Enabling Technology for Highly Aspheric or Free-Form Optics Manufacturing

Apr 25, 2013
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Additional Questions & Answers from the Webinar (below)

Dr. Dae Wook Kim
Assistant Research Professor
College of Optical Sciences
University of Arizona

Precision optics can be efficiently produced using a computer controlled optical surfacing (CCOS) process. Various new approaches advancing the current CCOS processes have been developed and implemented to manufacture highly aspheric or free-form optics such as 8.4m diameter Giant Magellan Telescope off-axis primary segment at the University of Arizona. The new technologies and theories including Rigid Conformal (RC) lap using non-Newtonian fluid, smoothing model for mid-to-high spatial frequency error control, edge removal effect for segmented optical systems, and non-sequential optimization using multiple tools simultaneously, are presented with actual data demonstrating the performance of the enhanced process to build next generation optical systems.

Dae Wook Kim is assistant research professor in the College of Optical Sciences, University of Arizona. He is principal scientist for projects that develop and implement advanced technologies for building and testing large optical systems and telescope mirrors, as well as ground- and space-based telescopes. He is chair or co-chair of the SPIE International Symposium on Optical Engineering + Applications (Optical Manufacturing and Testing) and the Topical Meeting on Optical Fabrication and Testing of the Optical Society (OSA). He is also an associate editor of Optics Express.

Additional Questions & Answers from the Webinar

Q. How can we take away from this presentation, practical guidelines to designing manufacturable aspheric surfaces, both reflective and refractive? (I.e. departure from sphere, diameters, diameter to thickness ratio) What algorithms are there to estimate cost? What production rates can be achieved?

The answer to this question depends on many factors such as the size of optics, target specification, computer controlled polishing machine type, and so forth. Unfortunately, it is not simple to give some general and well-defined references or guidelines.

Q. What is the material for the substrate?

The 4.2m Advanced Technology Solar Telescope off-axis substrate uses Zerodur from SCHOTT. The 8.4m Giant Magellan Telescope primary segment and the monolithic Large Synoptic Survey Telescope M1/M3 substrate uses borosilicate glass from Ohara.

Q. These surfaces are always at least part of a rotationally symmetric surface. Do you also deal with surfaces having no symmetry at all?

In general, the answer is no. For most astronomical large mirror applications, the surfaces are usually a part of rotationally symmetric surface. However, the aspheric departure and the lack of symmetry of off-axis mirrors are not much different from the other free-form optics, which have no symmetry at all. This is especially true from the fabrication point of view. All the discussions about the conformable tool, smoothing effect, edge model, and process optimization are still valid for any free-form surfacing processes. However, the optics metrology part might be much more challenging for the case with no symmetry at all.

Q. The Smoothing factor curve for two optics with same starting PVi: one smoothed with increments of 1 um DC removal and one with increments of 2 um DC removal. The 1 um DC SF curve will be higher than the 2 um DC SF curve. Therefore, you cannot compare SF for optics processed with different DC removal increments?

Yes, you can compare the Smoothing Factor for optics processed with different DC removal increments. The Smoothing Factor is basically normalized by the nominal removal depth (i.e. DC removal). In other words, it is defined by the ratio of smoothing to bulk removal. Please, check “Parametric smoothing model for visco-elastic polishing tools,” Opt. Express 18, 22515-22526 (2010) for more information.

Q. Have you compared the performance of your polishing approach to other small-tool approaches, such as MRF?

No. Our technology has been applied for the very large (actually the largest on the Earth) and one-of-a-kind mirrors. The other small tool approaches are practically not simple to be applied for our projects (e.g. limitations in available tool sizes, mid-spatial frequency error control for the very large optics, etc...). In general, different approaches have different pros and cons. You can find more discussions about this topic in Table-1, "Rigid conformal polishing tool using non-linear visco-elastic effect", Optics Express, Vol. 18 Issue 3, pp.2242-2257 (2010).

Q. Would it be possible to correct an aberrated wavefront due to a not-so-good big (20m) spherical primary mirror with a small (~cm) aspheric mirror?

If this question is only about the manufacturing capability for a ~cm aspheric mirror (with a given specification), yes, it can be done. However, the real challenge will be in the spatial frequency error requirement for the small mirror and the aberration correction for the whole field of view of the system.
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