The astrometric performance of a ground-based optical interferometer with air-filled delay lines has been investigated. Group delay tracking with zeroing the residual phase difference between two bands has been proposed, and shown to be only slightly dependent on the ambient parameters. With group delay tracking only, we have shown that dual-field astrometry at a precision level of a few 10-10 rad (or better than 0.1 mas) per observing sequence of a minute or so is achievable with air-filled delay lines and observation in the near-IR range. The interferometer baseline should not exceed a few 10 meters, for astrometric baseline calibration to be performed without compensation glass plates.Acknowledgements
The authors acknowledge very fruitful discussions with F. Cassaing, P. Dierich and P. Juncar. We are grateful to M. Colavita for his comments and suggestions. This study was partly supported by a special grant (Action Incitative) from Observatoire de Paris, which is greatly acknowledged.
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