Observatories Across the Electromagnetic Spectrum
Astronomers use a number of telescopes sensitive to different parts
of the electromagnetic spectrum to study objects in space. Even though
all light is fundamentally the same thing, the way that astronomers
observe light depends on the portion of the spectrum they wish to study.
For example, different detectors are senstive to different wavelenths
of light. In addition, not all light can get through the Earth's
atmosphere, so for some wavelengths we have to use telescopes
aboard satellites. Even the way we collect the light can change
depending on the wavelength. Here we briefly introduce observatories
used for each band of the EM spectrum.
A sample of telescopes (operating as of
February 2013) operating at wavelengths across the electromagnetic
spectrum. Several of these observatories observe more than one band
of the EM spectrum, and those are placed within the band of their
The represented observatories are: HESS,
Fermi and Swift for gamma-ray, NuSTAR and Chandra for X-ray, GALEX
for ultraviolet, Kepler, Hubble, Keck (I and II), SALT, and Gemini
(South) for visible, Spitzer, Herschel, and Sofia for infrared,
Planck and CARMA for microwave, Spektr-R, Greenbank, and VLA for
here to see this image with the observatories labeled.
Artist's concept of radio space
interferometry using the Spektr-R radio satellite. (Credit:
Radio waves can make it through the Earth's atmosphere without significant obstacles.
In fact, radio telescopes can observe even on cloudy days. In principle, then, we don't
need to put radio telescopes in space. However, space-based radio observatories
complement Earth-bound radio telescopes in some important ways.
A special technique used in radio astronomy is called "interferometry." Radio
astronomers can combine data from two telescopes that are very far apart and create images
that have the same resolution as if they had a single telescope as big as the distance
between the two telescopes. This means radio telescope arrays can see incredibly small
details. One example is the Very Large Baseline Array (VLBA), which consists of 10 radio
observatories that reach from Hawaii to Puerto Rico, nearly a third of the way around the
By putting a radio telescope in orbit around Earth, radio astronomers can make images
as if they had a radio telescope the size of the entire planet. The first mission
dedicated to space interferometry was the Japanese HALCA mission which ran from 1997 to
2005. The second dedicated mission is the Russian Spektr-R satellite, which launched in
Artist's conception of the European Space
Agency's (ESA's) Planck observatory cruising to its
orbit. (Credit: ESA/D. Ducros)
The Earth's atmosphere blocks much of the light in the microwave band, so astronomers
use satellite-based telescopes to observe cosmic microwaves. The entire sky is a source
of microwaves in every direction, most often referred to as the cosmic microwave
background (or CMB for short). These microwaves are the remnant of the Big Bang, a term
used to describe the early universe.
A very long time ago, all the matter in existence was scrunched together in a very
small, hot ball. The ball expanded outward and became our universe as it cooled. Since the
Big Bang, which is estimated to have taken place 13.7 billion years ago, it has cooled all
the way to just three degrees above absolute zero. It is this "three degrees" that we
measure as the microwave background.
The first precise measurements of the temperature of the microwave background across
the entire sky was done by the Cosmic Background Explorer (COBE) satellite from 1989 to
1993. Since then, the Wilkinson Microwave Anisotropy Probe (WMAP) refined the COBE
measurements, operating from 2001 to 2010. More recently, the Planck mission was launched
in 2009 and will further improve astronomer's understanding of the CMB.
Tell me more about WMAP
Artist's conception of SOFIA flying at
sunset (Credit: NASA)
Photograph of the Keck I and II domes at
dawn; the Keck telescopes operate in visible and infrared wavelengths.
(Credit: Rick Peterson/WMKO)
Infrared astronomy has to overcome a number of challenges. While some infrared
ratiation can make it through Earth's atmosphere, the longer wavelengths are blocked. But
that's not the biggest challenge everyting that has heat emits infrared light.
That means that the atmosphere, the telescope, and even the infrared detectors themselves
all emit infrared light.
Ground-based infrared telescopes reside at high altituteds in dry climates in an effort
to get above much of the water vapor in the atmosphere that absorbs infrared. However,
ground-based infrared observatories must still account for the atmosphere in their
measurements. To do this, the infrared emission from the atmosphere is measured at the
same time as the measurement of the cosmic object being observed. Then, the emission from
the atmosphere can be subtracted to get an accurate measurement of the cosmic object. The
telescopes, for both ground-based and space/airborne observatories, are also designed to
limit the spurious infrared radiation from reaching the detector, and the detectors are
cooled to limit their infrared emissions.
In 2003, NASA launched the Spitzer Space Telescope into an earth-trailing,
heliocentric orbit, where it does not have to contend with the comparatively warm
environment in near-Earth space. Spitzer ran out of coolant in 2009, but one of it's
instruments continues to operate in a reduced capacity. Another major infrared facility is
the Stratospheric Observatory for Infrared Astronomy (SOFIA). SOFIA carries a large
telescope inside a 747 aircraft flying at an altitude sufficient to get it well above most
of the Earth's infrared absorbing atmosphere.
Visible spectrum observatories
The Hubble Space Telescope just after it
was captures by the Space Shuttle Atlantis to be serviced in 2009.
Visible light can pass right through our atmosphere, which is why astronomy is as old
as humanity. Ancient humans could look up at the night sky and see the stars above them.
Today, there is an army of ground-based telescope facilities for visible astronomy (also
called "optical astronomy"). However, there are limits to ground-based optical astronomy.
As light passes through the atmosphere, it is distorted by the turbulence within the
atmosphere. Astronomers can improve their chances of a good image by putting
observatories on mountain-tops (above some of the atmosphere), but there will still be
limits to how crisp their images will be, especially for faint sources.
Visible-light observatories in space avoid the turbulance of the Earth's atmosphere. In
addition, they can observe a somewhat wider portion of the electromagnetic spectrum, in
particular ultraviolet light that is absorbed by the Earth's atmosphere. The Hubble Space
Telescope is perhaps the most famous optical telescope in orbit. Also in orbit is the
Kepler observatory. Kepler is using visible light to survey a portion of the Milky Way
galaxy to discover planetary systems. The Swift satellite also carries an UltraViolet and
Optical Telescope (the UVOT) to perform observations of
Artist's concept of the GALEX satellite in
orbit. (Credit: NASA/JPL-Caltech)
The Earth's atmosphere absorbs ultraviolet light, so ultraviolet astronomy must be done
using telescopes in space. Other than carefully-select materials for filters, a
ultraviolet telescope is much like a regular visble light telescope. The primary
difference being that the ultraviolet telescope must be above Earth's atmosphere to
observe cosmic sources.
The GALEX obseravtory is the most recent dedicated ultraviloet obseratory. It was
launched in 2003, and is still operating as of February 2013. Its goal was to observe the
history of star formation in our Universe in ultraviolet wavelengths, and it has observed
over a half-billion galaxies going back to when our Universe was just about 3 billion
In addition to GALEX, the Hubble Space Telescope and the UltraViolet and Optical
Telescope on Swift can both perform a great deal of observing at ultraviolet wavelengths,
but they only cover a portion of the spectrum that GALEX observes.
Artist's concept of the NuSTAR satellite.
X-ray wavelengths are another portion of the electromagnetic spectrum that are blocked
by Earth's atmosphere. X-rays also pose a particular challenge because they are so small
and energetic that they don't bounce off mirrors like lower-energy forms of light.
Instead, they pass right through. Unless they just barely graze the surface of the
mirror. (Read more about how X-rays are focused on the
Telescope Introduction page.)
Focusing X-ray telescope require long focal lengths. In other words, the
mirrors where light enters the telescope must be separated from the X-ray detectors by several meters. However. launching such a large observatory is costly and limits the launch vehicles to only the most powerful rockets (the Space Shuttle in the case of the Chandra X-ray Observatory).
In 2012, the Nuclear Spectroscopic Telescope Array (or NuSTAR for short), solved this
problem by designing an observatory with a deployable mast. In other words, NuSTAR was
designed with its mirror module and detector module on a mast, or boom, that could be
extended once it was in orbit. By doing that, NuSTAR could be launched on a low-cost
Artist's concept of the Fermi satellite.
One of the HESS telescopes. (Credit: HESS Collaboration)
Not only are gamma-rays blocked by Earth's atmosphere, but they are
even harder than X-rays to focus. In fact, so far, there have been no
focusing gamma-ray telescopes. Instead, astronomers rely on
alternate ways to determine where in the sky gamma-rays are produced.
This can be properties of the detector or using special "masks" that
cast gamma-ray shadows on the detector. (Read more about gamma-ray
telescopes on the
Gamma-ray Telescopes and Detectors page.)
The Swift satellite was launched in 2004 to help solve the mystery of gamma-ray bursts.
Swift has a gamma-ray detector that can observe half the sky at a time, and if it detects
a gamma-ray burst, the satellite can quickly point its X-ray and optical telescopes in the
direction of the burst. The Fermi Space Telescope was launched in 2008 and is designed to
study energetic phenomena from a variety of cosmic sources, including pulsars, black
holes, active galaxies, diffuse gamma-ray emission and gamma-ray bursts.
It might be surprising to know that astronomers can use ground-based astronomy to
detect the highest energy gamma-rays. For these gamma-rays, the telescopes don't detect
the gamma-rays directly. Instead, they use the atmosphere itself as a detector. The HESS
array has been in operation for over 10 years. The array began with four telescopes
arranged in a square, and recently added the HESS II telescope to its ranks. (Read more
about this type of telescope on the
Air Cerenkov Detector page.)
Updated: February 2013