Using Aspheric Optics in Complex Lens Designs
- Tuesday, 06 September 2011
Optical design engineers often discuss the use of aspheres to reduce element count, but they seldom put the discussion into real-time practice, often fearing that the more complex fabrication process of the asphere itself overwhelms the advantages that they might gain.
In 2008, however, when Lockheed Martin Space Systems contracted Light- Works Optics (Tustin, CA) to complete the optical and opto-mechanical design, analysis, fabrication, and testing of NASA’s Geostationary Lightning Mapper (GLM) high-performance lens assembly, design engineers saw the project as a perfect opportunity to introduce an aspheric optical element into the allfused silica, single-wavelength, wide-field lens design.
The Geostationary Lightning Mapper itself will fly on the National Oceanic and Atmospheric Administration (NOAA) GOES-R Series environmental satellites where it will monitor lightning on a global scale. The collected data will provide new insight into the formation, distribution, morphology, and evolution of storms, and help protect communities by increasing severe storm and tornado warning lead times. Long-term GLM observations will also enable a better understanding of the Earth’s climate system through investigations into the mechanisms at the core of the global water and energy cycle; the dynamics and life cycles of storms and weather systems; and the fast time-scale elements of atmospheric convection.
The decision to use aspheres produced two key outcomes: reduction in the narrow band filter’s angles of incidence (AOI) and a reduction in weight. The GLM monitors a specific wavelength and requires a very discriminating optical filter, one that operates at a 777.7 nm wavelength, just outside the visible spectrum. The filter requires near normal incident light in order to optimize its performance. If the filter’s angle of incidence is off by even a few degrees, the filter will “leak” other undesired wavelengths.
By implementing the correct angles, no light leaks occur, and lightning strikes can be seen much more clearly, and with less noise in the background. Design engineers were able to reduce the AOI on the filter by the critical halfdegree needed, which the all-spherical design could not have achieved. The new assembly also reduced the elements in Lockheed Martin’s original design by one large lens, resulting in reduced weight of the optics. The previous, all-spherical design used up the majority of the weight budget allocation and would have required the fabrication of the housings from an exotic material. The elimination of the single large lens allowed for the use of conventional aluminum for the large optical housings.
“The material that these lenses are made of — they’re very expensive. It’s a very high-quality fused silica,” said Kent Weed, vice president of engineering at LightWorks Optics. “By eliminating one lens, it basically paid for the extra complexity of the asphere.”
One reason that many companies, including Lockheed Martin, have hesitated with the use of aspheric lenses is because the unconventional lenses often require unconventional testing. It was up to LightWorks Optics to provide a testable lens assembly.
In-house, computer-controlled mechanisms, combined with a diamond-turned null, polished the 1000-fringe departure asphere to final specifications. The diamond-turning equipment provided a full-aperture null reflector that enabled the whole lens to be seen. To ensure the aspheric profile, a secondary check was conducted using a Taylor Hobson Form Talysurf, an instrument that scans the aspheric profile surface in a single direction. The final assembled lens passed all requirements during testing across the field of view, validating the accuracy of the design and fabrication processes.
Lockheed Martin Space Systems has ordered the next phase of fabrication of the flight units, and the GOES-R series satellite platform, which will house the lens assembly, is still in the prototype phase. The Geostationary Lightning Mapper, scheduled to launch in the next two years, will eventually operate in geosynchronous orbit over the northern hemisphere
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