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F.J. Vrba (USNOFS), A.A. Henden (USNOFS/USRA), C.B. Luginbuhl, H.H. Guetter, D.G. Monet (USNOFS)
The capability of carrying out astrometric observations at near-infrared wavelengths has been long sought, as the region between 1.2-2.2 microns offers smaller atmospheric refractive distortions and measurement of objects which are not easily detectable at optical wavelengths. The recent discoveries by 2MASS, DENIS, and SDSS of numerous nearby L- and T-dwarfs whose SEDs peak in the near-infrared makes the capability of determining parallaxes and proper motions at these wavelengths especially attractive. We have carried out astrometric test observations using a Rockwell 256x256 HgCdTe (NICMOS 3) array at the USNO 1.55-m telescope to gain understanding of the prospects and problems inherent in a long-term near-infrared astrometric program. We routinely obtain accuracies of about 10 mas for a single measurement for well-exposed stars between 1.2 and 2.2 microns. We expect this accuracy to allow distance determinations to 2% or better for the majority of known L and T dwarfs within a two to three year observation series. These tests were carried out in anticipation of the use of an ALADDIN 1024x1024 InSb array, which will provide a larger FOV and increased sensitivity at 2.2 microns, for routine near-infrared parallax observations at USNO beginning in summer 2000. We report preliminary astrometric results obtained with engineering-grade ALADDIN arrays employed in NOAO instrumentation.