Experimental investigation of adhesive bond strength between metal and optical glass

Within the general astronomical community as well as at the University of California Observatories, there has been a long history of using epoxy to mount optics within instruments such as spectrometers and telescopes. The Ken & Gloria Levy Spectrometer, part of the Automated Planet Finder (APF) telescope located at Mt. Hamilton's Lick Observatory, relies on epoxy-bonded joints to attach the instrument's large cross-dispersing prism and echelle grating to its Invar space-frame structure. Design constraints dictated that these large optics each be attached at only three points, and that the bond areas be as small as possible while maintaining an adequate strength factor of safety. Previous UCO instruments, such as the Keck Telescopes' primary mirror segments and the ESI Spectrometer, used Hysol's 9313 epoxy product for this purpose. Concerns over long-term reliability of such joints led us to re-examine this issue. We empirically investigated the roles played by epoxy selection and techniques such as surface preparation and the use of a primer, in creating a robust metal-to-glass bond. Bond strength data was generated, leading us to select a previously unused epoxy, and to implement particular techniques to ensure bond quality. Most notably, we found that bond strength data as typically reported on adhesive manufacturers' datasheets was not a reliable indicator of long-term bond reliability between metal and optical glass.