Conventional Shack-Hartmann Wavefront Sensing requires a point source such as a star to perform wavefront sensing. This software allows one to conduct such sensing using an extended-scene or scene-based image. The software allows a Shack-Hartmann Wavefront Sensor (SH-WFS) to be used with an extended scene. It estimates the shift between the positions of a reference and test extended-scene sub-images or cells.
It runs a series of six steps:
- It determines the centers of all of the sub-images, and stores these x and y positions in a [2×Ncell] matrix. Usually, the centroids of the point-source spot-images measured in the same system can serve this purpose.
- It chooses an N×N-pixels subaperture, S(x,y), within each sub-image with the x and y positions determined in step 1 as its center, where N is preferentially a power of two for this algorithm (e.g., N=32). Such a subaperture is referred to as a test cell. It also chooses one M×M-pixels reference cell, r(x,y), preferably near the center of the whole extended scene image, where M<N is preferentially also a power of two (e.g., M=16).
- It calculates the cross-correlation (CC) of r(x,y) and s(x,y), where s(x,y) is the central M×M-pixels portion of S(x,y), using FFT (fast Fourier transform). It determines the location of the CC-peak by fitting a quadratic curve to three points near and including the CC-peak in the xdirection, and doing the same in the y-direction. This is done analytically since there are three data for three unknown parameters in such a fit.
- It shifts the test cell S(x,y)by the amount determined in the previous step to match it with the reference cell.
- It repeats Steps 3 and 4 in an iteration loop, while accumulating the sub-image offsets dx and dy between the test and the reference cells, until a maximum iteration number is reached or the incremental change in sub-image offset becomes smaller than a pre-determined tolerance.
- It repeats Steps 2 to 5 for all of the test cells.
This work was done by Erkin Sidick of Caltech for NASA’s Jet Propulsion Laboratory.
The software used in this innovation is available for commercial licensing. Please contact Dan Broderick at
This Brief includes a Technical Support Package (TSP).
Adaptive Periodic-Correlation Algorithm for Shack-Hartmann Wavefront Sensing
(reference NPO-48003) is currently available for download from the TSP library.
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Overview
The document is a Technical Support Package from NASA's Jet Propulsion Laboratory (JPL) detailing the Adaptive Periodic-Correlation Algorithm for Shack-Hartmann Wavefront Sensing (SHWS). It is part of NASA Tech Briefs (NPO-48003) and aims to disseminate advancements in wavefront sensing technology that have broader scientific, technological, and commercial applications.
The Shack-Hartmann Wavefront Sensor (SHWS) is a critical tool in adaptive optics, used to measure wavefront distortions in optical systems. The document discusses the capabilities of SHWS in both point-source and extended-scene imaging. It highlights the use of a lenslet array and a Shack-Hartmann camera to capture wavefront distortions, which can be caused by local variations in the incoming beam. The SHWS can effectively handle a large capture range, making it versatile for various applications.
One of the key features discussed is the extended-scene SHWS testbed, which allows for the simulation of extended scenes using a uniform arc-lamp illumination. This setup includes a deformable mirror (DM) and an extended scene simulator, which are essential for testing and refining the wavefront sensing algorithms. The document also mentions the importance of vignetting, which can lead to non-uniform sub-images in the SHWS frame, affecting the accuracy of measurements.
The document provides a schematic diagram of the extended-scene SHS testbed, illustrating the components involved, including the phase retrieval camera and obscuration mask, which are crucial for establishing the system's optical parameters. It emphasizes the significance of these technologies in improving the performance of optical systems, particularly in aerospace applications.
Additionally, the document includes contact information for further inquiries and emphasizes compliance with U.S. export regulations, indicating that the information may contain proprietary content.
In summary, this Technical Support Package serves as a comprehensive overview of the Adaptive Periodic-Correlation Algorithm for Shack-Hartmann Wavefront Sensing, detailing its applications, the technology involved, and the importance of accurate wavefront measurement in advancing optical systems for aerospace and other fields.
