In order to understand what Hubble sees, it is necessary to understand what astronomical telescopes do, and how they differ from the ones
on the ground. All telescopes collect light and magnify it. When an
image is magnified, it gets dimmer. This is because the light coming
from the object is spread out over a larger area. If we try to magnify
the image too much, it gets so dim when we look through the telescope
we don't see anything at all.
Astronomers have come up with several creative ways to obtain more
light and make their images brighter. One way astronomers get around
this problem is to make the telescope larger. A larger telescope will
collect more light from the object, so the image is brighter too.
Another way is to use a recording device instead of the human eye.
Recording devices can be photographic films or electronic detectors.
Unlike the human eye, these methods have an advantage in that exposures
of faint objects can made over a very long time. The exposures can
be as long as 45 minutes per image. The images can then be added
together to add up to exposure times of up to 38 hours for each
filter. The longer an exposure is, the more light falls on the film.
As long as the image is held very still, eventually an image of
the faint target will appear. Another advantage of recording devices
is that a permanent copy of the observation is obtained.
Hubble looks at objects which are so distant and faint, that even
with highly sensitive CCD detectors (which are many times more sensitive
than the human eye) it must expose for long periods of time. These
exposures can be for 30 minutes or more. Sometimes hundreds of half-hour
exposures are added together to produce one very long exposure.
Observations using CCD detectors can also be added together, which
is something that cannot be done with photographic film.