  The Hubble Space Telescope's mission is summarized, with special emphasis
placed on the Space Telescope Science Institute's unique experience with
Hubble's behavior as an astronomical telescope in the environment of low earth
orbit for over two decades. Historical context and background are given, and
the project's early scientific expectations are described. A general overview
of the spacecraft is followed by a more detailed look at the optical design,
both as intended and as built. Basic characteristics of the complete complement
of science instruments are also summarized. Experience with the telescope
on-orbit is reviewed, starting with the major initial problems, solutions,
human servicing missions, and the associated expansion of the observatory's
capabilities over this time. Specific attention is then given to understanding
Hubble's optical quality and pointing/jitter performance, two fundamental
characteristics of a telescope. Experience with-and the important mitigation
of-radiation damage and contamination is also related. Beyond the telescope
itself, the advances in data reduction, calibration, and observing techniques
are briefly discussed, as well as the subsequent emergence of highly accessible
high-level archival science products. Hubble's scientific impact concludes the
discussion.
