This paper describes and reports uncertainties associated with the absolute flatness measurement for an Φ300 mm optical flat. Absolute flatness of 3 flats denoted as optical flat A, B and C was determined by conducting a three-flat test method. The measurements were carried out using a Fizeau interferometer. Sources of uncertainty that contributed to the measurement uncertainty were examined. The dominant contributions include off-center alignment during three-flat test measurement, temperature instability and deformation due to gravity. In our experiment, although the alignment was performed crucially, deviation up to 4 nm was observed from 6 sets of three-flat test. Since the optical flat was mounted using a sling type module, deformation due to clamping force can be neglected. Deformation due to gravitational force was investigated by comparing the contour map of the optical flat oriented at 0° and 180°. The maximum difference up to 99 nm was observed which is higher than accuracy of the reference flat (λ/20) itself.
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