Dilute Aperture Research Articles

Calculation and correction of subaperture tilt aberration modes in synthesis imaging

We extend the redundant spacings calibration method for finding piston coefficients affecting the elements of a dilute aperture array so that tilt phase coefficients can also be calculated and corrected without the need for assumptions about the object. The tilt coefficient retrieval method is successfully demonstrated in simulation, and the specifics of correction by image sharpness are discussed, showing that in dilute aperture systems this method does not necessarily produce a unique image.

Calculation and correction of piston phase aberration in synthesis imaging

The principle of redundant spacings calibration has previously been described for the purpose of calibrating piston phase aberration affecting elements of a dilute aperture array using a system of linear equations in terms of the aperture phases as well as object phase information. Here we develop matrices for the correction of piston phase aberration by use of image sharpness and also by phase retrieval. These are both presented in wavefront sensor formulation in order to draw analogy between the approaches. We then discuss solution ambiguity affecting both methods and describe array design criteria to prevent such ambiguity. The problem of increased image aliasing under image sharpness correction is also highlighted.

X-Ray Interferometry

For the astronomer, X-ray interferometry is the theory and practice of building dilute aperture telescopes for studying celestial X-ray sources. The short wavelengths and high surface brightness of X-ray sources will make the eventual scientific payoff very high, with direct imaging of the event horizons of black holes as the centerpiece. In this article, we review the history of X-ray interferometry and discuss the recent technical developments toward astronomical applications. We present several mission concepts and show they are achievable with today’s technology.

Dilute aperture diffraction imagery and object reconstruction: II. Deterministic aberrations

Replacement of a large continuum by a set of appropriately placed small subapertures offers a number of advantages if the object being viewed can be reconstructed reasonably accurately from the subaperture diffraction imagery. A previous paper was devoted to this problem and numerical calculations were carried out for the aberration-free situation. In the present paper, we extend the numerical calculations to determine the influence of deterministic wavefront aberration on the efficiency of object reconstruction in the presence of measurement noise. Our calculations seem to indicate that moderate amounts of wavefront aberrations do not appear to affect the inversion.

Extrasolar planet detection at infrared wavelengths from the Earth

We show that extrasolar planets in orbit around nearby stars can be detected from the ground or from a stratospheric telescope in the infrared region of the spectrum. We present calculations on the detectability of extrasolar planets, using Erich Grossman's Atmospheric Transmission (AT) code to compute atmospheric transmission at bands centered at 11 μm, 20 μm, 27 μm, 200 μm, 225 μm, 350 μm, 450 μm, 640 μm, 750 μm, and 870 μm. Detection limits for terrestrial and Jovian planets orbiting Sun‐like stars are presented by assuming Planck emission. We consider several potential sites including the south pole, a 5000‐m elevation site in the Atacama Desert in Chile, Mauna Kea in Hawaii, and an aerostat‐borne telescope flown near the poles. We consider extrasolar planet detection with a 10‐m class mid‐IR telescope and a dilute aperture stratospheric telescope consisting of 4‐m mirrors optimized for this task. Detection in the submillimeter, even with a 104 m2 collecting area array is extremely difficult because of low atmospheric transparency, and the decrease (∝ λ−2) in planet emission with increasing wavelength in the Rayleigh‐Jeans limit. We discuss critical technologies needed for this undertaking, including tethered aerostats and balloon‐borne telescopes, the development of mid‐IR nulling interferometry, actively cooled optics operating in the atmosphere, and optimized filters that are matched to the atmospheric transmission windows.

Imaging a coherently illuminated object through a random screen by using a dilute aperture

We consider the properties of the average-energy spectrum of an object that is coherently illuminated and then viewed after it passes through a random screen. We show that use of a dilute aperture permits the retrieval of diffraction-limited information for a statistically varying object. The factors influencing the accuracy of this procedure are discussed. Results of a computer simulation that are consistent with our theoretical predictions are presented.

Dilute aperture diffraction imagery and object reconstruction

Replacement of a large continuous aperture by a set of small apertures offers a number of advantages if the subapertures can be placed within the array so as to achieve minimal spatial frequency redundancy. Using an approach based on combinatorics and integer programming, we have found solutions for a linear array for N (number of subapertures) up to eight. In my opinion, too much emphasis has been placed on the subaperture positioning problem and not enough on the resultant diffraction imagery. To this end, we have calculated the diffraction images of extended incoherent objects for these optimum arrays. We then proceed to discuss inversion techniques whereby we determine the object intensity from the diffraction images.