Miniature instrument for the measurement of gap thickness using polychromatic interferometry

An optical instrument for measuring the thickness of a thin gap or transparent film is presented. It will be used to calibrate the adaptive secondary mirror under development for the Steward Observatory 6.5 m Multiple Mirror Telescope. Capacitative sensors in the mirror assembly measure dynamically the thickness of the nominally 50 micrometers air gap between the deformable mirror and a glass reference body. The miniature interferometer has been developed to accurately determine the gap thickness so that the capacitive sensor may be calibrated. Interference fringes are produced by illuminating an air gap, which is between two reflective surfaces, with monochromatic plane waves and observing the reflected light. Intensity variations are measured as the wavelength of illumination is varied over an octave. The film thickness is determined by correlating the observed fringes with those modeled for different gaps. Absolute measurement to an accuracy of a small fraction of a wavelength is possible.