Image Processing Reference
are not generating it through heating (as, for example, is the case with a light bulb
filament). Because stars generate most of their light by glowing incandescently at
a very high temperature, most of the light they emit is at much shorter wavelengths
than the radio band. They are weak radio emitters (except in special cases where
they “flare,” in which case the interaction of the flare's magnetic field and the hot
gas of the star results in powerful bursts of nonthermal radio emissions.)
This image of the radio sky is in galactic coordinates, which puts the center of
theMilkyWay(our galaxy) at the center of the image, and aligns the plane of the
Milky Way with the “equator” of the image. The horizontal band through the center
is radio emission from objects within the plane of the Milky Way and the intense
white blob in the center is Sagittarius A, the compact radio source mentioned
earlier. The large looping structures represented by light blue pseudocolor are
remnants of “bubbles” blown by star-forming processes within vast molecular
clouds of gas. If we could see light with a wavelength of 73 cm with our eyes,
the dominant structures in the sky would be these large bubbles and structures, all
having an apparent size much greater than either the full moon or the sun. The
image was made by combining data taken in the 1970s from three large radio
telescopes: Jodrell Bank in England, the Effelsberg 100-meter in Germany, and the
Parkes 64-meter in Australia. As the last two names imply, all of these telescopes
are massive dish antennae tens of meters across that can be steered around to scan
the sky. The first two antennae are in the Northern Hemisphere, the last in the
Southern, making it possible to obtain a data set of the entire sky.
For comparison, consider the sky we see with our eyes, as seen in Fig. 3.11.
This image, like Fig. 3.10, is made up of many small color photographs that
are “stitched” together by a computer algorithm into an Aitoff projection. The
broad band of stars is an edge view of the plane of our galaxy, what we call the
Milky Way. The Milky Way contains many radio sources, which is why Figs. 3.10
and 3.11 look similar along the “equator”; yet there are important differences.
Sagittarius A, located at the very center of this image, is not visible to our eyes,
since intervening dust clouds scatter and absorb the visible light emitted by it.
Since the wavelength of microwaves is much longer than the size of interstellar dust
particles, the microwave light used to make Fig. 3.10 propagates freely through the
dust. 9 The large loops and swirls of microwave-emitting hydrogen gas are too cold
to glow with visible light, and thus are absent from the visible sky.
Devices such as radio telescopes that “see” with microwaves and radio waves (1
cm to 10 m or more) do not work on the same principle as human eyes and standard
photographic cameras. Figure 3.12 is a schematic of a typical radio telescope.
Instead of a lens, these imaging systems have directional antennae that are sensitive
in a cone-shaped or fan-shaped beam pattern. Instead of an FPA of sensors similar
in function to the human retina, many radio astronomy imaging systems have a
9 As mentioned earlier, the scattering of waves by objects depends on the size of the objects relative
to the wavelength of the wave. Visible light is highly scattered by dust particles, which are typically
the order of a wavelength in size. Conversely, radio waves pass freely through dust, as they are many
times longer in wavelength than the size of the dust particles.