The in-focus PSF optimizer (IPO) is an algorithm for use in monitoring and controlling the alignment of the segments of a segmented-mirror astronomical telescope. IPO is so named because it computes wavefront aberrations of the telescope from digitized point-spread functions (PSFs) measured in in-focus images. Inasmuch as distant astronomical objects that behave optically as point sources can typically be seen in almost any astronomical image, the main benefit afforded by IPO may be to enable maintenance of mirror-segment alignments without detracting from valuable scientific-observation time.
IPO evolved from prescription-retrieval type algorithms. Prescription retrieval uses in-focus and out-of-focus PSFs to infer the state of an imaging optical system. The state, in this context, refers to the positions, orientations, and low-order figure errors of the optical elements in the system. Both prescription-retrieval and IPO use an iterative, nonlinear, least-squares optimizer to compute the optimal state parameters such that a digital computer-generated model image matches the digitized image acquired from the real system.
The difference between IPO and prescription-retrieval algorithms is that IPO is specifically designed to utilize in-focus images only. Although the restriction to in-focus images limits IPO to calculating only the lowest-order wave front aberrations, it also causes the resulting computation to take much less time because fewer degrees of freedom are included in the optimization process.
In the prescription retrieval software developed at JPL, the model images are generated using the ray-trace/physical optics program, MACOS. IPO, on the other hand, uses a linear sensitivity matrix to compute the exit-pupil wave front from the system parameters; the wave front is then converted into a complex pupil field, which is then propagated to the image plane via a fast Fourier transform. This approach is computationally faster and requires less computer memory than is needed for prescription retrieval.
This work was done by Catherine Ohara, David Redding, Fang Shi, Joseph Green, Philip Dumont, Scott Basinger, and Andrew Lowman of Caltech for NASA’s Jet Propulsion Laboratory and Laura Burns and Peter Petrone of Goddard Space Flight Center. For further information, access the Technical Support Package (TSP) free on-line at www.techbriefs.com/tsp under the Information Sciences category. NPO-30733
This Brief includes a Technical Support Package (TSP).
Measuring Low-Order Aberrations in a Segmented Telescope
(reference NPO-30733) 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, focusing on measuring low-order aberrations in segmented telescopes, specifically through the In-focus PSF Optimizer (IPO) algorithm. It outlines the development and application of this algorithm in optimizing the performance of segmented telescopes, which are crucial for high-precision astronomical observations.
The IPO algorithm is designed to monitor and control the Point Spread Function (PSF) of telescopes, ensuring that the images captured are as clear and accurate as possible. The document details the performance of the IPO on the WCT-2 (Wavefront Control Testbed 2), highlighting its capabilities in detecting piston, tip, and tilt aberrations. The IPO's two-wavelength approach extends the piston capture range, allowing for more effective correction of low-order aberrations.
The document also discusses various experiments and simulations conducted to assess the IPO's effectiveness. For instance, it mentions the scanning of segment 1 in piston steps of +50 nm and the collection of data at specific wavelengths (640 nm and 650 nm) to analyze the bandpass sensitivity. This data is crucial for understanding how different wavelengths affect the detection of aberrations.
Future work outlined in the document includes further simulations and experiments to explore the effects of signal-to-noise ratio (SNR), segment aberrations, and jitter between segments. Enhanced IPO algorithms are also proposed, such as the "Defocused IPO," which utilizes slightly defocused PSFs to improve detection capabilities, and the "Broadband IPO," which examines the impact of filter bandwidth on detection accuracy.
Overall, the document emphasizes the importance of these advancements in the context of the Next Generation Space Telescope (NGST) and other scientific observations that rely on broadband filters. It serves as a resource for understanding the technical aspects of wavefront control in segmented telescopes and highlights the potential for broader applications in aerospace technology and beyond.
For further information, the document provides contact details for the NASA Scientific and Technical Information (STI) Program Office, encouraging readers to explore additional resources related to aerospace developments.
