  During the Hipparcos mission in September 1992, I presented a concept for
using direct imaging on CCDs in scanning mode in a new and very powerful
astrometric satellite, Roemer. The Roemer concept with larger aperture
telescopes for higher accuracy was developed by ESA and a mission was approved
in 2000, expected to be a million times better than Hipparcos. The present name
Gaia for the mission reminds of an interferometric option also studied in the
period 1993-97, and the evolution of optics and detection in this period is the
main subject of the present report. The transition from an interferometric GAIA
to a large Roemer was made on 15 January 1998. It will be shown that without
the interferometric GAIA option, ESA would hardly have selected astrometry for
a Cornerstone study in 1997, and consequently we would not have had the
Roemer/Gaia mission.

Instrumentation and Methods for Astrophysics;Astrophysics of Galaxies