KEYWORDS: Modulation transfer functions, Data modeling, Wavefronts, Tolerancing, Image quality, Lenses, Point spread functions, Monte Carlo methods, Imaging systems
Ruda designs high performance imaging systems to meet difficult mission requirements, but these nearly diffraction limited systems often have small margins between the image quality of the nominal design and the required performance of the as-built system. Due to this we may spend significant resources designing and operating specialized test setups to ensure that results of MTF and Ensquared Energy (EE) measurements are well-calibrated and accurate. Alternatively, wavefront measurements – like those captured by wavefront sensors and interferometers – can be taken of the system to characterize the quality of the as-built system. Wavefront measurements are typically higher resolution, faster to setup, and quicker to measure than image quality metrics, making them particularly attractive for use when validating as-built system quality. Since the wavefront is related to the point spread function, and thereby the image quality, different wavefront measurements can contain information about the system MTF and EE. Thus, if the relationship between the wavefront and image quality metrics of interest can be established for an as-built system, it is possible to supplement or fully validate MTF and EE requirements from wavefront measurements. To investigate this relationship, we used Zemax OpticStudio to generate toleranced Monte Carlo trials of two nearly diffraction limited imaging systems designed by Ruda. The Monte Carlo models were then analyzed to form large data sets for statistical analysis. For wavefront data, the simulation produces single pass and double pass wavefront Zernike decompositions as well as wavefront root mean squared error over a range of object fields and visible wavelengths. For image quality data, the MTF at three spatial frequencies and the EE at two integration lengths are computed for the same fields and wavelengths as the wavefront data. These data sets are then processed to demonstrate that high degrees of correlation can exist between wavefront data and image quality metrics in toleranced high performance imaging systems, even when there is a difference in wavelength between the metrics. Sources of noise in these correlations are identified, and paths for supplementing or validating image quality requirement with correlated wavefront measurement data through machine learning are discussed.
Wide-field image correction of turbulence-induced phase requires tomographic reconstruction of each layer of turbulence. Before reconstruction can occur, the layers must be counted and ranged. A new signal-to-noise ratio metric for detecting a single layer of turbulence in a multi-layer atmosphere from SLOpe Detection And Ranging (SLODAR) measurements of Shack-Hartmann wave-front sensor (SHWFS) data is presented. 12,000 1-4 layer atmosphere profiles are procedurally defined by Fried length, layer altitude, and a minimum layer SNR requirement. Each profile is measured in simulation by a SHWFS in a 1.5 meter telescope with a 2.5 arcminute field of view over a 200 millisecond window. The simulation outputs are used as a 5-fold cross validation training data set for convolutional neural networks (CNNs) that count and range layers. The counting network achieved 92.6% accuracy and all ranging networks scored above 97.8% validation accuracy. We find that layers with SNR below 1 accounted for a majority of the misclassified points for all networks. We conclude that CNNs are a good candidate for wide-field image correction systems imaging through turbulence due to their ability to accurately profile the atmosphere from short time windows of collected data.
Complex-mask coronagraphs destructively interfere unwanted starlight with itself to enable direct imaging of exoplanets. This is accomplished using a focal plane mask (FPM); a FPM can be a simple occulter mask, or in the case of a complex-mask, is a multi-zoned device designed to phase-shift starlight over multiple wavelengths to create a deep achromatic null in the stellar point spread function. Creating these masks requires microfabrication techniques, yet many such methods remain largely unexplored in this context. We explore methods of fabrication of complex FPMs for a Phased-Induced Amplitude Apodization Complex-Mask Coronagraph (PIAACMC). Previous FPM fabrication efforts for PIAACMC have concentrated on mask manufacturability while modeling science yield, as well as assessing broadband wavelength operation. Moreover current fabrication efforts are concentrated on assessing coronagraph performance given a single approach. We present FPMs fabricated using several process paths, including deep reactive ion etching and focused ion beam etching using a silicon substrate. The characteristic size of the mask features is 5μm with depths ranging over 1μm. The masks are characterized for manufacturing quality using an optical interferometer and a scanning electron microscope. Initial testing is performed at the Subaru Extreme Adaptive Optics testbed, providing a baseline for future experiments to determine and improve coronagraph performance within fabrication tolerances.
Conference Committee Involvement (1)
Optical Modeling and Performance Predictions XV
3 August 2025 | San Diego, California, United States
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