5. OTHER ATTRIBUTES
Sensitivity and angular resolution are not the only qualities that determine the scientific performance of a telescope. Depending on one's scientific goals, one might want to trade off these properties in order to improve some other aspect of the performance of the telescope. For example, if you want to survey as much of the sky as possible in order to find some peculiar new object, you might want a greater field of view at the expense of sensitivity and/or angular resolution. Or, if you want to measure the wavelengths of spectral lines with great accuracy, you might want greater spectral resolution at the expense of sensitivity or spectral range, as we describe below.
Field of View: This term refers to the angular scope of vision. Your eyes have a very large field of view. Looking straight ahead, you can still notice things almost 40 degrees off to the side. Most cameras (unless equipped with a fish-eye lens) have a much smaller field of view than your eyes. When you use a telephoto lens, binoculars, or a telescope, the field of view decreases.
Generally, the greater the angular resolution a telescope has,
the smaller is its field of view. That is why smaller "finder" telescopes are
mounted on the telescopes at the Sommers-Bausch observatory. Since the finder
telescope has a greater field of view, you can use it to find and center an
object in the field of view of the bigger telescope. The field of view of a
telescope is usually limited by the size of the detector in its camera. For
a typical large telescope, it might be a square area about 20 arcminutes on
a side. On the Hubble Space Telescope, it is a square about 3.3 arcminutes on
a side. This area represents only a tiny fraction (about 7.6 ´
10-8) of the whole sky. It would take about 500,000 years for the
HST to map the entire sky with the sensitivity of the Hubble Deep Field (which
required a 10 day exposure)!
If astronomers want to study a known source in the sky, a telescope with a wide field of view may not be necessary. But if they want to discover rare objects, or make statistical studies of large numbers of sources of a certain type (for example, binary stars or distant galaxies), they should survey large areas of the sky. Obviously, the Hubble Space Telescope is very poorly suited for such work. Instead, astronomers build telescopes especially designed to take pictures (and spectra) with a wide field of view. One of the most important such telescopes is the Sloan Digital Sky Survey telescope, a 3.5-meter telescope with a field of view equal to about 30 full moons. The SDSS is mapping the northern sky with far greater sensitivity than the previous all sky survey (which was done in 1950-1955) and will take spectra of more than a million sources. This 5-year project is about half complete.
Above right: a comparison of the fields of view of the Sloan Digital Sky Survey (large square), a typical large telescope (middle square), and the Hubble Space Telescope (square dot).
In Section 4 of this lesson, we described adaptive optics systems that can correct for the distortion of infrared waves by the Earth's atmosphere and provide images with angular resolution limited only by the telescope diameter. But the field of view over which such adaptive optics systems can sharpen the image is very small -- less than one arc minute. The only way we know to obtain very sharp optical or infrared images over a wide field of view is to put the telescope on a spacecraft.
Spectral Range: Your eyes are sensitive to photons having wavelengths ranging from 4000 to 7000 Angstroms (1 Angstrom (A) = 10-10 meters). [Scientists often use another unit, the nanometer (1 nanometer = 10 A), to define wavelength.] Photons with wavelength 4000 A appear violet; those with 7000 A appear red; 5000 A are green. Photons with wavelengths from about 3000 A (ultraviolet) to 13000 A (infrared) can reach the ground, as can infrared photons in "windows" of longer wavelength and radio photons with wavelength > 1 cm. Ground-based optical/infrared telescopes equipped with the right detectors can observe these photons from cosmic sources. But a detector that is sensitive to infrared photons will probably not be sensitive to ultraviolet photons, so the astronomer will have to choose a camera with a more limited spectral range in order to detect faint sources.
Moreover, with telescopes in space we can observe gamma rays, X-rays, ultraviolet, and infrared photons that cannot penetrate the Earth's atmosphere. So, you see that your eyes are sensitive to only a tiny fraction of the electromagnetic spectrum. As you can see in the Multiwavelength Milky Way, the sky looks completely different in different wavelength bands. In the following sections, we discuss telescopes designed to work in other spectral ranges.
Spectral Resolution: This term refers to the ability to discriminate among wavelengths. With your naked eyes in bright light, you might be able to distinguish the colors of light with wavelengths differing by about 10% (say, about ten colors in the rainbow). But with modern spectrometers, astronomers can distinguish wavelengths of starlight differing by less than one part in 105. These spectrometers are usually made with diffraction gratings.
Of course, if you are going to analyze the light from a cosmic object into thousands of different wavelength components, you will need to capture thousands of more photons than you would need simply to detect the object. For that reason, the Hubble Space Telescope cannot take spectra of the faintest objects it can see (for example, in the Hubble Deep Field). Large ground-based telescopes can do better because they have greater apertures. The spectra of many of the faint galaxies in the Hubble Deep Field have been obtained with the Keck Telescopes.
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Last modified January 19, 2003
Copyright by Richard McCray