Mappit: Optical Interferometry with the Anglo-Australian Telescope

Abstract

Turbulence in the Earth's atmosphere usually prevents large optical telescopes from achieving their full resolving power. Instead, the image of a star is blurred to a seeing disk about one arcsecond in diameter. Several techniques are available which use interferometry to overcome atmospheric seeing and restore diffraction-limited performance. This thesis describes the design, construction and use of an optical interferometer located at the coude focus of the 3.9m Anglo-Australian Telescope. The instrument, known as MAPPIT (Masked APerture-Plane Interference Telescope), uses the technique of non-redundant masking. This involves placing a mask with several small holes over the aperture of the telescope and recording a succession of short-exposure interferograms. These interferograms are analysed to determine the power spectrum and closure phases of the object, which are used to reconstruct a diffraction-limited image. One advantage of using a non-redundant aperture mask is that, at least for bright objects, it increases the signal-to-noise ratios of the power spectrum and closure phase measurements relative to observations with an unobstructed aperture. This is despite the fact that much of the light is blocked by the mask. Another advantage is that it improves the accuracy with which one can correct for variations in atmospheric seeing, something which is often the limiting factor in high-resolution imaging. The main drawback of non-redundant masking is a less efficient coverage of spatial frequencies. However, for simple objects such as multiple and barely resolved stars, adequate spatial frequency coverage can be obtained by combining observations made with different masks and with the masks rotated to several different position angles on the sky. An important feature of MAPPIT is the use of a prism to disperse the interference pattern in wavelength. This overcomes a restriction common to all forms of interferometry, namely the requirement that one observe over a narrow band of wavelengths. By using an aperture mask with a linear array of holes, one obtains a one-dimensional fringe pattern which can then be dispersed in the direction parallel to the fringes. In MAPPIT, a prism and a cylindrical lens produce wavelength-dispersed fringes using a novel combination of image-plane and pupil-plane imaging. Results presented in this thesis include observations of several close double stars. One of these is delta Sco, and I provide the first image of this star and the first determination of its orbit. Another bright star, sigma Sgr, is found to a barely-resolved double with a separation of just 12 milliarcsec. Observations of two resolved single stars, alpha Sco and beta Gru, are also reported. The measured angular diameter of aSco agrees well with published values. For the M5 III giant beta Gru, which has not previously been resolved, I find a uniform-disk diameter of (27±3) milliarcsec.