Research for Diffraction Limited Imaging with Large Optical Telescope

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In this research, we have developed an aperture synthesis method with a non-redundant aperture mask (NRM) to obtain diffraction limited images with a large optical telescope. In 1988, we constructed a data acquisition system, investigated the design method of 2-dimensional NRM, and conducted a fundamental optical simulation. In 1989, based on these results and using the data using the data acquisition system, we observed single stars and binaries with the 91 cm telescope at Dodaira Astronomical Observatory, National Astronomical Observatory (NAO). From these observations, we found that the atmospheric fluctuations are very severe in Japan and an image stabilizing system must be required. Thus, in 1990, we developed an aperture masking system with an image stabilizing function. Using the system for NRM observations with the 188 cm telescope at Okayama Astrophysical Observatory, NAO, we confirmed good response of the imaging stabilizing function and obtained a lot of fundamental data for diffraction limited imaging. From our investigations, we obtained following results :1. The radius of the coherence patch is less than 3 cm in Japan. Since the number of sub-apertures of NRM is small, total aperture area of NRM of which sub-aperture radius is equal to that of coherence patch is very small Therefore, it is required to use the mask with small redundancy for weak objects.2. At longer baselines, visibility amplitude decreases severely, because the delay difference between sub-apertures is large. Some portion of this effect can be canceled out by an image stabilizing system. In the case of bad seeing, however, it is desired to use a simple adaptive optics.