Looking Deep 

Narrator:  It took a dark, unassuming patch of sky to find young galaxies
twinkling at the edge of the cosmos.

Ferguson:  For the Hubble Deep Field it was important that we get an
essentially unblocked view of the distant universe.  We wanted to be
unblocked by anything in our own galaxy, that is, bright stars or dust in
our galaxy.  We wanted to avoid any nearby galaxies that might have
essentially dominated the picture.  And so we had to do some studies to
find a field of sky that was essentially boring in every way so that we
could look at the distant galaxies.

Nar:  The team targeted Hubble’s Continuous Viewing Zone—narrow patches of
sky near the North and South Poles where Hubble can stare for several days
without being blocked by the Earth.  The long exposure brings out wispy
detail in galaxies as far as 12 billion light years away.  But the
telescope also captures light that astronomers need to filter out.

Ferguson:  The problem is that the telescope itself is subject to radiation
from charged particles, most of which come from the solar wind.  And those
also leave little spots of light on the detector.  So when you see one of
the Hubble images it’s covered with these spots like measles.  So you have
to take a whole bunch of pictures.  These spots will move around because
they’re not on the sky, but the galaxies and the stars will stay there.
So you take a whole bunch and combine them together, keeping only the
things that don't change from image to image.

Nar:  In all, it took 350 images, with 3 color filters, for the team to
piece together the final Hubble Deep Field image.  

Ferguson:  Well the thing that amazes me when I look at the Hubble Deep
Field, first of all, is just how many galaxies there are.  This tiny spot
on the sky, and almost everything you see in the image is a galaxy.  Each
of those galaxies has anywhere from a billion to a hundred billion stars in
it.  And so we have before us in this image the history of the universe if
we can figure out what it’s trying to tell us.  And, in many ways, it’s
like going out on a very dark night and staring up at the sky.  And you
realize how tiny a part of the sky it is, and yet you’re seeing these vast systems of stars.  What an amazing universe it is we’re looking at.