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We will restrict this discussion to visual bandpass astrometric systems that are capable of the measurements necessary to derive a high precision annual parallax. Recent technological advances have resulted in astrometric systems with the precision necessary to determine stellar parallaxes with standard errors of 0.5 to 2 mas (1 mas = 0".001). Example of these are: the European astrometric satellite, HIPPARCOS; the U.S. Naval Observatory's Astrometric CCD program on the Strand 1.55-m reflector; the Astrometric programs conducted with the Hubble Space Telescope; the MARK III Stellar Interferometer; and the Multichannel Astrometric Photometer (MAP) at the University of Pittsburgh. We will discuss the basis of each of these remarkably different systems briefly.
Future astrometric systems now under discussion would utilize the most recent technological advances to enhance the best underlying principles of the systems mentioned above. These include a possible second HIPPARCOS mission, the employment of highly calibrated large format CCD cameras, the possible deployment of ground based and space borne narrow field astrometric telescopes, ground and space borne interferometers, and an improved version of the MAP to be employed on large aperture telescopes. The new ground based systems will generally be limited by atmospheric turbulence to study precisions of 0.03 to 0.1 mas but will be able to measure the parallaxes of objects as faint as the 21st magnitude. Space borne systems will generally be limited to brighter targets but should be able to achieve study precisions of 0.01 to 0.5 mas. The parallax precision of current astrometric systems is now sufficient to determine the trigonometric distances of the nearest star clusters, future systems will extend that reach to neighboring arms of the galaxy.
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