A Diffraction-limited f/2 1.5 Meter Diameter Liquid Mirror

The Airy-like pattern of a 1.5-m-diameter liquid mirror has been observed directly. Measurements show that the dark rings of the Airy pattern are at the predicted locations, the first ring being at a radius of 0.11 arcsec. Interferometric data give Strehl ratios of order 0.8 and rms surface deviations of order lambda/20, confirming what the direct observation of the Airy pattern indicates: that the mirror is essentially diffraction-limited. Direct observations of the point-spread function also show that the mirror does not suffer an excessive amount of scattered light. The optical path of the test light beam goes through 23 optical surfaces, used for testing purposes, from pinhole to detector; therefore, the liquid mirror alone should have a surface quality actually better than indicated by the above measurements.