Using the new observation mode “cube-mode” (where the frames were directly saved in cubes with nearly no loss of time during the readout), we obtain several thousand frames within half an hour. To reach a high precision we use a statistical approach. Using an adaptive optics system we correct for atmospheric turbulences and furthermore by using a narrow band filter in the near infrared we can suppress differential chromatic refraction effects. By measuring the separation of double or triple stars we want to determine the astrometric signal of an unseen substellar companion as a periodic change in the separation between the stellar components. 150 μ as with ground-based and single-aperture observations. We present an observation method to obtain a relative astrometric precision of about 100. We present preliminary parallax results for this and two other ultracool subdwarfs (USDs) from the Calar Alto Omega 2000 parallax program. SSSPM 1444, with the extremely large proper motion of 3.5 arcsec/yr, is one of the nearest among these subdwarfs with masses at the substellar boundary. However, alternative deep optical HPM surveys based on archival photographic data are not only sensitive enough to detect some of the nearest representatives, they do also uncover many of the rare class of ultracool halo objects crossing the Solar neighborhood at large velocities. Deep optical sky survey (SDSS) and near-infrared sky surveys (DENIS, 2MASS), played a major role in the search mainly based on colours, since BDs emit most of their light at longer wavelengths. Hundreds of BDs have been identified for the last decade. The astrometric orbital monitoring of ε Indi Ba+Bb, for which we know an accurate distance from the Hipparcos measurement of its primary, ε Indi A, will allow the determination of individual masses of two low-mass BDs. Both were meanwhile found to be binary BDs. This technique led to the discovery of the first free-floating BD, Kelu 1, and of the nearest BD, ε Indi B. A direct astrometric detection of the hidden isolated BDs in the Solar neighborhood is possible with deep high proper motion (HPM) surveys. Resolved faint BD companions of nearby stars can be identified by their common proper motion (CPM). Accurate astrometry can be used to detect them indirectly as companions to stars by the signature of the so-called astrometric wobble. Therefore, it was only about 10 to 20 years ago that such ultracool objects began to be detected. Different from their low-mass stellar brothers, the red dwarfs, BDs cool down with their lifetime to very faint magnitudes. As a result of failed star formation, brown dwarfs (BDs) do not reach the critical mass to ignite the fusion of hydrogen in their cores.
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