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C.A. Hummel, D.J. Hutter, N.M. Elias II (USNO)
We present recent results of the Navy Prototype Optical Interferometer from observations of double stars and astrometry over wide angles. The former is relative astrometry based on the visibility amplitude and phase and their transformation into maps, the latter astrometry is based on the geometrical delay of the fringe position and its transformation into star positions.
The NPOI has produced maps at optical wavelengths which resolved spectroscopic double stars with separations down to a few milliarcseconds, and we present maps and preliminary orbits on a number of them. We also report on the status of the NPOI program to determine relative positions of stars over wide angles. We address the issues of correction for atmospheric refractive index fluctuations, internal path length variations, and siderostat pivot motions. We show that the precision achieved separately on every correction is sub-micron, if stars sufficiently bright are chosen for the atmospheric correction. On a twenty meter baseline of the astrometric sub-array of NPOI this would yield positions better than 10 milliarcseconds. Actual results have not yet reached this level of precision, however, and we offer some possible explanations.
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