It is essential that strict wavefront error (WFE) budgets be met when designing and manufacturing space telescope optics. These performance thresholds dictate a telescope’s ability to clearly and accurately monitor Earth from orbit or capture images of distant celestial objects.
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Rather than simply focusing on precision, optical manufacturers must maintain ultra-low surface figure and irregularity to ensure mission-critical reliability in the extreme conditions of space.
This article features practical insights from the manufacturing floor, highlighting how Avantier delivers precision optics that meet the stringent WFE budgets for both small- and large-scale space systems.
Understanding Wavefront Error Budgets (WFEs)
WFEs are system-level allocations of permissible optical aberrations across the components in an optical path. These budgets are generally expressed in nanometers RMS and include contributions from:
- Optical surface figure and irregularity
- Thermal and mechanical deformation
- Alignment tolerances
- Coating-induced stress
Manufacturers are responsible for ensuring a minimal WFE contribution for each component to ensure the entire system is kept within spec. These are typically under λ/20 or λ/50 at 633 nm.
Manufacturing Challenges in Meeting WFE Budgets
Tight Surface Figure Tolerances
The achievement of ultra-precise surface figure accuracy is typically the most significant contributor to total WFE. It is important that optics conform to highly specific shapes with nanometer-level accuracy, including spherical, aspheric, and off-axis parabolas.
Avantier leverages a combination of sub-aperture tools, computer-controlled polishing (CCP), and deterministic figuring guided by interferometric metrology to meet surface figure targets without compromising edge or slope transitions.
Controlling Optical Surface Irregularity
Surface irregularity can scatter light and degrade imaging performance, most notably in terms of high-frequency errors. These are often the most difficult part of the spec to effectively manage.
Using magnetorheological finishing (MRF) and ion beam figuring (IBF), Avantier successfully eliminates mid- and high-spatial-frequency errors without introducing new artifacts. This approach is key to maintaining a uniform, clean optical surface.
Managing Coating-Induced Deformation
Thin-film optical coatings are key to reflectivity or transmission, but these coatings can warp optics due to residual stress, resulting in subtly distorted wavefronts.
Avantier simulates coating stress during design, applying symmetrical or balanced coatings wherever possible. In-situ monitoring also enables the early detection and correction of distortions.
Material Selection for Thermal Stability
Materials exhibiting poor thermal behavior can potentially shift figure under orbital temperature changes, exceeding WFE tolerances after launch.
Avantier recommends using ultra-low CTE substrates, such as Zerodur®, ULE®, or SiC, for space-bound optics. The company verifies substrate performance via environmental testing and finite element thermal modeling.
Verifying Wavefront Quality with Advanced Metrology
It is only possible to achieve nanometer-level surface precision with equally precise measurement tools. To ensure this, Avantier uses:
- Phase-shifting interferometry for full-aperture WFE measurement
- Custom CGHs or deflectometry for off-axis and freeform optics
- Stitching profilometry for aspheres and large optics
Where required, measurements are performed in a controlled environment designed to simulate space-bound thermal and vacuum conditions.
Partnering with Designers on WFE Budget Allocation
Early collaboration between system engineers and optical manufacturers is key to project efficiency.
Reviewing the full wavefront error budget in partnership with its clients allows Avantier to suggest trade-offs based on practical manufacturing capabilities, for example, relaxing certain specs to improve alignment tolerances.
Conclusion
More than just precision polishing is necessary to accommodate wavefront error budgets in space telescope optics. A closed-loop process of design-for-manufacturing, advanced metrology, and deep material knowledge is required.
Avantier offers the tools and expertise to ensure that every optic delivered meets the performance targets of the mission in question, from launch to orbit.
Acknowledgments
Produced from materials originally authored by Avantier Inc.
This information has been sourced, reviewed, and adapted from materials provided by Avantier Inc.
For more information on this source, please visit Avantier Inc.