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TELESCOPES
Portals of Discovery
© Sierra College Astronomy Department
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Telescopes: Portals of Discovery
The Eye: The Everyday Light Sensor

The eye is made of a lens, pupil,
and a retina
– The pupil allows a certain amount of
light to enter the eye.
•
The pupil is constricted (and lets in less
light) when it is bright and dilates (and
lets in more light) when it is dim
– The lens focuses light to point on the
retina
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Telescopes: Portals to Discovery
Reflection and Refraction
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Light travels in a straight line as long as it
remains in the same medium (i.e., the
material that transmits light).
Reflection is the redirecting of light off a
surface
– Incident angle = reflection angle

Refraction is the bending of light as it
crosses the boundary between two
materials in which it travels at different
speeds.
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Telescopes: Portals to Discovery
Reflection and Refraction

The amount of refraction is determined by
two factors:
– Relative speeds of light in the two materials
(e.g., air and glass)
•
The ratio of the speed light in vacuum to its speed in
some material is called the index of refraction
– The angle between rays of light and the
surface (i.e., the smaller the angle between the
ray of light and the surface, the more the light
bends on passing through the surface).
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Telescopes: Portals to Discovery
Reflection and Refraction
Different colored light beams refract at
slightly different angles.
 Dispersion is the separation of light
into its various wavelengths upon
refraction (it’s what a prism does).
 This effect can seen when light is
refracted (and reflected) in water
droplets, producing a rainbow

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Telescopes: Portals to Discovery
Refraction and Image Formation
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Focal point (of a converging lens or mirror)
is the point at which light from a very
distant object converges after being
refracted or reflected.
Focal length (F) is the distance from the
center of a lens or a mirror to its focal point.
Image is the visual counterpoint of an
object, formed by refraction or reflection of
light from the object.
Focal plane is where the image focuses
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Telescopes: Portals to Discovery
Refraction and Image Formation
While dispersion can be useful in
examining the colors coming from an
object, it also introduces an inherent
problem
 Chromatic aberration is a defect of
optical systems that results in light of
different colors being focused at
different places. The resulting image
will be fuzzy at the edges.

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Telescopes: Portals to Discovery
Basic Refracting Telescopes

The objective is the main light-gathering
element - lens or mirror - of a telescope. It
is also called the primary. It is
characterized by its diameter (D).
– An example of a basic refracting telescope in
nature: the human eye

An eyepiece (which may be a combination
of lenses) added just beyond the focal
point of the telescope’s objective acts as a
magnifier to enlarge the image.
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Telescopes: Portals to Discovery
The Powers of a Telescope
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Angular size of an object is the angle
between two lines drawn from the viewer to
opposite sides of the object.
Magnifying power (or magnification) is the
ratio of the angular size of an object when it
is seen through the instrument to its angular
size when seen with the naked eye.
M  Fobjective Feyepiece
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Telescopes: Portals to Discovery
The Powers of a Telescope
Long focal length eyepieces (>25 mm)
produce less magnification; short
focal length eyepieces (<10 mm)
produce more magnification.
 Field of view is the actual angular
width of the scene viewed by an
optical instrument.

– As magnification increases the field of
view decreases.
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Telescopes: Portals to Discovery
The Powers of a Telescope
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Light-gathering power is a measure of the
amount of light collected by an optical
instrument (the area of the objective lens or
mirror).
Light-gathering power is related to the size
of the objective, which is usually given as a
diameter. [Remember that the area of a
circle is proportional to the (diameter)²].
2
o
2
o
D
L.G.P. 
d
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Telescopes: Portals to Discovery
The Powers of a Telescope
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Diffraction is the spreading of light upon passing the
edge of an object. This also depends on the color of
light
Resolving power (or resolution or diffraction limit) is
the smallest angular separation detectable with an
instrument. It is a measure of an instrument’s ability to
see detail
 wavelength of light 

 diameter of telescope 
Resolving power or diffraction limit = 250,000  
in arcseconds
Resolving Power (or diffraction limit) is in seconds of arc and l and
D must be in the same units
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Telescopes: Portals to Discovery
The Powers of a Telescope
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Resolution Details
– The best human eyes can resolve is about 1 arcminute or
1/60 of a degree.
– A 15-cm (6-inch) telescope has a maximum resolving power of 1
arcsec or 1/3600°.
– Because of atmospheric turbulence (which causes the stars to
twinkle), even the largest Earth-based telescopes have a
practical resolution of between 1 and ½ arcsec.
– Operating above the atmosphere, Hubble Space Telescope has
a resolving power of 0.1 arcsec or better.

Two distant objects can resolved if the telescope’s
resolution (may not be the diffraction limit) is smaller
than the angular separation of the two objects.
– Recall that angular separation is determined by the small
angle formula:
angular separation = physical separation x
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2 x distance
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Telescopes: Portals to Discovery
Basic Reflecting Telescopes
An inwardly curved - or concave –
primary mirror can bring incoming
light rays to a focus and is used to
construct reflecting telescopes.
 The reflecting telescope was invented
by Isaac Newton, who also used a
small flat secondary mirror placed in
front of the objective mirror to deflect
light rays out to the eyepiece.
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Telescopes: Portals to Discovery
Basic Reflecting Telescopes
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A Cassegrain focus reflecting telescope has a
secondary convex mirror that reflects the light
back through a hole in the center of the primary
mirror.
A Newtonian focus reflecting telescope has a
plane mirror mounted along the axis of the
telescope so that the mirror intercepts the light
from the objective mirror and reflects it to the side.
A Nasmyth/Coude focus reflecting telescope uses
a third mirror to reflect light out the side but lower
down than the Newtonian design
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Telescopes: Portals to Discovery
Reflectors vs Refractors
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Reflectors can be made larger (and less
expensively) than refractors because:
– There are fewer surfaces to grind, polish, and
configure correctly
– Reflecting mirrors do not exhibit chromatic
aberration as do lenses
– Light does not transmit through a mirror so
imperfections in the glass are not critical
– Mirrors can be supported on their backs;
lenses must be supported along their rims
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Lecture 6: Telescopes: Portals of Discovery
What to do with a Telescope?
So we have a telescope! What do we do
with it?
 Most telescopes are set up to do imagery
 We must consider the location of
telescope
 Telescopes may also do timing and
spectroscopy
 Telescopes not restricted to the optical
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Telescopes: Portals to Discovery
Techniques and Instrumentation
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Imaging
– Camera with photographic film or plates.
•
Keys: aperture (area of lens or mirror) and exposure time
– Charge-coupled device (CCD)
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•
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Electronic “camera” with an array of pixels emit electrons
when struck by incoming photons.
The data collected is formed into images by a computer.
Advantages over cameras
– More sensitive to light
– Wider dynamic range
– Image can be manipulated via image processing (can be used
to create “false-color” images from non-visible observations)
– Filters for selecting desired frequencies
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Telescopes: Portals to Discovery
Large Optical Telescopes
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Timing
– A light curve is a plot of an object’s intensity
(over some wavelength range) with respect to
time.
– Photometry is the practice of creating these
light curves.
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Early photometers were like a camera’s light meter
Modern photometers use a CCD for greater speed
and accuracy
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Telescopes: Portals to Discovery
Large Optical Telescopes
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Spectroscopy
– A spectrometer or spectrograph is a device
that uses a diffraction grating (or other devices)
to separate light into its various wavelengths
for recording by some detector (e.g., a CCD).
– The recorded spectrum from a spectrometer is
also know as a spectrograph or spectrogram.
– The amount of information that can be gleaned
from a spectrogram is dependent on the
spectrometer’s spectral resolution (the higher
the resolution, the more detail can be seen).
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Telescopes: Portals to Discovery
Observation Conditions
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Considerations for Ground-Based Observations
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Daylight and weather
Light pollution
Twinkling and atmospheric turbulence
Limited wavelength transmission
Solutions
– Place telescopes on top of mountains in dry, clear climates,
and away from artificial lights.
– Active/Adaptive optics is a system that monitors and changes
the shape of a telescope’s secondary (or even a third or
fourth) mirror to produce the best image.
– Place telescopes in space
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Telescopes: Portals to Discovery
Radio Telescopes
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Radio waves
– have less intensity than visible light and have much longer
wavelengths (which leads to less resolution).
– are the only other wavelengths (besides the visible band)
that can be observed from the Earth’s surface.
– are not prone to atmospheric distortion, but do compete
against man-made radio pollution.

To detect radio waves with meaningful resolution
– very large parabolic dishes are required.
• The largest radio telescope, the Arecibo radio dish,
stretches 305 meters, but only has a resolution of about 1
arcminute at its commonly observed wavelength of 21 cm.
– several radio telescopes are linked together in an array.
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Telescopes: Portals to Discovery
Interferometry
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Interferometry is a procedure that allows a
number of telescopes to be used as one by
taking into account the time at which
individual waves from an object strike each
telescope.
– Interferometry is possible because extremely
accurate atomic clocks allow for precise timing
between radio telescopes.
– Interferometry increases the resolution of the
resulting image because the size of the objective is
effectively the size of the furthest separated dishes.
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Telescopes: Portals to Discovery
Interferometry
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Interferometry is a well established technique in radio
astronomy
– The Very Large Array (VLA) near Socorro, New Mexico, is
the most famous example
•
There are twenty-seven 25-m dishes which can be
separated by as much as 40 km.
– The Very Large Baseline Array (VLBA) comprise of eleven
25-m dishes spread across the United States
•
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A radio antenna has been put in space to extend the resolution
further
For smaller wavelengths, timing the signals is very
difficult, but there has been some success in the
infrared and visible regions (and there plans to extend
the technique into the X-ray band).
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Telescopes: Portals to Discovery
Detecting Other EM Radiation
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Infrared Telescopes
– These telescopes collect and focus infrared much the same
way as the visible light telescopes, with the practical limitation
of the Earth’s atmosphere.
•
Some observations can be made on the ground through limited
“infrared windows” in the atmosphere
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High and dry mountain tops allows for a broader range of infrared
signals to be detected (e.g., Mauna Kea in Hawaii)
•
Generally, infrared observations are best done high in the
atmosphere (e.g., from aircraft such as the SOFIA [Stratospheric
Observatory for Infrared Astronomy], NASA’s Airborne Observatory)
and in space (e.g., IRAS [Infrared Astronomical Spacecraft] and
the Spitzer Space Telescope even the Hubble Space Telescope)
– Infrared telescopes must be cooled so heat (IR radiation) from
the surroundings does not mask the signals received from
space.
– There is also some specialization with respect to the near-,
mid-, and far-infrared regions.
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Telescopes: Portals to Discovery
Detecting Other EM Radiation
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Ultraviolet Telescopes
– Like infrared telescopes, these telescopes
collect and focus most UV the same way as
the visible light telescopes.
– Ozone is the chief absorber of wavelengths
shorter than about 300 nm. Ultraviolet
telescopes must be located in space.
•
Three major UV telescopes: FUSE (Far
Ultraviolet Spectroscopic Explorer), GALEX
(Galaxy Evolution Explorer), and the Hubble
Space Telescope
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Telescopes: Portals to Discovery
Detecting Other EM Radiation
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X-Ray and Gamma-Ray Telescopes
– Like UV telescopes, X-ray telescopes and gamma-ray
telescopes also must be placed above the atmosphere is
orbiting satellites.
– X-Rays and gamma-rays penetrate many materials
and make focusing them a real challenge.
•
X-ray telescopes employ a grazing incidence technology
– NASA’s Chandra X-Ray Observatory consists of several nested
grazing incidence mirrors and offers the best in angular
resolution
– The European XMM-Newton telescope has a larger collecting
area
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Gamma-ray telescopes utilize massive detectors so that the
photons do not simply pass through the instrument
– 17-ton Compton Gamma Ray Observatory (1991-2000)
– NASA’s Swift launched in 2004 with source direction capabilities
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Telescopes: Portals to Discovery
The Hubble Space Telescope (HST)
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HST is designed to observe across the spectrum
from infrared to ultraviolet.
HST sees light before it encounters the
atmospheric turbulence of the Earth
HST underwent successful repair in 1993,
functioned at design specifications for many
years, but lack of service from the Shuttle may
doom it in the near-future.
The Next Generation Space Telescope (NGST)
now called the James Webb Space Telescope
(JWST) with a planned launched in 2018.
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Telescopes: Portals to Discovery
Looking Beyond Light
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Other “cosmic messengers” to detect
– Neutrinos
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Extremely light-weight and neutral atomic particles associated with
stellar nuclear reactions
“Neutrino telescopes” typically located in deep mines or under water
or ice
– Cosmic rays
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Very high-energy subatomic particles with uncertain origin and
composition
Satellites and ground-based detectors are employed
– Gravitational waves
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Oscillations in spacetime predicted by Einstein’s General Theory
of Relativity
Expected to be produced by exotic objects like orbiting neutron
stars and black holes
Detectors are up and running in Washington, Louisiana, Italy,
and Germany
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