Interferometric Test of an f/8, 24-inch (60.96 cm) Diameter Paraboloidal Mirror in the Atmosphere

A 24-inch (60.96 cm) paraboloidal mirror having a vertex radius of curvature of 32 feet (9.75 m) was tested on-axis with a laser interferometer in a normal atmosphere. A compensating null lens corrected the spherical aberration. Simultaneous observation of the fringe pattern and point spread function produced consistent, high quality interferograms. The residual opti-cal path differences obtained from ten interferograms were averaged. Analytically removing astigmatism present in the system produced a residual root-mean-square optical path difference of .014 of the wavelength of the light. The coma amplitude measured was .04 of a wavelength. The computed modulation transfer function at the mid-spatial frequency with astigmatism analytically removed is .002 below that of a diffraction limited system.