Parabolic liquid mirrors in optical shop testing

Abstract

A parabolic liquid mirror obtained by the rotation of a mercury bath around a vertical axis has been built and its optical surface characteristics measured to demonstrate that it can be used in optical shop testing as a reference surface. A linear Hartmann test allowed us to check that the focal length is well related to the rotation velocity, following the theory, and that no spherical aberration is present, as assumed by previous authors. The spherical aberration has been found to be smaller than λ/50 at 633 nm. An interferometric test of the mirror compared with a null lens gave information about the quality of the optical surface for which the rms wave-front error, when the random errors are averaged, is ~λ/25. Because modifying the mirror diameter is cheap and fast and adjusting its focal length within a large range is straightforward, the parabolic liquid mirror can become a highly adaptable tool in optical metrology. In particular, it can be used in optical shop testing as a reference surface to test null correctors, to check any system developed to control the shape of large parabolic or quasiparabolic top-quality solid-state mirrors, or to make holographic references of such surfaces.

Optical tests of a 25-m-diameter liquid mirror: behavior under external perturbations and scattered-light measurements

“…A Lidar facility has also been built and operated by the University of California at Los Angeles 7 . Liquid mirrors are also used as reference surfaces to test conventional optics 8 .…”

<title>Liquid mirror telescopes: a progress report</title>

“…Liquid mirrors are also used to generate reference surfaces to test optical surfaces. The work of Ninane 14 is particularly of interest, not only because of her original use of a liquid mirror, but also because she carried out independently optical shop tests that confirm the high optical quality of liquid mirrors reported in 3 and 4 . It also worthwhile to point out an interesting application of active flat mercury mirrors 15 .…”

The 4m international liquid mirror telescope (ILMT)

“…Following the suggestion that modern technology (Borra 1982) gives us tracking techniques that render liquid mirrors useful to astronomy, research and development programs were begun to assess the feasibility of the concept. Mirrors up to a diameter of 2.5m were extensively tested and showed the high surface quality of such mirrors Borra, Content & Girard 1993;Ninane & Jamar 1996; Girard & Borra 1997). For a well tuned mirror, the root mean square values of the deviations from a perfect paraboloid are typically λ/20.…”