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The Narrabri Stellar Intensity Interferometer (NSII) was an optical interferometer, specifically an intensity interferometer in operation at Narrabri Observatory in New South Wales, Australia. It consisted of two 6.5-meter reflector telescopes of one pixel each on a circular track 188 meters in diameter. Rather than using the interference due to the wave-nature of light, it correlated the variations in the intensity of the source, using variation-frequencies up to about 100 MHz (which can produce correlations according to the Hanbury Brown and Twiss effect). It was in operation from 1963 to 1974, used to measure the angular diameters of 32 stars, which were on the order of a milli-arcsecond.