CHAPTER 6 | LIGHT AND TELESCOPES 105
Resolution and Precision
What limits the detail you can see in an
image? All images have limited resolution.
You see this on your computer screen because
images there are made up of picture ele-
ments, pixels. If your screen has large pixels,
the resolution is low, and you can’t see much
detail. In an astronomical image, the size of
a picture element is set by seeing and by dif-
fraction in the telescope. You can’t see detail
smaller than that resolution limit.
This limitation on the detail in an image
is related to the limited precision of any
measurement. Imagine a zoologist trying to
measure the length of a live snake by holding
it along a meter stick. The wriggling snake
is hard to hold, so it is hard to measure
accurately. Also, meter sticks are usually
not marked fi ner than millimeters. Both
factors limit the precision of the measure-
ment. If the zoologist said the snake was
43.28932 cm long, you might be suspicious.
The resolution of the measurement technique
does not justify the accuracy implied by all
those digits.
Whenever you make a measurement you
should ask yourself how accurate that mea-
surement can be. The accuracy of the mea-
surement is limited by the resolution of the
measurement technique, just as the amount of
detail in a photograph is limited by its resolu-
tion. If you photographed a star, you would
not be able to see details on its surface for
the same reason the zoologist can’t measure
the snake to high precision.A high-resolution image of Mars reveals details
such as mountains, craters, and the southern
polar cap. (NASA)6-1
It is a Common Misconception that the purpose of
an astronomical telescope is to magnify the image. In fact, the
magnifying power of a telescope, its ability to make the image
bigger, is the least important of the three powers. Because the
amount of detail you can see is limited by the seeing condi-
tions and the resolving power, very high magnifi cation does
not necessarily show more detail. You can change the magnifi -
cation by changing the eyepiece, but you cannot alter the
telescope’s light-gathering power or resolving power without
changing the diameter of the objective lens or mirror, and that
would be so expensive that you might as well build a whole
new telescope.
You can calculate the magnifi cation of a telescope by divid-
ing the focal length of the objective by the focal length of the
eyepiece:M =Fo
__
Fe
For example, if a telescope has an objective with a focal length of
80 cm and you use an eyepiece whose focal length is 0.5 cm, the
magnifi cation is 80/0.5, or 160 times.
Notice that the two most important powers of the telescope,
light-gathering power and resolving power, depend on the diam-
eter of the telescope. Th is explains why astronomers refer to
telescopes by diameter and not by magnifi cation. Astronomers
will refer to a telescope as an 8-meter telescope or a 10-meter
telescope, but they would never identify a research telescope as
being a 200-power telescope.
Th e quest for light-gathering power and high resolution
explains why nearly all major observatories are located far fromVisual-wavelength image■ Figure 6-9
The left half of this photograph of a galaxy is from an image recorded on a
night of poor seeing. Small details are blurred. The right half of the photo
is from an image recorded on a night when Earth’s atmosphere above the
telescope was steady and the seeing was better. Much more detail is visible
under good seeing conditions. (Courtesy William Keel)
Th at’s why stars look like fuzzy points of light no matter how big
your telescope. All measurements have some built-in uncertainty
(How Do We Know? 6-1), and scientists must learn to
work within those limitations.