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gathering power(LGP). Much larger even
than the Hale Telescope is the Keck
Telescope that has an effective diameter of
10 meters. Its light gathering power is two
and a half million times the amount a typical
eye can receive. Although NASA's Hubble
Space Telescope, in orbit above Earth's
atmosphere, has only a LGP of 144,000, it
has the advantage of an unfiltered view of
the universe. Furthermore, its sensitivity
extends into infrared and ultraviolet
wavelengths. Once the telescope collects the photons, the detection method becomes important. Telescopes are collectors, not detectors. Like all other telescopes, the mirror of the Hubble Space Telescope is a photon collector that gathers the photons to a focus so a detector can pick them up. It has several filters that move in front of the detector so that images can be made at specific wavelengths. In the early days, astronomers recorded what they saw through telescopes by drawing pictures and taking notes. When photography was invented, astronomers replaced their eyes with photographic plates. A photographic plate is similar to the film used in a modern camera except that the emulsion was mounted on glass plates instead of plastic. The glass plate collected photons to build images and spectra. Astronomers also employed the photo-multiplier tube, an electronic device for counting photons. The second half of this century saw the development of the Charge Coupled Device (CCD), a computer-run system that collects photons on a small computer chip. CCDs have now replaced the photographic plate for most astronomical observations. If astronomers require spectra, they insert a spectrograph between the telescope and the CCD. This arrangement provides digital spectral data.
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Driving each of these advances is the need
for greater sensitivity and accuracy of the
data. Photographic plates, still used for
wide-field studies, collect up to about 5
percent of the photons that fall on them. A
CCD collects 85 to 95 percent of the
photons. Because CCDs are small and can
only observe a small part of the sky at a
time, they are especially suited for deep
space observations. Because the entire electromagnetic spec-trum represents a broad range of wave-lengths and energies (See figure 1, page 24.), no one detector can record all types of radiation. Antennas are used to collect radio and microwave energies. To collect very faint signals, astronomers use large parabolic radio antennas that reflect incom-ing radiation to a focus much in the same way reflector telescopes collect and concen-trate light. Radio receivers at the focus convert the radiation into electric currents that can be studied. Sensitive solid state heat detectors measure infrared radiation, higher in energy and shorter in wavelength than radio and micro-wave radiation. Mirrors in aircraft, balloons, and orbiting spacecraft can concentrate infrared radiation onto the detectors that work like CCDs in the infrared range. Be-cause infrared radiation is associated with heat, infrared detectors must be kept at very low temperatures lest the telescope's own stored heat energy interferes with the radia-tion coming from distant objects. Grazing-incidence mirrors that consist of a mirrored cone collect ultraviolet radiation reflected at a small "grazing" angle to the mirror surface and direct it to detectors placed at the mirror's apex. Different mirror coatings are used to enhance the reflectivity of the mirrors to specific wavelengths. |