Transcript Astronomical Instruments

Astronomical Instruments
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Early Telescopes
Ancient cultures used special sites,
observatories, to observe the sky
At these observatories, they were able to
measure the position of celestial objects
visible with the naked eye
Telescopes were first used by Galileo Galilei
and are a relatively recent
addition to the tools used by
astronomers
Their use however completely
revolutionized our views and
understanding of the Universe
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Galileo’s Telescopes
Galileo first used a telescope
to observe the sky in 1610
His telescopes were simple
tubes held by hand
They were also small in comparison to
the telescopes in use today
The use of these small telescopes
allowed Galileo to revolutionize the
field of astronomy (and get into a lot
of trouble with church authorities)
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Why Use Telescopes
to Observe the Sky?
Celestial objects, such as stars, planets, and
galaxies, emit light in all directions
Only a minuscule fraction emitted (or
reflected) by celestial objects is perceived by
the human eye, with its tiny opening
Telescopes enable astronomers to gather the
light emitted by a distant object over a much
larger surface
The mount of light gathered by a telescope is
much, much larger than what can be gathered by
a naked eye
Also, they provide for much higher quality images
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Telescopes of all kinds…
Stars and other celestial objects emit all
wavelengths of radiations, not only visible
light
Nowadays telescopes enable astronomers
to observe electromagnetic radiations that
are not visible such as radio-waves,
infrared, ultraviolet, x-rays, and even
gamma rays
Telescopes enable us
to see more light
to see waves that one could not otherwise
perceive with our human senses
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Aperture
The light-gathering ability of a
telescope is determined by its
aperture
The aperture of a telescopes
corresponds to the width of its
primary lens or reflector
Basically, the larger the aperture of a
telescope, the larger its ability to
gather light
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Need for Images
The study of astronomical objects
requires the formation of their images
Each of the images can be
looked at directly with the naked eye
imprinted on a photographic film
detected and recorded with various lightsensitive devices
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Telescope Images in History
Before the 20th century, telescope “images”
were simply looked at with the naked eye
This is a rather inefficient and unreliable way of
gathering/collecting and preserving the
information
Nowadays astronomers actually rarely look
through the larger telescopes
Images are recorded electronically on computers
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Formation of Image by Lens or Mirror
A lens is a transparent piece of
material that bends parallel rays of
light passing through it - bringing
them to a focus or focal point.
Focal length
Parallel light rays
focus
lens
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Eyepiece
Telescopes use a combination of lenses and
mirrors to produce images
An image formed by
the primary lens of
a telescope can be
viewed by using a
second smaller lens
called an eyepiece
Nowadays, the
eyepiece of a
telescope
is usually
replaced by a
camera or electronic light detector
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Magnification
The eyepiece can magnify the image
Stars are typically so distant that they
appear as points of light, and so
magnification does not do much
Planets are however much closer and
galaxies much bigger than stars so that
magnification is actually quite useful to
see the shape and structure of planets
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Concave Mirrors
Rays can also be focused to form an image with a
concave mirror, which
Has a curved surface where the light is reflected has a
parabolic shape
Has a surface coated with a thin layer of metal to make if
light reflecting
Reflects incoming rays parallel through its focus
Thus images are produced by a mirror exactly as
they are by a lens
Focal length
Parallel light rays
focus
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Concave mirror
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Telescope Types
Refracting telescopes
Reflecting telescopes
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Refracting Telescopes
They use a lens as the primary light
gathering device
Galileo's telescopes were all refractors
Binoculars and opera glasses are refractors
Refractors not good for most astronomical
applications
Difficult to support large telescope structure
Quite difficult to make a large piece of glass
with the exact right surface shape to reflect
light without distortions
Not much in used by modern astronomers
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Reflecting Telescopes
Conceived by James Gregory in 1663
First successful model was built by Newton in
1668
Concave mirror placed at the bottom of a
tube or open framework
The mirror reflects the light back up the tube
to form an image near the front end at a
location called the prime focus
Images can be observed at the prime focus
directly or via various systems of auxiliary
mirrors and lenses to bring it to a more
convenient location
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Reflecting Telescope Variations
• Newtonian mode
– A small secondary mirror
is angled to reflect the
light off to the side of the
telescope tube
Parallel light rays
• Cassegrain mode
– The secondary mirror
reflects the light back
down the tube, through
the primary mirror, and to
a focus below the
telescope tube
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Parallel light rays
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Brightness
A measure of the amount of light that
is concentrated in a unit area
Depends on the amount of light
focused by the primary mirror
Doubling the aperture of the primary
mirror
Increases its area by a factor of four (4)
Results in images that are 4 times
brighter
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Large-Aperture
Telescopes
Two Keck telescopes
at Mauna Kea, Hawaii,
both have apertures of 10 m
33 feet long
twice the aperture of the
older 5-m Hale telescope
at Mt. Palomar
have images four times
brighter than those of
the Hale telescope
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Palomar
Keck
Keck
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Resolution
Resolution refers to the fineness of
detail present in the image
Astronomers seek to get as many
details about the object they observe
as possible
The larger the telescope aperture, the
sharper the image will be
The resolution of images is however
also limited by external factors
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Important Factor Limiting Resolution
Fluctuations of the Earth's atmosphere above the
telescope introduce a blurring of the images
produced by a telescope
Important to place telescopes at high altitudes
where such distortions are minimized
Distortions observable with the naked eye
twinkling of the stars
Telescopes mounted in outer space, above the
Earth's atmosphere do not have this twinkle
light of the stars is steady
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Resolution Measurement Units
The resolution of an astronomical
image is measured by the angular size
of a point source such as a star
This size is expressed in seconds of
arc, or arcsec
One arcsec is 1/3600 degree
One arcsec is how a quarter would
look like seen from a distance of 5 km
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Telescope Fabrication (1)
Purpose: collect light
from very faint sources
Use as large an aperture
as achievable
KeckFabricated out of one or
multiple large disk or piece of glass
Keck
Glass is ground and polished to produce a
concave mirror of high optical quality
Shape is critical
All parallel rays, no matter where they hit the
mirror, must be reflected into a single focus
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Telescope Fabrication (2)
Primary mirror mounted in a support
structure
Strong, complex, and steady support
structure to provide
a steady support
ability to move the
telescope rapidly
yet accurately to
any desired
direction in the sky
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5-m Hale telescope at Mt. Palomar
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Telescope Fabrication (3)
Since Earth is constantly rotating
Use motorized system
Move telescope backward at the exact same rate
the Earth is moving so that it continuously
points to the designated direction
Sophisticated instruments needed
to analyze and record the light collected at the
telescope's focus
All machinery housed
in a dome to protect
it from weather
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Telescope Usages
Astronomers use large telescopes in
three basic ways:
Imaging
Consists in photographing or recording the
appearance of a small portion of the sky
Brightness and color measurements
Consist in a determination of the intensity and
the dominant color of the light received from an
object
Spectroscopy
Consists in a measurement of the spectra of
astronomical sources
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Photographic and Electronic
Detectors
Photographic films or plates used throughout most of the
20th century for direct imaging and for spectroscopy
In a plate, a light sensitive chemical coating is applied to
a piece of glass which when developed provides a lasting
record of the image
Although photographic films represent a large
improvement over the human eye, they still present
serious limitations
films are inefficient: only about one per cent of the light
incident on the plates contributes to an image, the rest is
wasted
Astronomers now use much more efficient electronic
detectors to record images
These are most often charge-coupled devices or CCDs
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CCDs
The photons of light incident on a CCD generate a
stream of charged particles (electrons)
The electrons are accumulated (stored) and
counted at the end of the exposures
CCDs record as much as 60 to 70 percent of the
photons that strike them : this allow to detect
objects that are more than 60 times fainter (with a
given exposure time) than would be possible with
films
It also enables more accurate measurement of
the brightness of the objects
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Infrared Observations
Pose many additional challenges
The infrared extends from wavelength
near 1 micrometer out to 100 mm or
longer
Infrared radiation is basically heat
radiation
The human body emits heat in the
infrared range
A big challenge to observe! Why?
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The Infrared Challenge
Typical temperature near the Earth's
surface ~ 300 kelvin
According to Wien's law, the telescope, the
observatory, and surrounding sky are
radiating infrared energy with a peak
wavelength of about 10 micrometers
The challenge is to detect/distinguish faint
cosmic sources from this background
It is like trying to observe the star (in the
visible spectrum) during day time
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Infrared Observations
Astronomers must protect the infrared
detector from nearby sources of radiation,
just as one would shield photographic films
from exposure to ambient bright day light
The detector must be isolated in cold
surroundings and are often held near
absolute zero temperature (1-3 kelvin) by
immersing them in liquid helium
Astronomers also attempt to reduce the
radiation emitted by the telescope structure
and optics and block their radiation
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Spectroscopy
Spectroscopy is one of the most powerful
techniques used in astronomy
More than half of the observation time at
large observatories is used for
spectroscopy
The many wavelength present in light can
be separated by passing it through a prism
to form a spectrum
A spectrometer is an instrument designed
to produce and record such a spectrum
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Principle of the Spectrometer
Light from the source (or its image) enters the instrument
through a small hole or narrow slit and is collimated by a
lens
Light passes through a prism and produces a spectrum
Different wavelengths directed in different directions
A second lens placed behind the prism to focus the many
different images of the slits of a CCD
Nowadays, prism replaced by diffraction grating
A grating is a piece of transparent material with thousands of
grooves in its surface
The grooves cause the light waves to interfere with each other with
the result that the light also spreads out in to a spectrum
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A Prism Spectometer
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A Simple Spectrometer
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Factors Considered When Choosing
Observation Site
Weather
Cloud, rain, etc
Best sites have clear weather as much as 75% of the time
Atmosphere
Dust & Humidity
Atmosphere filters out a fraction of starlight
Absorption in infrared due principally to humidity
Prefer dry sites, at high altitudes
Dark Sky - Light Pollution
Near cities, the air scatters the glare from lights producing an
illumination that hides the faintest stars
Unsteady air - Bad Seeing
Light passing turbulent air is disturbed
Produces images distortions
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Funding
Telescopes built during the 1st half of
the 20th century were funded by
private donations and were essentially
available to but a few astronomers
In recent years, the National Science
Foundation (NSF) provides funds to
support astronomy research – built
various facilities around the world in
collaboration with other countries
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Radio Telescopes
Radio emission was discovered by Karl G.
Jansky, an engineer of the Bell Telephone
Laboratories, in 1931
Grote Reber, built in 1936, the first amateur
radio ham, using galvanized iron and wood
He was able to receive cosmic radio waves
Over the years, he built several antennas
and conducted a pioneering survey of the
sky for celestial radio sources
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Radio Waves
In commercial radio broadcasting, sound
information is encoded at the source and
decoded at the receiving ends - the
listener's radios where they are played into
head phones or speakers
Radio waves from space do not contain
music or other types of human information
The waves nonetheless carry some
elementary information about the chemistry
and physical conditions of their source
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Radio Astronomy
Radio waves can produce an electric current in
conductors (such as metals)
An antenna is such a conductor
It intercepts the path of waves which in turn induce a
small current in it
The current is then amplified in a radio receiver and
recorded
Receivers can be used similarly to TVs or
Radios, that is by tuning on single frequency
An astronomical radio telescope provide radio
spectra, giving information about how much
radiation we receive at each wavelength or
frequency
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Radio Telescopes
A radio-reflecting telescope
consists of a concave metal
reflector, called a dish, quite
analogous to an optical
telescope mirror
The radio waves collected by
the dish are reflected to the
focal point of the reflector
where a receiver detects the
waves and record them
Astronomers often construct a pictorial
representation of the radio sources they observe in
order to more easily communicate and visualize
their data
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A Radio Image
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Green Bank Telescope
World's largest fully
steerable radio telescope
under construction at the
National Radio Astronomy
Observatory's site in Green
Bank, Pocahontas County,
West Virginia
Dimensions of the surface
are 100 by 110 meters
The overall structure of the
GBT is a wheel-and-track
design that allows the
telescope to view the entire
sky above 5 degrees
elevation
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Arecibo Telescope (1)
It consists of a 305m
fixed reflecting surface,
made up of 40,000
individual panels
Suspended in a natural limestone sinkhole in
northwestern Puerto Rico
Incoming rays are reflected back from the
surface to two additional reflectors located
450 feet above on the "platform", a 500 ton
structure supported by cables from three
towers
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Arecibo Telescope (2)
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Radio Interferometry (1)
A telescope resolution depends primarily on its
aperture
It also depends on the wavelength of the wave
being detected
The longer the waves, the harder it becomes to detect fine
details.
Radio waves have very large wavelength
Substantial challenges for astronomers who need good
resolution
The largest radio dishes cannot have poorer
resolution than small optical telescopes
To overcome this difficulty, astronomers have
learned to link two or more radio telescopes
together electronically, and succeed in greatly
sharpening the images they get
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Radio Interferometry (2)
An array of
telescopes
linked together
in this way is
called an
interferometer
The word indicates that these devices
operate via a measurement of the degree of
interference between different waves
Interference is a technical term for the way waves
that arrive in a detector at slightly different times
interact with each other
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Interferometer Resolution
The resolution of an interferometer depends
on the separation of the telescopes - not on
their individual apertures
Two small telescopes separated by 1 km provide
the same resolution as would a single dish 1 km
wide
However they clearly cannot gather the same
quantity of waves
Even better resolution can be achieved by
combining more than two reflectors into an
interferometer array
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VLA
The most extensive
such instrument is
the National Radio
Astronomical
Observatory's Very
Large Array (VLA)
near Socorro, New Mexico
It consists of 27 movable radio telescopes,
each having an aperture of 25 m, spread over
a total span of about 36 km
The telescopes signals are combined
electronically and permit astronomers to obtain
pictures of the sky with resolution comparable
to those obtained with an optical telescope with
a resolution of about 1 arc second
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Observations Outside Earth's Atmosphere
Earth's atmosphere blocks most radiation at
wavelength shorter than visible light
It is thus possible to make astronomical observation at
these wavelength only from space
Getting above the disturbing effects of the
atmosphere is also a great advantage at visible
and infrared wavelengths
Since stars do not twinkle in empty space, the
resolution is far superior than that on Earth
The resolution thus becomes solely limited by
the size and quality of the instrument used to
collect the light
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Hubble Space Telescope (1)
Launched in April 1990
Permitted a giant leap
forward in astronomy
It has an aperture of
2.4 m
Largest telescope put
in space to date
Aperture limited by the payload of the space
shuttle, used to put it in orbit
Named after Edwin Hubble, the astronomer
who discovered the expansion of the Universe
in the 1920s
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Hubble Space Telescope (2)
Operated jointly by NASA Goddard SpaceFlight Center and the Space Telescope
Science Institute in Baltimore
First orbiting observatory designed to be
serviced by shuttle astronauts
Visits by astronauts in
1993, 1997, and 1999
allowed improvements
and replacements of
initial instruments
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Chandra X-Ray Telescope
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