A detailed laboratory experiment has been completed which models a simultaneous multiple beam Fourier telescopy
(FT) technique capable of imaging rapidly changing objects. Fourier telescopy uses multiple beams that illuminate the
target with a complex fringe pattern that sweeps across it due to frequency differences between beams. Using this
method, the target spatial frequency components are encoded in the temporal signal that is reflected from the target.
Previous work has concentrated on system designs where the target is illuminated with 3 individual beams in order to use
a standard phase closure process. Data processing and image reconstruction for this laboratory experiment invoked a
novel reconstruction algorithm that has been previously developed. The algorithm compensates for atmospheric phase
fluctuations affecting the large number of beams transmitted simultaneously and includes a new type of global phase
closure which allows image reconstruction from the time history of measured total reflected intensity from the target.
The reconstruction algorithm also solves for hundreds of image Fourier components simultaneously, permitting rapid
reconstruction of the image. This multiple beam laboratory experiment includes effects from realistic photon and speckle noise. Additional effects have been expanded to include uplink turbulence, piston jitter, and beam scintillation on the target, which will be encountered in an actual FT imaging system. Experimental results have obtained reconstructed image Strehl values which are greater than 0.9 under scaled system conditions.
Fourier telescopy (FT) is an active imaging technique that is a candidate for high resolution imaging systems which can be used to obtain satellite images out to geosynchronous target ranges. Fourier telescopy uses multiple beams that illuminate the target with a fringe pattern that sweeps across it due to a set frequency difference between beams. In this way the target spatial frequency components are encoded in the temporal signal that is reflected from the target. The FT receiver can then be composed of a large area "light bucket" collector, since only the integrated temporal signal is necessary to reconstruct the target image. The GEO Light Imaging National Testbed (GLINT) system was previously designed to obtain satellite images at geosynchronous ranges by using this technique. The "light bucket" receiver was designed use forty heliostats, each having a collection area of ten meters square, and composed of a 16 x 16 grid of two foot square mirrors. The heliostats would redirect the return light from the target onto a large spherical concentrator array composed of hexagonal mirror segments. This concentrator would then focus the return light onto a photomultiplier tube (PMT) detector. The FT Field experiment presented in this paper uses one 10-meter square heliostat and a single PMT, plus a scaled down secondary array to provide the optical elements of the receiver for the FT field experiment. In this paper, we will describe the performance characteristics of the heliostat, secondary, and PMT detector. Performance characteristics include optical wavefront, alignment, and alignment stability of the optical elements. Finally, results will be presented after the receiver was integrated with a transmitter system that provided the modulated FT signal from various targets. Image reconstructions will show that even using low quality "Light bucket" receiver optics and a 1.5 km horizontal path through the atmosphere, the modulated signal can still produce good image quality of the targets. Image reconstruction will also be presented for different SNR values in the received signal.