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Problem Set 2 Solutions
Review and Discussion:
2, 4, 6
List three advantages of reflecting telescopes over refracting telescopes.
The largest telescopes are reflecting telescopes, primarily because of 3 distinct disadvantages of the
refracting telescope. When light passes through a lens, light of different wavelengths focus at slightly
different places. This is known as chromatic aberration and can produce seriously out of focus images. It
is not easy to correct when making large lenses. Note, however, that camera lenses are quite successful in
correcting this aberration; otherwise all your color photos would be rather blurry. A second problem is the
glass lens absorbs certain wavelengths of light that the astronomers needs to observe. In the infrared, for
instance, glass is not transparent like it is for visible light. In the infrared, the glass lens blocks light from
entering the telescope. Lastly, it is difficult to keep a lens bigger than about one meter from bending in its
support. Glass is flexible and lenses can only be supported around their edge. When a large glass lens
bends due to its own weight, its curvature changes and so does the focus, thus ruining the image.
How does Earth’s atmosphere affect what is seen through an optical telescope?
The Earth's atmosphere smears out images seen in telescopes. "Seeing" is the blurring in the image of an
object, such as a star, as its light passes through the Earth's atmosphere. Instead of the star image being
very small, limited by the diffraction of the telescope, the image is blurred to many times this size. The
atmosphere of the Earth is not homogeneous; it is turbulent and contains layers of varying temperatures
and density. Light passing through these layers is refracted into many slightly different paths. Fortunately
for astronomers, the Earth's atmosphere is really rather thin and so the images are not completely blurred
What are the advantages of a CCD over a photographic plate?
A CCD is a charge-coupled device. It has thousands of individual detectors arranged in a grid, each much
more sensitive than a photographic plate. The light level of each detector is read out by computer. A
primary advantages are its high sensitivity to light, its linear response to light (twice as much light
produces twice as much signal, unlike a photographic plate, which is highly non-linear), and the ease with
which the image can be processed by computer software.
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A certain telescope has a 10'
10' field of view that is recorded using a CCD chip having 2048
2048 pixels. What angle on the sky corresponds to 1 pixel? What would be the diameter of a typical
seeing disk (1" radius), in pixels?
A 10' angle is equal to 600". But there are 2048 pixels along this angle, so each pixel is 0.29". A typical