192
wavelengths bending more
than others. As a result, the
straight beam of light that traveled
to Earth across the cosmos starts
to follow an ever-changing and
haphazard zigzagging path
through the air. A telescope or
a naked eye focused on it will
see a fluctuation in brightness
as some of the light is directed
in and out of that line of sight.
The impact twinkling has
on capturing sharp astronomical
images is called “seeing.” When
the atmosphere is very still and
seeing is good, the image of a
distant star in a telescope is a
small steady disk. When seeing
is poor, the image breaks up into
a squirming cluster of dots. An
image taken over a period of time
is smeared out into a larger disk.
The effect is similar to the
telescope being out of focus.Improving the view
Observational conditions change
constantly with the atmosphere.
Before the 1990s, observers simply
waited until distortions were at
a minimum. For instance, high
winds clear away turbulence,
creating near-perfect viewing
conditions. In the late 1940s,
astronomers started to use
movie cameras to film the sky
in the hope that, among the
thousands of frames filmed over
time, there would be the odd “lucky
image” that captured the sky in
crystal clarity. Another solution
was to go higher. Today, the
world’s most effective terrestrial
observatories are invariably built
at the top of high, arid mountains,
where cloud cover is minimal, and
the air above is generally calm.SPACE TELESCOPES
With the advent of powerful
computers in the 1990s, earthbound
astronomers began using adaptive
optics (AO) to correct the problems
of astronomical seeing. AO
measures distortions in the arriving
light and evens it out, just as a
distorted mirror might be used to
correct a deformed image to make
it look like the original image prior
to deformation. AO systems use
minutely adjustable mirrors and
other optical devices, but they
also rely heavily on computers to
filter out the atmospheric “noise”
from images. Despite the dramatic
improvements brought about by
AO, however, a large telescope in
orbit, which could observe in multiple
wavelengths of the spectrum,
including visible light, was the
ultimate goal for astronomy.The road to Hubble
As the leading voice in the field,
Spitzer had been made head of
NASA’s task force for developing
the Large Space Telescope (LST)
program in 1965. In 1968,
NASA scored its first space-
telescope success with the
Orbiting Astronomical Observatory
(OAO-2), which took high-quality
images in ultraviolent (UV)Adaptive optics requires a clear star
as a reference point. As these are hard
to find, a sodium laser creates a “star” by
lighting up dust in the high atmosphere.
Our knowledge of stars and
interstellar matter must be
based primarily on the
electromagnetic radiation
which reaches us.
Lyman Spitzer Jr.