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X-ray spacecraft also use grazing-incidence
mirrors and solid state detectors while
gamma ray spacecraft use a detector of an
entirely different kind. The Compton
Gamma-Ray Observatory has eight 1-meter-sized
crystals of sodium iodide that detect
incoming gamma rays as the observatory
orbits Earth. Sodium iodide is sensitive to
gamma rays, but not to optical and radio
wavelengths. The big crystal is simply a
detector of photons--it does not focus them. Today, astronomers can choose to collect and count photons, focus the photons to build up an image, or disperse the photons into their various wavelengths. High energy photons are usually detected with counting techniques. The other wavelengths are detected with counting (photometry), focusing methods (imaging), or dispersion methods (spectroscopy). The particular instrument or combination of instruments astronomers choose depends not only on the spectral region to be observed, but also on the object under observation. Stars are point sources in the sky. Galaxies are not. So the astronomer must select a combin-ation that provides good stellar images or good galaxy images. Another important property of astronomical instruments is resolution. This is the ability to separate two closely-space objects from each other. For example, a pair of automobile headlights appear to be one bright light when seen in the distance along a straight highway. Close up, the headlights |
resolve into two. Since telescopes, for
example, have the effect of increasing the
power of our vision, they improve our
resolution of distant objects as well. The
design and diameter of astronomical
instruments determines whether the
resolution is high or low. For stellar work,
high resolution is important so the
astronomer can study one star at a time.
For galaxy work the individual stars in a
galaxy may often not be as important as the
whole ensemble of stars. Unit Goals
Because many of the wavelengths in the electromagnetic spectrum are difficult or dangerous to work with, activities in this section concentrate on the visible spectrum, the near infrared, and radio wavelengths. Several of the activities involve lenses and mirrors. The Lenses and Mirrors activity provides many tips for obtaining a variety of lenses and mirrors at little or no cost. |