Matthew A. Davies, The University of North Carolina at Charlotte; Brian S. Dutterer, The University of North Carolina at Charlotte; Steven Swagler, The University of North Carolina at Charlotte; Eann LAwing, The University of North Carolina at Charlotte; Nicholas Horvath, The University of North Carolina at Charlotte; Jannick Rolland, The Institute of Optics, University of Rochester; Aaron Bauer, The Institute of Optics, University of Rochester
Keywords: Freeform optics, Three-mirror-anastigmat, Ultraprecision diamond machining, Metrology, Opto-mechanics
Abstract:
Freeform optics, containing optical surfaces with no axis of rotational invariance (within or beyond the optical part), introduce additional degrees of freedom into optical design are having a radical effect on optical design [1]. The implications are potentially very disruptive: reduced size and weight, improved performance, reduced system cost and improved manufacturability, and often entirely new optical functionality [2,3]. For imaging, wide-field of view, large aperture and unobstructed systems with optimization of aberrations over the full field of view are possible using freeform systems but judicious choice of starting geometry in the optical design is critical [4,5]. Further, to realize the benefits freeform optics in the final physical imaging systems requires a concurrent engineering approach where optical design, fabrication, metrology, opto-mechanics and desired field performance are considered simultaneously [6]. A concurrent approach can have radical impact on system cost with little change in performance.
In this work, we describe the application of a concurrent engineering design approach for a three mirror anastigmat (TMA) off-axis in-plane optical design. The system is 250 mm aperture-class three-mirror imager operating at F/3 over a 2° × 2° full FOV, with near diffraction-limited performance over the visible spectrum for a system volume of about 72 liters, a target mass of 15 kg, and a 36.4 mm by 27.6 mm detector with 4.6 ?m pixel spacing. The fabrication, metrology and preliminary testing of an aluminum system is described, emphasizing the use of concurrent engineering methods to improve manufacturability and reduce overall system cost. Many tradeoffs and questions are discussed and include the following.
What are the effects of targeting diffraction versus detector limited performance on required tolerances?
How can alignment degrees of freedom be introduced judiciously to improve manufacturability of the optics while not overcomplicating the opto-mechanical design?
Can a balance be struck between rigid body mirror placement tolerances (opto-mechanics) and surface prescription tolerances that enables the system to be fabricated more efficiently?
We show how asking and answering these questions can directly affect the system costs, potentially by an order of magnitude or more, without changing the overall system performance.
The aluminum system prototype is fabricated on a mix of precision equipment (Makino A51 machining center) and ultraprecision, multi-axis diamond machining equipment (Moore Nanotechnology 650FG, five axis machining center). All of the mirrors are placed in the system using precision kinematic mounts allowing them to be removed and replaced with little retuning required. The adjustments in the system are three orthogonal linear degrees of freedom on the second mirror and defocus on the detector, both accomplished with off-the-shelf positioning stages. It is shown how these adjustments were adequate to make the tolerances achievable with the existing equipment. Manufacturing was performed using coordinated axis machining (three-axis turning). Metrology is performed on all of the mirrors both with an on-machine system and an independent metrology instrument. A kinematic mounting system which matches the opto-mechanical designs is utilized on both the manufacturing and metrology platforms allowing deterministic closed loop manufacturing and metrology of the optics until they are within the needed tolerances required to meet the target system performance. Sub-micrometer mirror form tolerance specifications were met with the errors scaling with the clear apertures of the mirrors. The relatively minimal mirror positioning for tuning during assembly is enabled by the closed-loop, deterministic and concurrent approach to system design and fabrication. Initial imaging results are shown indicating near detector limited performance.
Because of the deterministic fabrication and metrology methods, we demonstrate that making another system could be done at comparatively lower cost when compared to more classic assembly approaches that allow multiple degrees of freedom of adjustment on each optic in the system. The system design is also suitable to be fabricated in other materials. In particular we target a silicon carbide design, due to its high strength to weight ratio, robustness, and future work will involve the deterministic micro-grinding of CVD-silicon carbide mirrors that can be tested in the existing prototype platform. We will also be performing quantitative wavefront testing and wavefront repeatability testing (on assembly/disassembly) to further quantify system performance.
[1] Thompson K.P., Rolland J.P. (2012) Freeform optical surfaces: A revolution in imaging optical design. Optics and Photonics News 23(6):30-35.
[2] Plummer, W. T. Baker, J. G., Van Tassell, J. (1999) Photographic optical systems with nonrotational aspheric surfaces, Appl. Opt. 38, 35723592.
[3] Smilie, P.J., Dutterer, B. S., Lineberger, J. L., Davies, M. A., Suleski, T. J. (2012) Design and characterization of an infrared Alvarez Lens, Optical Engineering 51(1), 013006.
[4] Rolland, J. P., Davies, M. A., Suleski, T. J., Evans C.J., Bauer A., Lambropoulos J. C., Falaggis K. (2021) Freeform optics for imaging, Optica 8, 161-176. https://doi.org/10.1364/AO.410350.
[5] Bauer A., Schiesser E. M., Rolland J. P. (2018) Starting geometry creation and design method for freeform optics, Nat. Commun. 9, 1756.
[6] Horvath N. W., Davies M. A. (2019) Concurrent engineering of a next generation freeform telescope: mechanical design and manufacture, Proc. SPIE 10998, 109980X.
Date of Conference: September 14-17, 2021
Track: Optical Systems & Instrumentation