Optical sparse aperture imaging

Abstract

The resolution of a conventional diffraction-limited imaging system is proportional to its pupil diameter. A primary goal of sparse aperture imaging is to enhance resolution while minimizing the total light collection area; the latter being desirable, in part, because of the cost of large, monolithic apertures. Performance metrics are defined and used to evaluate several sparse aperture arrays constructed from multiple, identical, circular subapertures. Subaperture piston and/or tilt effects on image quality are also considered. We selected arrays with compact nonredundant autocorrelations first described by Golay. We vary both the number of subapertures and their relative spacings to arrive at an optimized array. We report the results of an experiment in which we synthesized an image from multiple subaperture pupil fields by masking a large lens with a Golay array. For this experiment we imaged a slant edge feature of an ISO12233 resolution target in order to measure the modulation transfer function. We note the contrast reduction inherent in images formed through sparse aperture arrays and demonstrate the use of a Wiener-Helstrom filter to restore contrast in our experimental images. Finally, we describe a method to synthesize images from multiple subaperture focal plane intensity images using a phase retrieval algorithm to obtain estimates of subaperture pupil fields. Experimental results from synthesizing an image of a point object from multiple subaperture images are presented, and weaknesses of the phase retrieval method for this application are discussed.

Deep-turbulence wavefront sensing using digital-holographic detection in the off-axis image plane recording geometry

“…In practice, digitalholographic detection allows for the estimation of the complex field in the presence of an extended noncooperative object via speckle averaging and image sharpening algorithms or the angular diversity created by using multiple transmitters and receivers. [7][8][9][10][11][12][13][14][15][16][17][18] This versatility allows for long-range imaging, 19 three-dimensional imaging, 20 laser radar, 21 and synthetic-aperture imaging. 22 In general, the applications are abundant.…”

Active Sparse-Aperture Millimeter-Wave Imaging Using Digital Correlators

“…However, the autocorrelation is not an invertible function and, as such, an array that yields a desired MTF cannot be calculated analytically. Numerical optimizations can be used to calculate the arrays, but are computationally expensive [14].…”

“…Golay arrived at these configurations using algorithms based on random guessing and by limiting those solutions to various grid patterns [14]. These configurations can be applied to arrays that have different fill factors and have been applied to imaging systems having individual apertures of varying sizes [16].…”

Experimental demonstration of enhanced resolution of a Golay3 sparse-aperture telescope

“…For traditional monolith telescopes, the cost and volume undesirably increase with increasing aperture. The concept of sparse aperture imaging, which is aimed at achieving a high-resolution telescope by combining several smaller telescopes, has grown out of the quest for addressing the problem [3,4] . As the simplest sparse-aperture configuration that can improve the resolution along all the directions and serve as a good start for other complex designs, Golay3 telescope systems have captured the interests of many scientists and have been researched a lot.…”

“…There are many choices of single characteristic MTF cutoff frequency such as ρ max , ρ min , or a mean of ρ min and ρ max . Considering the inevitable system errors, such as aberrations or magnification error, we select ρ min as the cutoff frequency, the most conservative measure also chosen for other sparse-aperture system [4,12] . Then the effective diameter D eff can be defined according to [4] …”