computation-efficient source analysis, and direct abstraction of physical features. We introduce the basic concepts of RST and illustrate the use of RST for multidimensional imaging based on a geometric radiation model.

Reference structure tomography.

The degree of linear polarization in solar flares has not yet been precisely determined despite multiple attempts to measure it with different missions. The high energy range, in particular, has very rarely been explored, due to its greater instrumental difficulties. We approached the subject using the Reuven Ramaty High Energy Spectroscopic Imager (RHESSI) satellite to study six X-class and 1 M-class flares in the energy range between 100 and 350 keV. Using RHESSI as a polarimeter requires the application of strict cuts to the event list in order to extract those photons that are Compton scattered between two detectors. Our measurements show polarization values between 2 and 54%, with errors ranging from 10 to 26% in 1σ level. In view of the large uncertainties in both the magnitude and direction of the polarization vector, the results can only reject source models with extreme properties.

/pdf/x-ray-polarization-of-solar-flares-measured-with-rhessi-k5pczg2me6.pdf

X-Ray Polarization of Solar Flares Measured with Rhessi

This paper describes a compressive sensing strategy developed under the Compressive Optical MONTAGE Photography Initiative. Multiplex and multi-channel measurements are generally necessary for compressive sensing. In a compressive imaging system described here, static focal plane coding is used with multiple image apertures for non-degenerate multiplexing and multiple channel sampling. According to classical analysis, one might expect the number of pixels in a reconstructed image to equal the total number of pixels across the sampling channels, but we demonstrate that the system can achieve up to 50% compression with conventional benchmarking images. In general, the compression rate depends on the compression potential of an image with respect to the coding and decoding schemes employed in the system.

/pdf/compressive-imaging-sensors-1ajijvj52g.pdf

Compressive imaging sensors

We present mid- and far-infrared spectral energy distributions (SED) for 10 objects from the CfA Seyfert sample, obtained with the Infrared Space Observatory (ISO). We apply an inversion Bayesian method to recover the observed SEDs, assuming that the mid- and far-IR emission of Seyfert galaxies is of thermal origin. The result is that the SED of these galaxies can be reproduced with thermal emission from two or three different sources: a warm component (T ~ 150 K) that originates in dust close to the active galactic nucleus (AGN), a cold component (T ~ 40-50 K) due to dust heated in star-forming regions, and a very cold component (T ~ 10-20 K) due to dust heated by the general galactic interstellar radiation field. We also note that the temperature of the warm component for the type 2 Seyfert galaxies tends to be lower than that of the type 1 Seyfert galaxies.

The Mid- and Far-Infrared Spectral Energy Distribution of Seyfert Galaxies*

Imaging with mirrors is difficult at high photon energies, and this has led to the development by X- and gamma-ray astronomers of a variety of alternative approaches to forming images. Most of these involve 'coding' the sky brightness distribution, such that a single bright point source at a given position produces a complex but characteristic signature on the detector. The mathematical description of the imaging process, and the advantages and disadvantages of focusing and non-focusing approaches are discussed. The problem of image reconstruction in the focusing case is similar to that in familiar optical imaging applications, though complicated by variations in point spread function with position and energy. The authors concentrate on the inversion of coded images, which are more characteristic of high-energy astronomy. The design of the coding process, which is crucial to achieving good imaging performance, is discussed, as are techniques for image reconstruction in the case of 'perfect coding', and a variety of remedies for making the best use of imperfectly coded data.

https://iopscience.iop.org/article/10.1088/0266-5611/11/4/004/pdf

T J Ponman

Papers

Inverse problems in X-ray and gamma-ray astronomical imaging