Sparse Aperture Telescope Research Articles

All-purpose automatic image stacking for sparse aperture telescopes using dynamic metric stochastic parallel gradient descent algorithm

Image stacking is the first step of alignment of the sparse aperture telescopes. We propose an all-purpose automatic image stacking technique using dynamic metric stochastic parallel gradient descent algorithm (DM-SPGD). With the image stacking modeled as an optimization problem, the dynamic metric functions consisting of a distance metric and an energy metric are defined. According to the judgment of the spot state, the two metrics, both computed from the focal spot at each step, are switched dynamically to perform SPGD optimization. By use of the dynamic metric functions, DM-SPGD can exactly and quickly correct any-amplitude tip/tilt errors, regardless of the array arrangements, aperture numbers and coherence as verified by simulation. In addition, jitter and changes in piston brought about by the environment have no effect on the image stacking. Multiple closed-loop image stacking experiments are performed, demonstrating that the offsets of randomly distributed spots can be all controlled below 0.5 pixels within 300 iterations (within 30 s).

Model-based large-dynamic iterative piston correction using extended objects.

Cophasing is crucial for segmented or sparse aperture telescopes to achieve high resolution. In this Letter, we propose a novel, to the best of our knowledge, model-based piston correction method that can remove large-scale piston errors within a few iterations using extended objects. The relation between the piston error and a metric function is derived theoretically under broadband illumination. The metric function is based on the image's power spectral density at the spatial frequency where the sidelobe peak of the modulation transfer function (MTF) appears. The piston error is iteratively estimated and corrected by introducing positive and negative piston biases. The dynamic range of piston correction can be as large as the coherence length of light. The correction accuracy in experiments is affected by the image noises and the accuracy of the introduced piston biases.

Demonstration of a Fizeau Directly-Imaging Sparse-Aperture Telescope with Pointing and Tracking Capabilities

At present, the majority of sparse-aperture telescopes (SATs) are unable to observe moving targets. In this paper, we describe the construction of and present the results obtained using a Fizeau directly-imaging sparse-aperture telescope (FDISAT) that permits pointing and the tracking of moving targets. The telescope comprises three sub-apertures, each of which is equipped with a Risley prism system that permits a maximum tracking range of 5° and has independent boresight adjustment capability. On targets in various positions, experiments with pointing and tracking are conducted. The maximum root-mean-square error (RMSE) of pointing in the sub-apertures was found to be 8.22 arcsec. When considering a target moving at 0.01°/s for approximately 320 s, the maximum RMSE of tracking in the sub-apertures was found to be 4.23 arcsec. The images obtained from the focal plane detector exhibit clear interference fringes while tracking. The experimental results demonstrate that the system can effectively track moving targets, providing a method for SAT observation of moving targets.

Research on the method of co-focus detection for sparse aperture telescope based on curvature sensing

Research on the method of co-focus detection for sparse aperture telescope based on curvature sensing

Decenter Error Sensing Technology of Sparse Aperture Telescope Systems

The imaging quality of sparse aperture telescope systems is degraded by several factors including design, manufacturing, and arrangement errors. To achieve a high-resolution, the optical system requires an accurate control of not only the wave-front errors such as the piston and tip-tilt but also the decenter errors. Decenter diminishes the field of view of synthetic systems heavily and causes frequency shifting as well. In this paper, we firstly present an approach to sense the decenter of the sparse aperture telescope using the multi-beam interference method based on phase shift and centroid extraction algorithm. The most attractive point of our method is that it can sense not only the decenter but also the other wave-front errors simultaneously, but does not need to install any additional sensors. The numerical simulations are described and preliminary experiments are performed for validation, demonstrating that this technique is efficient and accurate and could be used to guide future sparse aperture telescope error correction.

An off-axis Golay3 sparse aperture telescope with a freeform secondary mirror

The application of freeform surface in an optical system can effectively increase the freedom of optical design and reduce the number of optical elements. In this work, a two-mirror off-axis sparse aperture telescope is proposed. The primary mirror is made of three sub-mirrors arranged in the Golay3 configuration while the secondary is a freeform surface defined by a polynomial of two variables X and Y. The off-axis configuration is used to remove the obstruction of the secondary mirror. The results indicate that the fill factor of the Golay3 primary mirror increases to 58.4%, which is significantly higher than that of the on-axis configuration. The image quality is improved within 2.5° full field of view from its spot diagram, distortion, modulation transfer function, and encircled energy.

基于串联机械臂的稀疏孔径望远镜装调

基于串联机械臂的稀疏孔径望远镜装调

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

In this Letter, we report a Golay3 sparse-aperture telescope newly built in the Key Laboratory of Optical Engineering, Chinese Academy of Sciences and present the experimental results of enhanced resolution. The telescope consisting of 3 collector telescopes of 127 mm diameter can achieve a theoretical resolution corresponding to a monolithic aperture of 245 mm diameter. It is shown by the experimental results that the resolution is improved to 3.33 μrad with respect to the diffraction limit of 6.07 μrad for a single aperture using the Rayleigh criteria at 632 nm. The compact optical configuration and cophasing approach are also described.

Electromagnetic Formation Flight for Multisatellite Arrays

The use of propellant to maintain the relative orientation of multiple spacecraft in a sparse aperture telescope such as NASA’s Terrestrial Planet Finder (TPF) poses several issues. These include fuel depletion, optical contamination, plume impingement, thermal emission, and vibration excitation. An alternative is to eliminate the need for propellant, except for orbit transfer, and replace it with electromagnetic control. Relative separation, relative attitude, and inertial rotation of the array can all be controlled by creating electromagnetic dipoles on each spacecraft, in concert with reaction wheels, and varying their strengths and orientations. Whereas this does not require the existence of any naturally occurring magnetic fields, such as the Earth’s, such fields can be exploited. Optimized designs are discussed for a generic system and a feasible design is shown to exist for a five-spacecraft, 75-m baseline TPF interferometer.

Image quality of sparse-aperture designs for remote sensing

Sparse-aperture telescopes can be designed to capture the same resolution as a filled aperture but with a significant reduction in size and weight. image-chain models have been generated for several sparse-aperture remote sensing system concepts. These models are used to generate image simulations, which are used to quantify the image quality as a function of design parameters associated with various sparse-aperture designs. This study generated image simulation models for the Annulus, Golay 6, and Tri-Arm sparse-aperture designs. A psychophysical evaluation was conducted to determine the integration time necessary to maintain the image quality as the fill factor is decreased for these designs. Image simulations were also generated to determine the accuracy of the generalized image quality equation to predict the image quality differences between the different designs.

Low Cost Space Structure Experiment Optical Phasing Control System

A description is presented of a sparse aperture telescope system designed to automatically control piston and tilt errors of the optical wavefront. A 1/4 scale design of the flight telescope and full scale equivalents of the phasing control system were built for the laboratory test. The tilt control loops have demonstrated 100 Hz bandwidth and 3.5 microradian residuals. The ability of the piston control loop to correct optical path differences to 12 nm has also been demonstrated.