QUESTION:
How many different Galaxies can the Hubble take pictures of?

ANSWER from Bryan Miller, May 1, 1996
Essentially an unlimited number of galaxies can be observed with the
Wide Field and Planetary Camera 2 (WFPC2) on the Hubble Space
Telescope.  The number that we can detect is limited in a practical
sense only by the amount of observing time that we have.  There are
perhaps a few thousand bright, nearby galaxies that would completely
fill the field of view of the WFPC2.  A few hundred of these galaxies
have been observed.  However, there are millions of faint, distant
galaxies, a few of which appear in every image taken away from the
plane of the Milky Way.  I study star clusters in nearby galaxies and
it is sometimes hard to distinguish star clusters from compact
background galaxies.

Last December HST was used to make the most sensitive image of galaxies
every made.  The telescope pointed at the same piece of sky, known as
the Hubble Deep Field, for 150 orbits (roughly 225 hours).  This is a
very long time considering that most projects spend less than 5 orbits
on a single target.  Over 1500 galaxies were found in this small patch
of sky.  The faintest galaxies detected are about 10 times fainter than
can be detected from the ground.  We are seeing these galaxies as they
appeared very early in the universe so that studying them will help us
to understand how galaxies evolve.