CHAPTER 6 | LIGHT AND TELESCOPES 107
Infrared telescopes have fl own to high altitudes under balloons
and in airplanes to get above absorption by water vapor. NASA is
now testing the Stratospheric Observatory for Infrared Astronomy
(SOFIA), a Boeing 747SP that will carry a 2.5-m telescope, con-
trol systems, and a team of astronomers, technicians, and educa-
tors into the dry fringes of the atmosphere. Once at that altitude,
they can open a door above the telescope and make infrared
observations for hours as the plane fl ies a precisely calculated
path. You can see the door in the photo in Figure 6-11.
To reduce internal noise, the light-sensitive detectors in astro-
nomical telescopes are cooled to very low temperatures, usually
with liquid nitrogen, as shown in Figure 6-11. Th is is especially
necessary for a telescope observing at infrared wavelengths.
Infrared radiation is emitted by heated objects, and if the telescope
is warm it will emit many times more infrared radiation than that
coming from a distant object. Imagine trying to look for some-
thing at night through binoculars that are themselves glowing.
Beyond the other end of the visible spectrum, astronomers
can observe in the near-ultraviolet at wavelengths of about 290
to 400 nm. Your eyes don’t detect this radiation, but it can be
recorded by specialized detectors. Wavelengths shorter than
about 290 nm, the far-ultraviolet, are completely absorbed by the
ozone layer extending from about 15 km to 30 km above Earth’s
surface. No mountaintop is that high, and no airplane can fl y to
such an altitude. To observe in the far-ultraviolet or beyond at
X-ray or gamma-ray wavelengths, telescopes must be in space
above the atmosphere.pointing systems are available for a price on many small tele-
scopes. A good telescope on a poor mounting is almost
useless.
You might be buying a telescope to put in your backyard,
but you must think about the same issues astronomers consider
when they design giant telescopes to go on mountaintops.
Observing Beyond the Ends of the
Visible Spectrum
Telescopes in mountain-top observatories usually observe at
visual wavelengths, but important observations also can be made
from Earth at some infrared and ultraviolet wavelengths.
Beyond the red end of the visible spectrum, some infrared
radiation leaks through the atmosphere in narrow, partially open
atmospheric windows ranging from wavelengths of 1200 nm to
about 20,000 nm. Infrared astronomers usually measure wave-
length in micrometers (10−6 meters), so they refer to this wave-
length range as 1.2 to 30 micrometers (or microns for short).
Even in this range, much of the radiation from celestial sources
is absorbed by water vapor, carbon dioxide, and ozone molecules.
Nevertheless, some infrared observations can be made from
mountaintops where the air is thin and dry. For example, a num-
ber of important infrared telescopes observe from the 4200-m
(13,800-ft) summit of Mauna Kea in Hawaii. At this altitude,
the telescopes are above much of the water vapor in Earth’s atmo-
sphere (■ Figure 6-11).
Infrared astronomers can often
observe with the dome lights
on. Their instruments are not
usually sensitive to visible light.Adding liquid nitrogen to the
camera on a telescope is a
familiar task for astronomers.SOFIA will fly at roughly 12 km
(over 40,000 ft) to get above
most of Earth’s atmosphere.■ Figure 6-11
Comet Hale–Bopp hangs in the sky over the 3-meter NASA Infrared Telescope
Facility (IRAF) atop Mauna Kea. The air at high altitudes is so dry that it is
transparent to shorter infrared photons. SOFIA will fl y at altitudes up to 14
km ( 45,000 feet) where it will be able to observe infrared wavelengths that
cannot be observed from mountaintops. Most astronomical CCD cameras must
be cooled to low temperatures, and this is especially true for infrared cameras.
(IRTF: William Keel; SOFIA: NASA; Camera: Kris Koenig/Coast Learning Systems)