Mr. Noah R. Block Profile

Noah R. Block

at Rochester Inst of Technology

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Proceedings Article | 19 May 2005 Paper

Using multispectral information to decrease the spectral artifacts in sparse-aperture imagery

Noah Block, Robert Introne, John Schott

Proceedings Volume 5798, (2005) https://doi.org/10.1117/12.604002

KEYWORDS: Optical transfer functions, Modulation transfer functions, Image restoration, Image quality, Multispectral imaging, Telescopes, Signal to noise ratio, Systems modeling, James Webb Space Telescope, Space telescopes

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Optical sparse-aperture telescopes represent a promising new technology to increase the effective diameter of an optical system while reducing its weight and stowable size. The sub-apertures of a sparse-aperture system are phased to synthesize a telescope system that has a larger effective aperture than any of the independent sub-apertures. Sparse-apertures have mostly been modeled to date using a "gray-world" approximation where the input is a grayscale image. The gray-world model makes use of a "polychromatic" optical transfer function (OTF) where the spectral OTFs are averaged to form a single OTF. This OTF is then convolved with the grayscale image to create the resultant sparse-aperture image. The model proposed here uses a spectral image-cube as the input to create a panchromatic or multispectral result. These outputs better approximate an actual system because there is a higher spectral fidelity present than a gray-world model. Unlike its Cassegrain counterpart that has a well behaved OTF, the majority of sparse-aperture OTFs have very oscillatory and attenuated natures. When a spectral sparse-aperture model is used, spectral artifacts become apparent when the phasing errors increase beyond a certain threshold. This threshold can be based in part on the type of phasing error (i.e. piston, tip/tilt, and the amount present in each sub-aperture), as well as the collection conditions, including configuration, signal-to-noise ratio (SNR), and fill factor. This research addresses whether integrating a restored multispectral sparse-aperture image into a panchromatic image will decrease the amount of spectral artifacts present. The restored panchromatic image created from integrating multispectral images is compared to a conventional panchromatic sparse-aperture image. Conclusions are drawn through image quality analysis and the change in spectral artifacts.

Proceedings Article | 1 September 2004 Paper

Image quality analysis of a spectra-radiometric sparse-aperture model

Noah Block, Robert Introne, John Schott

Proceedings Volume 5418, (2004) https://doi.org/10.1117/12.542419

KEYWORDS: Optical transfer functions, Image quality, Modulation transfer functions, Systems modeling, Point spread functions, Signal to noise ratio, Telescopes, Spatial frequencies, Space telescopes, Signal detection

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Sparse aperture (SA) telescopes represent a promising technology to increase the effective diameter of an optical system while reducing overall weight and stowable size. Although conceptually explored in the literature for decades, the technology has only recently matured to the point of being reasonably considered for certain applications. In general, a sparse aperture system consists of an array of sub-apertures that are phased to synthesize a larger effective aperture. The models used to date to create predictions of sparse aperture imagery typically make use of a "gray world" assumption, where the input is a resampled black and white panchromatic image. This input is then degraded and resampled with a so-called polychromatic system optical transfer function (OTF), which is a weighted average of the OTFs over the spectral bandpass. In reality, a physical OTF is spectrally dependent, exhibiting varying structure with spatial frequency (especially in the presence of optical aberrations or sub-aperture phase errors). Given this spectral variation with spatial frequency, there is some concern the traditional gray world resampling approach may not address significant features of the image quality associated with sparse aperture systems. This research investigates the subject of how the image quality of a sparse aperture system varies with respect to a conventional telescope from a spectra-radiometric perspective, with emphasis on whether the restored sparse aperture image will be beset by spectral artifacts.

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