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Chapter 3
Telescopes
The tools of Astronomy
Very Large Array (VLA), National Radio Astronomy Observatory (NRAO),
Socorro, New Mexico
(Radio telescope: 27 antennas , Y configuration, 25 meters diameter each)
Reading assignment: Chapter 3
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Let’s start with something basics
How does your eye form an image?
Light is refracted (bend) by the lens (converging lens) and forms an image
in the retina. The pupil control the amount of light accepted by the lens.
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What is refraction?
surface
Incident
ray
Refracted ray
Reflected
ray
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• Refraction is the bending of
light when it passes from one
substance into another
(substances with different
refraction index).
• Example: Light passing from
air to glass.
• For the ray to bend, it must
hit the surface at an angle less
than 90 degrees from the
surface.
• The lenses in your eyes uses
refraction to focus light and
produce an image.
Refraction in a prism
A simple model of a converging lens
Refraction of light by a prism
Keep in mind that light of different
wavelengths (colors) are refracted at different angles
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An approximation of a converging (convex) lens by
prisms and sections of prisms
Focusing light with a converging lens
• Refraction can cause parallel light rays to converge
to a focus and form an image.
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Image Formation
• The focal plane is where light from different
directions comes into focus and form an image.
• The image behind a single (convex) lens is actually
upside-down!
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Focusing light, recording an image
Digital cameras detect
light and record
images with an
electronic device
called ChargeCoupled Device
(CCD).
• A camera focuses light like an eye and captures the image
with a detector.
• The CCD detectors in digital cameras are similar to those
used in modern telescopes.
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What are the two basic designs of
telescopes?
• Reflecting telescope: focuses light with
mirrors. The curved (concave) mirror
reflect light and forms an image
• Refracting telescope: focuses light with
lenses. The lens “bends” light by refraction
and forms an image
• The main (or primary) mirror or lens is also
known as the objective of a telescope
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eyepiece
Reflecting Telescopes
opening
Secondary
mirror
•
Primary Mirror
(Concave)
A reflecting telescope has a primary mirror and secondary mirror. The secondary can be flat
or curved.
• The primary mirror is curved (concave) and can be supported from the back in several
places so it can maintain the curvature.
• The primary mirror can be spherical or parabolic. Some spherical mirror requires a corrector
plate at the opening
• Reflecting telescopes can have much greater diameters.
•© 2010Modern
telescopes are reflectors.
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Mirrors in Reflecting Telescopes
Twin Keck telescopes on
Mauna Kea in Hawaii
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Segmented 10-meter mirror
of a Keck telescope. The
main mirror is composed of
36 hexagonal segments
Refracting Telescopes
• A refracting telescope uses a lens instead of a mirror
Some disadvantages of refracting telescopes:
• The lens separate light into different colors. It
focuses light at different distances along the optical
axis. This is known as chromatic aberration.
• To correct for chromatic aberration it is necessary to
use an objective composed two or three elements
• Refracting telescopes need to be very long (large
focal length), with large, heavy lenses.
• Heavy lenses can be supported only from the edges .
The weight flex the lens and distort the shape
causing aberrations in the images.
• Light passing through a lens can get absorbed.
Absorption can be severe at UV and IR wavelengths
• To manufacture a lens it is necessary to machine and
polish at least two surfaces or more if the objective
has 2 or 3 elements. More expensive.
The largest refracting telescope is the 40 inch diameter Alvan Clark telescope at Yerkes observatory
The UF Campus Observatory owns an 8” refracting telescope. It was manufactured about 90 years ago.
The lens was built by the Alvan Clark company, the same company that build the 40” at Yerkes.
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The telescope size or diameter
What are the two most important properties of a
telescope?
1.
2.
3.
Light-collecting area: Telescopes with a larger mirrors
or lenses (larger diameter) have a large collecting area. It
will act as a “light bucket”. A large collecting area can
gather a greater amount of light in a shorter time. A
telescope with a large collecting area can observe fainter
object
Angular resolution: Telescopes that have larger mirrors
or lenses are capable of taking images with greater
detail.
Both the light-collecting area and the angular
resolution are increased by a larger lens or mirror.
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Light-Collecting Area
( Or Light-Gathering Power)
• A telescope’s diameter tells us its light-collecting
area:
A = π (D/2)²
A= Area
D= Diameter
The diameter of the telescope D (the diameter of the lens or mirror) is normally referred
as the aperture of the telescope
• The light-gathering power is proportional to the area of the objective
• The light-gathering power is proportional to the diameter square
•
•
The largest telescopes currently in use have diameters in the range of 8-10 meters.
The largest single mirror telescope is the 10.4 meter diameter telescope in the
Canary Island (Spain). The mirror is composed of 36 small hexagonal mirror.
UF is a partner in the GTC (Gran Telescopio de Canarias)
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Question
How does the collecting area of a 10-meter telescope
compare with that of a 2-meter telescope?
a) It’s 5 times greater.
b) It’s 10 times greater.
c) It’s 25 times greater.
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Question
How does the collecting area of a 10-meter
telescope compare with that of a 2-meter
telescope?
A = π (D/2)²
a) It’s 5 times greater.
b) It’s 10 times greater.
c) It’s 25 times greater.
Ratio of the diameters is 10/2 = 5
The ratio of the collecting area is 5² =25
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Angular size and angular separation
Read “More precisely 0-1, Angular Measure” (See page 12)
Angular size of an object depends on two parameters
• The physical size of the object
• The distance to the object
Angular size is measured in units of angle (degrees, arcmin
and arcsec)
Angular size = Physical Size
Distance
Approximate formula (valid for small angles):
Angular Size = Physical size
360 degrees
2 π x distance
Angular size = Physical size x 360 degrees/ (2 π x distance)
Example: Physical size of the Moon
Angular size = 0.5 degrees
Distance = 380,000 km
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The resolving power of a telescope
Angular Resolution
• The resolving power
is the minimum
angular separation
that the telescope can
distinguish
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Angular Resolution
• The ultimate limit to resolution comes
from interference of light waves within a
telescope.
• Larger telescopes (larger diameter of
lens or mirror) are capable of greater
resolution.
• The angular resolution is proportional to
the wavelength  and inversely
proportional to the diameter D of the lens
or mirror
• Angular resolution = 0.25 (/D)
•  in micrometer (10^-6 m), D in meters
• High resolution means that the telescope
can distinguish details that are separated
by small angular distance
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The effect of the telescope aperture (diameter of lens or
mirror) in the resolution
Example: Resolving a binary star
Small aperture
Medium aperture
Large aperture
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Angular Resolution
•
Airy disc
Diffraction rings
•
•
•
•
•
•
•
•
Close-up of the image of a star taken by
the Hubble Space Telescope
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The image of a star produced by a telescope is not
a point source
The central spot and the rings in this image of a
star are produced by the phenomenon called
diffraction of light (Light behave like a wave)
Light passing through a circular aperture produce
this diffraction pattern.
The opening of a telescope is circular
The central bright spot, called the Airy disc
increases in diameter when the diameter of a
telescope decreases.
Larger diameter telescopes produce a small
Airy disc and have better resolving power
There is a limit in the angular resolution that a
telescope can achieve. It depends on its diameter.
This limit in the resolution is known as the
diffraction limit.
Angular resolution = 0.25 (/D)
The angular resolution of a telescope is given in
arc seconds
Airy discs and the resolution of a telescope
Airy disc produced by a green laser
beam on a circular aperture.
In a telescope, the diameter of the Airy
disc produced by a star increases if the
diameter of the aperture decreases
Intensity distribution
of Airy discs of two
sources (stars)
Airy discs patterns, point
spread function and
resolution.
An example: stars in a
stellar cluster
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An example of images of the Andromeda Galaxy
with different resolutions
(Using telescopes of different apertures)
Aperture of a
telescope: the diameter
of the mirror or lens.
10 arc minutes (10’)
(very small aperture)
1 arc minute (1’)
5 arc seconds (5”)
Resolution of the human
eye: ~0.5 arc minutes (0.5’)
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1 arc second (1”)
(Large aperture)
The focal length of a converging lens
f is the focal length
f is the distance from the
lens to the focal point, where
the image forms. Valid for
parallel rays (distant objects)
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The magnification of a telescope
•
•
•
•
•
•
•
The magnification of a telescope is how many times bigger an object looks
through a telescope compared to how it looks with the naked eye.
The focal length of a telescope (Ft) is the equivalent distance from the lens or
mirror to the plane where the image forms.
The focal length of an eyepiece (Fe) is the distance between the lens of the
eyepiece and the point where the image forms
The magnification of a telescope is the ratio M = Ft/Fe
The magnification is a number, it has no units
The magnification of a telescope can be changed by changing the eyepiece.
Eyepieces of different focal length (Fe) provide different magnifications
Magnifications around 250-350 are a practical limit for a telescope. Under good
sky conditions and with good optics, it can be as high as 600
Increasing the magnification of a telescope does not increase its resolution
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What do astronomers do with
telescopes?
• Imaging: taking images of object or large area of the sky
• Spectroscopy: breaking light into spectra and analyze the
spectral lines
• Photometry: measure the intensity of light and variation in
the intensity or brightness of the light from an object.
Obtaining the light curve of an object. The brightness of an
object can be expressed in an scale called magnitude.
• Astrometry: Measure the position (distance and angle) of an
object in the sky or the position respect to another object
• Timing: measuring how the light output from an object
varies with time and keeping a precise timing of those
variations
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A CCD (Charge Coupled Device)
used in astronomy
• Electronic cameras used
in Astronomy use a CCD’s
•The small elements in a
CCD sensitive to light are
called pixels.
•A CCD has millions of
pixels
•When light strike a pixel
it will develop an electric
charge proportional to the
intensity of light.
•The charge in each pixel
is read by the electronics
controlled by a computer
and stored as an array of
numbers
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An image
of a CCD
How a CCD store the information and
produces an image?
• The charge in each pixel is
read and stored as an array of
numbers in a computer
• To display the image, the
array of number is send to a
computer monitor or screen.
• The value of each number is
converted into intensity and
displayed in the screen.
•The intensity is proportional
to the value of the number.
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Imaging
•
•
•
•
•
•
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At the present the
astronomical detectors for
taking images in astronomy
are CCD (Charge Couple
Device)
No more use of photographic
plates in professional
astronomical observatories.
Astronomical detectors
generally record only one
color of light at a time. They
generate a black/white image.
Several images taken with
color filters must be
combined to make full-color
pictures.
Normally the color filters are
red, green and blue (RGB)
The software combining the
images assign the colors to
each of them and produce a
color image.
Imaging
• Some astronomical
detectors can record
forms of light our
eyes can’t see.
• Example: X-rays,
Gamma rays. IR, UV
• Color is sometimes used
to represent different
energies of non-visible
light.
• Some of the “color”
images may not
represent actual colors.
Different wavelength
are assigned different
colors. Images at some
wavelengths can be at
radio, UV, X-rays or
Gamma-rays
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Example of “color” images of Mars taken by the MAVEN spacecraft
after its arrival in September, 2014. The images were taken in UV
light reflected from Hydrogen and Oxygen
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Spectroscopy
• A spectrograph
separates the different
wavelengths of light
before they hit the
detector.
• A diffraction grating
is used to separate the
colors or wavelengths.
• A diffraction grating
has a few thousand
lines per mm
• The diffraction grating
shown here in the
spectrograph is a
reflection diffraction
grating
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Spectroscopy
• Graphing the
relative
brightness of
light at each
wavelength
shows the
details in a
spectrum.
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Photometry and timing
The example to
the right is of the
light curve of a
star with variable
brightness
•
•
•
•
•
A light curve represents a series of brightness measurements made over a period of
time.
The technique use a reference star that show no change in brightness. The brightness
of the variable star is calibrated against the reference star
This technique can detect variable stars and variability in astronomical objects
An important application: Measuring the variation of brightness from a star allows
the detection the transit of an exoplanet in front of a star
Exoplanet: Planet in orbit around other stars
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A summary
• What are the two most important properties of a telescope?
– Collecting area determines how much light a telescope can
gather.
– Angular resolution is the minimum angular separation a
telescope can distinguish.
– Both properties, collecting area and angular resolution
improve when the diameter of the lens or primary mirror
increases
• What are the two basic designs of telescopes?
– Refracting telescopes focus light with lenses.
– Reflecting telescopes focus light with mirrors.
– The vast majority of professional telescopes are reflectors.
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Summary
• What do astronomers do with telescopes?
– Imaging
– Spectroscopy
– Photometry
– Astrometry
– Timing
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Atmospheric Blurring
How does Earth’s atmosphere affect groundbased observations?
• The best ground-based sites for
astronomical observatories are:
– calm atmosphere (not too windy, atmosphere
less turbulent)
– High altitude (less atmosphere to see through,
less absorption)
– dark sky (far from city lights, low light
pollution)
– dry (few cloudy nights)
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Light Pollution
• Scattering of human-made light in the atmosphere
is a growing problem for astronomy.
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Effect of atmospheric blurring in the
formation of an image
The light coming from a star comes from a point source, the
rays will add (constructive interference) or subtract
(destructive interference) , causing the image to “twinkle”
The light coming from a planet comes from many points in
the disk. Planets normally do not twinkle
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• Rays of light are
distorted by the turbulence
in the terrestrial
atmosphere.
•The rays are deflected
and travel using slightly
different path.
•The rays interfere, adding
or subtracting which
causes the image to
“twinkle”
• And individual image
forms, each lasting for a
fraction of a second but its
position changes
continuously
• In a long exposure, the
individual images form a
disk.
• If the atmosphere is
turbulent, the diameter of
the disk is large ( A
condition called bad
seeing)
Twinkling and Turbulence
Bright star field viewed with
ground-based telescope
Same star field viewed with
Hubble Space Telescope
Turbulent air flow in Earth’s atmosphere and distorts our view,
causing stars to appear to twinkle. In a long exposure, the image of a
star looks bigger due to the random motion of the instantaneous
location of the image around a central location
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An example of “bad” seeing
The turbulence in
terrestrial atmosphere
distort the image of
these lunar craters. The
large crater is Clavius
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Adaptive Optics
A technique to reduce the effect of the atmosphere
Without adaptive optics
With adaptive optics
A technique to reduce the effect of the atmosphere is called Adaptive
optics. It consist in rapidly changing the shape of a telescope’s mirror
to compensate for some of the effects of turbulence.
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Location of observatories: in a calm,
high, dark and dry place
Summit of Mauna Kea, Hawaii
Altitude 14,000 feet (4,000 m)
Some of the telescopes: Gemini North: 8.1 m,
Keck twins: 10 m, Subaru: 8.3 m
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• The best observing
sites are atop remote
mountains.
• Some of the best
places are:
 Hawaii (Mauna Kea)
 North of Chile (Cerro
Tololo, Cerro Pachon,
Cerro Paranal)
 The Canary Island
 South-West US
(Texas, New Mexico)
Why do we put telescopes into space?
Telescopes in space are not
affected by the blurring effect,
absorption of light and light
pollution of the terrestrial
atmosphere
The Hubble Space Telescope
• In orbit since 1990. Still in operations
• 2.4 m diameter mirror (solid)
• It can observe in near UV, visual and
IR
• Cost about 2.5 billions US $
• Next space telescope: James Webb
Telescope
• 6.5 m diameter, 18 hexagonal mirrors
. Projected launch date : 2018
•It will have instruments for mainly
the IR
•Estimated cost: 8 billion US$
(maximum authorized by Congress)
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Hubble Ultra-Deep field
Image of a field taken by Hubble Telescope in the constellation Fornax.
The size of the field is about 1/10 the size of the full Moon
A good example of the high resolution that can be achieved in space.
Very faint galaxies can be imaged (no light pollution!)
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The TMT (Thirty Meter Telescope) telescope
The primary mirror is composed of 492
segments
It will be located at the Mauna Kea
observatory (Hawaii)
Projected date for beginning of operations
is 2022.
Cost: around 1.2 billion US dollars
The size of the mirrors in the 30 m TMT, the
10 m Keck and the 5 m (200 inch) Hale
(Mount Palomar) telescopes
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The ELT 39 meter European telescope in Cerro Armazones (Chile)
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Transmission in Atmosphere
• Only the longer wavelengths of radio and the visible light
pass easily through Earth’s atmosphere.
• We need telescopes high in the atmosphere to observe in the
IR part of the spectrum
• To extend the observations into the UV, X-Ray and Gamma
ray it is necessary to have telescopes in space
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Summary
• How does Earth’s atmosphere affect ground-based
observations?
– Telescope sites are chosen to minimize the problems of
light pollution, atmospheric turbulence, and bad
weather.
• Why do we put telescopes into space?
– Some wavelength of light other than radio and visible
do not pass through Earth’s atmosphere.
– Much sharper images are possible because there is no
atmospheric turbulence.
– No light pollution from light scattered in the terrestrial
atmosphere allows long exposure for recording very
faint objects
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6.4 Telescopes and Technology
Let’s explore the following topics:
• How can we observe invisible light (radio,
IR, UV, X-rays and Gamma rays)?
• How can multiple telescopes work together
and what is the advantage of multiple
telescopes working together?
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How can we observe invisible light?
• A standard satellite
dish is essentially a
telescope for
observing radio
waves (It is called a
radio telescope)
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Radio Telescopes
• A radio
telescope is
like a giant
mirror that
reflects radio
waves to a
focus.
• The image
shows the 300
meter (1000 ft)
diameter radio
telescope in
Arecibo
(Puerto Rico)
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ALMA
(Atacama Large Millimeter Array)
• The ALMA radio
telescope (Interferometer)
is located in the Atacama
desert (North of Chile) at
an altitude of 5,000 meter
(16.500 feet) .
• It consist of 66 antennas,
each 7 to 12 m diameters
used as an interferometer
to achieve a resolution
about 5 times better than
the Hubble telescope .
•It work in the wavelength
range from 0.3 to 9.6 mm
•Cost: about 1.5 billion
US dollars.
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ALMA
A view of some of the antennas.
In the background, the Magellanic Clouds, the Southern Cross and the Milky Way
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Infrared and Ultraviolet Telescopes
SOFIA (IR)
•
•
•
Spitzer (IR)
Infrared and ultraviolet light telescopes operate like visible-light telescopes but
need to be high or above the atmosphere to “see” at these wavelengths.
A UF Ph.D. graduate, Dr. Jim De Buizer is a scientist with SOFIA observatory.
UV telescopes were carried in Space Shuttle Missions ASTRO 1 and ASTRO 2.
Crew member in charge of the telescope on those missions was the astronaut and
a UF Ph.D. graduate Dr. Ron Parise (deceased)
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X-Ray Telescopes
• X-ray
telescopes
also need to
be above the
atmosphere.
• X-rays are
absorved by
the terrestrial
atmosphere
Chandra X-Ray Observatory (launched in 1999)
Named after the Indian-American astrophysicist Subramanyan Chandrasekhar (Nobel prize in
physics)
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X-Ray Telescopes
• Focusing of X-rays requires special mirrors.
• Mirrors are arranged to focus X-ray photons through
grazing bounces off the surface.
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Gamma-Ray Telescopes
Fermi Gamma-Ray Observatory
Launched in 2008
Named after Enrico Fermi
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• Gamma-ray telescopes
also need to be in
space. Gamma
radiation doesn’t reach
the ground.
• Focusing gamma rays
is extremely difficult.
• For the Fermi
telescope, the detectors
are scintillation
detector, an array of 12
crystals (Sodium
Iodide)
• The detectors are
arranged in a 3-D
matrix. That allow to
determine the direction
from where the gamma
ray came from
How can multiple telescopes
work together?
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Interferometry
• Interferometry is a
technique that consist in
linking two or more
telescopes and
combining the signal
from them.
What is the advantage?
• The resolution obtained
is equivalent to the
resolution of a single
large radio telescope of
an equivalent diameter
equal to the separation.
A couple of examples:
 The ALMA radio
telescope
 The VLA array in
Socorro, New Mexico
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Interferometry
Very Large Array (VLA) in Socorro, New Mexico
Part of the National Radio Astronomical Observatory
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• Easiest to do with radio
telescopes because of
the longer wavelengths
• Now possible with
infrared and visible-light
telescopes
• Interferometry consist of
combining the light (or
signals) from two or
more telescopes.
• In long wavelength
radio telescopes the
signals are transmitted
by coaxial cables or
waveguides.
• In optical, IR and short
wavelength radio
telescopes, the signals is
transmitted by optic
fiber
Interferometry in visible and IR light wavelengths
An example: The ESO (European Southern Observatory) VLT telescopes in Cerro Paranal
(Chile)
The VLT (Very Large Telescopes) are 4 telescopes that can combine the light
to make up a large telescope with an equivalent diameter equal to the
separation between them. The primary mirror of each telescope has a
diameter of 8.2 meters
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Future of Astronomy in Space?
• The Moon would be
an ideal observing
site.
• The lack of
atmosphere make it
ideal for observing at
almost all
wavelengths (radio,
UV,IR, X-Ray and
Gamma-Rays)
• Radio telescopes
located in the far side
of the Moon will be
shielded from radio
interference
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Summary
• How can we observe invisible light?
– Telescopes for invisible light are usually
modified versions of reflecting telescopes.
– Telescopes used for observing invisible light
such as X-ray, Gamma rays, UV and IR must
be in space.
• How can multiple telescopes work together?
– Linking multiple telescopes using the
interferometer technique enables them to
produce the angular resolution of a much larger
telescope.
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