Thermal instability of BK7 and how it affects the manufacturing of large high precision surfaces

Michael Hyman; Matthew Brunelle; Nohl Schluntz; Michael Gregory; Mark Notargiacomo; Greg Frisch; Jeremiah Triftshauser

doi:10.1117/12.2279807

16 October 2017 Thermal instability of BK7 and how it affects the manufacturing of large high precision surfaces

Michael Hyman, Matthew Brunelle, Nohl Schluntz, Michael Gregory, Mark Notargiacomo, Greg Frisch, Jeremiah Triftshauser

Author Affiliations +

Proceedings Volume 10448, Optifab 2017; 104481M (2017) https://doi.org/10.1117/12.2279807
Event: SPIE Optifab, 2017, Rochester, New York, United States

Abstract

When manufacturing precision optical surfaces of relatively larger sizes it is critical to understand the thermal stability of the substrate material. The material properties associated with thermal homogenization are commonly reviewed and soak schedules are created. These schedules ensure a surface under test is in a stable state and is ready for wavefront measurement with an interferometer. However with some materials such as N-BK7, standard soak schedules may not be enough. This paper shows the thermal challenges associated with manufacturing precision optical surfaces when the substrate material is N-BK7, and how the issue can be easily confused with poor metrology. Throughout the manufacturing of precision optical surfaces, the substrates are exposed to varying heat sources and loads. During the manufacturing of lenses greater than 4 inches in diameter we have observed permanent deformation of the optical surface as a result of exposure to temperatures well below the glass strain point. While the reasons why the change occurs is not yet well understood, the result is well documented and was recently observed during the manufacturing of a 15 inch diameter spherical mirror. We use this lens as a case study highlighting the challenges associated with this phenomenon.

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Michael Hyman, Matthew Brunelle, Nohl Schluntz, Michael Gregory, Mark Notargiacomo, Greg Frisch, and Jeremiah Triftshauser "Thermal instability of BK7 and how it affects the manufacturing of large high precision surfaces", Proc. SPIE 10448, Optifab 2017, 104481M (16 October 2017); https://doi.org/10.1117/12.2279807

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KEYWORDS

Mirrors

Surface finishing

Manufacturing

Polishing

Optics manufacturing

Metrology

Coating

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