Bad Astronomy: Misconceptions and Misuses Revealed, from ...

HUBBLE TROUBLE 227
A DROP IN THE BUCKET
A related misconception is that a telescope’s most important function
is to magnify an object, or make it look “closer.” That’s only
partly true. It helps to make a small object look bigger, of course,
but the real reason we make telescopes bigger is to collect more
light. A telescope is like a rain bucket for light. If you are thirsty
and want to collect rainwater, it’s best to use a wide bucket. The
wider the bucket you use, the more rain you collect. It’s the same
for telescopes: the bigger the mirror, the more light you collect
from an object. The more light you gather, the fainter an object
you can see. The unaided eye can pick out perhaps 10,000 stars
without help, but with the use of even a modest telescope you can
see millions. With a truly big telescope billions of stars become
detectable.
The biggest telescopes on Earth have mirrors about 10 meters
(33 feet) across, about the width of a small house. There are currently
plans to build much larger telescopes. One design calls for a
mirror 100 meters (109 yards) across! It’s called the OWL, for
Overwhelmingly Large Telescope. It’ll cost a lot, but probably still
less than Hubble did. A lot of that cost will probably go into simply
finding a place to put it.
So Hubble may be small, but remember, it’s above the atmosphere.
The air glows, which washes out faint objects when viewed
from the ground (see chapter 11, “Well, Well: The Difficulty of Daylight
Star Sighting”). Hubble has darker skies and can see much
fainter objects. The atmosphere also moves, so stars seen from the
ground wiggle and dance (see chapter 9, “Twinkle, Twinkle, Little
Star”). This spreads out the light from stars, making faint ones
even more difficult to detect, especially if they are near brighter
stars, which overwhelm them. With Hubble above the atmosphere,
it avoids this effect and can more easily spot fainter stars. Between
the much darker sky and ability to see faint objects, it holds the
record for detecting the faintest objects ever seen: in a patch of sky
called the Southern Deep Field, one of Hubble’s cameras spotted
objects ten billion times fainter than you can see with your unaided
eye. That’s a pretty good reason to loft a telescope a few hundred
kilometers off the ground.