The invention relates generally to a method of testing the surface quality of convex mirrors, and, in particular, to using an optical lens quality material as the mirror substrate, polishing the rear surface of the substrate to a precise surface figure to thereby transform it into a lens, and testing this lens by standard interferometric or wavefront methods prior to applying a reflective coating to the convex surface.
Large convex mirrors are typically used as secondary mirrors in large reflecting telescopes. For example, the NASA 3-meter telescope on Mona Kea uses a 244-mm diameter secondary mirror having a hyperbolic surface figure. Currently the standard method for testing convex mirrors is the Hindle sphere test or the improved version, the Hindle-Simpson test. The Hindle test uses a spherical mirror that is significantly larger in diameter than the convex mirror under test and it must be perforated at its center. A diagram of the test set-up is shown in FIG. 1.
The convex mirror under test, the test optic 10, is tested at the same conjugates as used in the telescope by employing a Hindle Sphere 11, a spherical mirror with a central perforation. The center of curvature (CoC) of the Hindle Sphere is positioned at the near focus 12 of the convex surface under test. The diameter of the Hindle Sphere has to be greater than that of the test optic. Light from an interferometer 13 is brought to the null test point 14 at the far focus of the convex surface of the test optic. After reflections off the test optic 10 and the Hindle Sphere 11, the light re-traces its path back to the interferometer 13 where it produces fringes on a monitor 15 depicting the wavefront aberrations of the test optic.
A schematic of the Hindle-Simpson test set-up is shown in FIG. 2. This test makes use of a meniscus-shaped Hindle Sphere 20 and a concave calibration mirror 21. All surfaces in the arrangement are spherical. By designing the ancillary optics, in this case the meniscus-shaped Hindle Sphere and the concave calibration mirror, to lie close to the convex mirror under test 22, the diameters of these optics are minimized with corresponding reduction in cost of fabrication. Nonetheless, the diameters still have to be somewhat larger than the diameter of the test optic.
In large telescopes, astronomical or otherwise, the secondary mirror often directs the light to a focus through a central hole in the primary. The distance from the vertex of the secondary mirror to this focus can be many meters, perhaps more than 10 meters. To reduce the total length of the test setup, a shortening lens 30 is often used as shown in FIG. 3. The lens is often a plano-convex lens with spherical convex surface. Again, this lens has to have a diameter greater than the diameter of the mirror under test 31, further adding to the complexity and cost of the test setup.
There is a need for a less complex and less expensive method of testing the surface of a convex mirror to enable accurate measurement and characterization of its surface figure.