Abstract

A broadband technique for measuring the surface accuracy of near millimeter wavelength radio telescopes has been developed. The near millimeter wavelength radiation from a distant astronomical point source is first focused by the telescope’s primary and secondary mirrors and then recollimated by an off-axis tertiary paraboloid for passage through a Twyman-Green interferometer. The end mirrors of the interferometer are situated so that the combination of tertiary paraboloid and beam splitter creates two identical images of the primary on their surfaces. By rotating one of the end mirrors out of its plane, the wavefronts are sheared with respect to each other, with the result that the focal plane pattern is scanned across the detector, where it interferes with the reference arm pattern. A 2-D Fourier transform of the focal plane data then yields the telescope’s aperture plane electric field. To separate the effects of different frequencies, the second end mirror is scanned. The resultant sheared interferograms, after being inverted, give the frequency dependence of the electric fields. Thus a 3-D Fourier transform of a grid of sheared interferograms simultaneously provides a measurement of the telescope surface at a number of frequencies.

© 1988 Optical Society of America