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A bane and a blessing
Tapping, Ken
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Skygazing: Astronomy through the seasons, 2017-02-14
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A BANE AND A BLESSING
Ken Tapping, 14th February, 2017
Have you ever looked through a really cheap
telescope or pair of binoculars? In addition to a
probably dim, blurry image, you will have noticed
that every object has a coloured border, red or
blue. You can change the colour of that border by
adjusting the focus, but you cannot get rid of it.
These false colours, which can be quite attractive,
are produced by a process called “chromatic
aberration”. Dealing with this has been one of the
main thrusts of lens and telescope design for
centuries. The work continues today, although
modern, high-quality refracting telescopes (ones
that use lenses to form the image), have to be
pushed hard to show that problem.
The light we see with our eyes ranges in
wavelength from about 400 to 800 nanometres
(billionths of a metre). Our brains interpret that
wavelength range as the familiar rainbow of
colours extending from blue to red.
A lens collects all the light from each point in a
distant object and focuses it all to a corresponding
point in the image. It achieves this by bending the
light so that it converges. This bending of the light
is called “refraction”, and the extent to which
anything bends light is described as a number the “refractive index”. The problem is that in most
materials, such as glass, the value of the refractive
index depends on the wavelength. This makes the
blue light in an image focus a different distance
from the lens than does the red light. You cannot
get all the wavelengths in focus at the same time,
so the image formed by the focused light is fringed
by the colours that are not focused.
This problem has been reduced and almost
eliminated by making telescope, binocular and
camera objectives from combinations of lenses
made from different glasses with different
refractive indices. The first lenses developed to
beat the chromatic aberration problem were known
as achromats. The current generations of these
lenses are made out of even more exotic materials
and are called apochromats. These show very
little, if any chromatic aberration. It is intriguing
though to note that chromatic aberration also
turned out to be a blessing that revolutionized
astronomy. It all started with Isaac Newton.
One day Isaac Newton passed a beam of sunlight
through a prism (a glass wedge). Since the
refractive index of the glass depended on the
wavelength of the light, blue light was bent more
than red light. The prism had taken the white light
and broken it up into the band of colours making
up light we perceive as white. It enabled us to
analyze light by breaking it up into its colour
components. Different atoms and molecules
absorb or radiate particular wavelengths, leaving
their signatures in the light. By analyzing that light
using a prism we can determine what stars are
made of, their temperatures and what sort of
material the starlight is passing through on its way
to us. Chromatic aberration led us to astronomical
spectroscopy, an essential part of astronomy.
Isaac Newton, who first harnessed chromatic
aberration for science, also devised an elegant
cure for it. Instead of forming the image in a
telescope with a lens, he used a mirror. A concave
mirror with a reflective coating on its surface forms
an image without the light passing through any
glass at all. Therefore there is no chromatic
aberration. His invention, the “reflecting” telescope,
is now the mainstay of astronomical research. The
Gemini Telescopes, the Canada France Hawaii
Telescope and the Hubble Space Telescopes all
use mirrors rather than lenses. We often put
spectroscopes on reflecting telescopes, combining
two of Newton’s inventions, one avoiding
chromatic aberration and the other exploiting it.
Mars and Venus lie in the Southwest after sunset.
Venus is very bright. Mars, redder and fainter, lies
close to its left. Jupiter rises around 10pm. The
Moon will reach Last Quarter on the 18th.
Ken Tapping is an astronomer with the National
Research Council's Dominion Radio Astrophysical
Observatory, Penticton, BC, V2A 6J9.
Tel (250) 497-2300, Fax (250) 497-2355
E-mail: [email protected]