Imaging Space System Architectures Using a Granular Medium as a Primary Concentrator
- Tuesday, 01 July 2014
Higher-resolution optics provide improved hyperspectral imaging for ocean and land monitoring, as well as exoplanet detection.
NASA’s Jet Propulsion Laboratory, Pasadena, California
Typically, the cost of a space observatory is driven by the size and mass of the primary aperture. Generally, a monolithic aperture is much heavier and complex to fabricate (hence, more costly) than an aperture of the same size but composed of much smaller units. Formation flying technology, as applied to swarm systems in space, is an emerging discipline.
The solution proposed uses a method to construct an aperture in space in which the nonlinear optical properties of clouds of micron-sized particles, shaped into a specific surface through the interaction of light pressure, and electromagnetic confinement fields, form a very large and lightweight aperture of an imaging system, hence reducing overall mass and cost.
The optical imaging system, or “back end,” is contained on its own spacecraft separate from the primary optics that can both measure and correct many kinds of errors that are anticipated with a granular medium optic. This design combines several layers of sensing and control to adapt to possible misalignments and shape errors in the granular medium aperture. This innovation also combines the light from several of these “clouds” to synthesize a large, multiple-aperture system to increase light throughput and resolution, and is robust enough to use three different kinds of primary optics: reflective, refractive, and diffractive.
In the reflective optical system design, starlight reflects off of the granular patches, and is slowly focused toward the formation-flying spacecraft that collects, corrects, and combines the light from individual patches to a single detector. Light from all patches converges at an intermediate focus. The light then reflects off of a secondary mirror (Gregorian) and the light from each patch becomes collimated. The collimated light from each patch then continues to a separate adaptive optics system. A fast-steering mirror and a deformable mirror correct pointing and low- to mid-spatial frequency aberrations. An optical delay line is used to correct phasing differences between the patches, and enables Fourier transform spectroscopy. A beam-splitter is included to allow some of the light to go to a Shack-Hartmann sensor to measure aberrations in the system, and to provide a feedback mechanism to the deformable mirror. The main portion of the light continues to the collector system, a Cassegrain telescope, which combines and focuses the light from all the patches onto the science detector.
While at the smaller scale of the grain spacing there can be a certain amount of spatial disorder within each patch, at the scale of the aperture diameter the granular patches are held in formation to form a regular arrangement (in a Golay sparse aperture pattern, for example). Computational optics techniques based on speckle interferometry are used to remove the noise induced by the granularity of the patches, so an image can be synthesized at the focal plane.
The uniqueness and innovation of the concept lies in that it would be a very lightweight system, and one granular patch could combine with other patches to form much larger apertures than, for example, the 6.5-meter size of the James Webb Space Telescope. It would be easy to transport and deploy, not requiring structural elements for a backing structure (except for the confining electromagnetic system), and line-of-sight retargeting and figure control would be realized at-a-distance electromagnetically. Contrary to a monolithic aperture, this distributed system can be highly fault tolerant, self-healing, and relatively easy to package and deploy.
This work was done by Scott A. Basinger, Marco B. Quadrelli, and Mayer Rud of Caltech; and Grover Swartzlander of Rochester Institute of Technology for NASA’s Jet Propulsion Laboratory. NPO-49188
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