Image Quality of Liquid‐Mirror Telescopes

Abstract

The quality of astronomical images obtained with the 3 m liquid-mirror telescope (LMT) of the NASA Orbital Debris Observatory (NODO) and with the University of British Columbia 6 m Large Zenith Telescope (LZT) is assessed and compared to that of conventional instruments. Analysis of star images in longexposure drift-scan data indicates that the profile of the image core is primarily set by atmospheric turbulence. Defocused star images reveal the presence of low-amplitude waves on the surface of the mercury, also seen in laboratory tests. The effect of these waves is to diffract light into the wings of the point-spread function. Analysis of the intensity profiles of stellar images can therefore probe the structure of the mirror surface on scales smaller than the atmospheric coherence length, which is about an order of magnitude larger that the characteristic wavelengths of the surface waves. It is found that the rms surface height error produced by these waves was approximately 37 nm for the NODO LMT. Improvements to the rotational speed stability of liquid mirrors, reduction of the thickness of the mercury layer, and use of a protective Mylar cover have allowed the LZT to reduce this source of error to approximately 9 nm rms, thereby achieving an image quality approaching that of conventional telescopes. © 2007. The Astronomical Society of the Pacific. All rights reserved.