Glyndwr Innovations Limited (GIL) has a requirement to use a Ritchey-Common optical test during the processing and final verification of a large elliptical flat mirror. The geometry of the test requires a reference sphere that is larger in diameter than the minor axis of the optical flat under test. In this implementation, the reference sphere measures approximately 60 inches in diameter, has an aspect ratio of 9:1, is a meniscus shape and has a mass of 430kg. The reference sphere support uses a tunable Schwesinger design, supporting the mirror radially, in a horizontal line of sight. FEA predicts the full aperture optical surface deformation resulting from the effects of gravity and the support of the mirror to be 95.9nm RMS, once optimized. Once integrated and tuned the interferometric testing delivers an actual optical surface deformation of 134mn RMS, with the key Zernike terms being spherical and trefoil aberrations. The final optical form of the reference sphere, measured over its illuminated diameter within the optical test, meets our test wavefront requirements.
The M6C mirror of the Extremely Large Telescope is a Zerodur elliptical flat with wavefront specification of 60 nm RMS at 45 degrees angle of incidence over an optical clear area of 694 x 504 mm. The mirror is being processed at Glyndwr Innovations Ltd via small tool CNC polishing, with a sub-aperture interferometric stitching test being used to measure the surface error. Here, the design and performance of the interferometric stitching test is discussed, complete with a full uncertainty budget to ensure conformance to the specification. Current work on the stitching test has found measurements of the M6C to be repeatable to 4 nm RMS surface error, and an overall uncertainty of measurement of 12.2 nm RMS. Using this test the optic will be polished to a measured surface error of less than 17.5 nm RMS.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.