Measuring very small angles is best done measuring differential displacements in a small field under large magnification. In the past this led to the development and use of large refractor telescopes for measuring parallaxes. Within a small field, however, the displacement due to parallax has the same direction for all stars; the amount of displacement varies with the distances of the stars. A differential measurement in a small field will generally be with respect to one or more stars that show the same, but preferably smaller, displacement. In the 1960, Pierre Lacroute developed the idea to combine two fields from different parts of the sky in a simultaneous measurement. It was soon clear that this could not be done from the ground, and further development ultimately led to the first astrometric satellite mission, Hipparcos, and its follow-up Gaia. Both satellites have a telescope with two fields of view, projecting on the same focal plane. The two fields scan the sky along a great circle, which ultimately allows for an accurate calibration of the angle between the two fields of view, the so-called basic angle. With the basic angle measured, the differential measurements within the focal plane between stars for different fields of view become measurements of large arcs on the sky. The parallax displacement direction on either side of any such arc will be different and is fully resolved when combining data from different arcs and different stars. This way Hipparcos and Gaia are able to measure directly absolute parallaxes.
Page last updated: 23 December 2013