Instruments and missions in the 21st century

Current and future techniques relevant to binary-star observations are reviewed. Binary-star science is not driving present and future instrumentation, but does benefit from it. Data mining opens new perspectives. Increased data flow and sample sizes call for a new approach to maintaining the catalogs, which otherwise will collapse. We discuss high-resolution and high-contrast imaging with adaptive optics, speckle interferometry, HST, and long-baseline interferometry. Perspectives of astrometry are mostly linked to GAIA mision, matched by substantial advances in the ground-based techniques. Presice radial velocities are of little use (and used little) for binary-star work, synoptic RV monitoring with dedicated robotic telescopes is much needed instead. The importance of following binaries in a systematic way is currently under-appreciated by the community. Modern techniques permit, for the first time, to obtain complete (un-biased) statistics of binary stars and higher hierarchies, advancing the star-formation theory and general astrophysics. 1 General Considerations An overview of current and future instrumental capabilities relevant for binarystar astronomy is presented, covering next ∼10 years. Only optical and infra-red (IR) techniques are considered. The progress of observational astronomy is fast and impressive, with a new generation of extremely large telescopes and exciting space missions on the horizon. Binary stars are rarely (if at all) mentioned in scientific justification of present and future facilities. The astronomical community recognizes binaries mostly as a useful auxiliary tool (e.g. to measure stellar masses or distances) rather than a research subject of its own merit. One notable exception is the study of extra-solar planets (exo-planets), considered to be important by astronomers, general public, and funding agencies. Essentially all methods of exo-planet observations (radial velocities, transits, direct imaging) were derived from the binary-star techniques and pushed to the extreme. The binary-star astronomy will undoubtedly benefit from the technical progress in the exo-planetary studies (as well as from other new capabilities), without being their driver. We simply “piggy-back” on the existing and future projects. Progress of observational techniques cannot be described by a single number because several parameters are important: angular resolution, sensitivity (limiting magnitude), dynamic range (smallest contrast of faint details near a bright source), precision (in radial velocity or astrometry), coverage in the number of objects or fraction of sky, and, finally, the time coverage. This last parameter is of value to binary stars which often have long orbital periods. Thus, we are forced to use nowadays historical data of low quality, such as old micrometer