Download Telescopes: Windows to the Universe

Midterm!
In 2 weeks Part I (online exam, Mastering
Astronomy, 50 pts) will be available, due
October 26th, 11:59pm
 In 3 weeks, Part II (in class exam, 50 pts.)

– Taken in 3rd hour (week of 10/22 to 10/25)
– Bring SCANTRON (882 form) and #2 pencil
– Based on “Review Questions” handout,
available soon!

Also: 10 of the 25 extra credit points are
due by October 26th, noon.
Lecture 5b: 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.
Eye
The
pupil is constricted (and lets in less
light) when it is bright and dialates (and
lets in more light) when it is dim
– The lens focuses light to point on the
retina
© Sierra College Astronomy Department
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Lecture 5b: Telescopes: Portals of 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.
© Sierra College Astronomy Department
Lens
Focusing
Demo
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Lecture 5b: Telescopes: Portals of Discovery
Refraction and Image Formation
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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
Reflection
– 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. Web Site
Refraction1
Refraction2
© Sierra College Astronomy Department
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Lecture 5b: Telescopes: Portals of Discovery
Refraction and Image Formation

The amount of refraction is determined by
two factors:
1. 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
2. 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).
© Sierra College Astronomy Department
Demo
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Lecture 5b: Telescopes: Portals of Discovery
Refraction and Telescopes
Dispersion
 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 in water droplets, producing a
rainbow
© Sierra College Astronomy Department
dispersion
6
Lecture 5b: Telescopes: Portals of Discovery
Refraction and Telescopes
Chromatic Aberration (ab-ĕ-RAY-shun)
 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.
© Sierra College Astronomy Department
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Lecture 5b: Telescopes: Portals of Discovery
The Camera and Recording Images
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The camera works like an eye except it
can make a permanent record of what it
sees.
A detector is any device that records
light: flim, digital-imaging chips
A camera has two way of controlling the
amount of light which enters it
Eye
– Aperture or opening size of the camera
(like the pupil of the eye)
– Exposure time which is amount of time
the camera can collect light.
© Sierra College Astronomy Department
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Lecture 5b: Telescopes: Portals of Discovery
Large Optical Telescopes
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Charge-coupled device (CCD) is an
electronic “camera” that serves as a light
detector by emitting electrons when struck
by incoming photons. The chip which
collects the photons is usually divided into
pixels (short for picture elements). The
data collected is formed into images by a
computer.
CCDs have a much wider dynamic range
and greater sensitivity than film cameras
– 90% of photon that strike the chip are recorded
(only 10% of photons on film are recorded)

Also digital images may be manipulated
after they are taken
© Sierra College Astronomy Department
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Lecture 5b: Telescopes: Portals of Discovery
The Refracting Telescope
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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 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.
© Sierra College Astronomy Department
Simple
Telescope
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Lecture 5b: Telescopes: Portals of 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.
Magnifying power:
M = Fobjective / Feyepiece
© Sierra College Astronomy Department
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Lecture 5b: Telescopes: Portals of Discovery
The Powers of a Telescope
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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.
© Sierra College Astronomy Department
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Lecture 5b: Telescopes: Portals of Discovery
The Powers of a Telescope
Light-Gathering Power
 Light-gathering power or Lightcollecting area 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.
© Sierra College Astronomy Department
LGP
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Lecture 5b: Telescopes: Portals of Discovery
The Powers of a Telescope

The angular separation between two
points of light depends on the actual
separation and their distance from us
(See Mathematical Insight 6.1)
Small
angle
360
angular separation = physical separation 
2  distance
If the angles are in arcseconds:
separation
angular separation = 206265  physical
distance
© Sierra College Astronomy Department
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Lecture 5b: Telescopes: Portals of Discovery
The Powers of a Telescope
Resolving Power (Angular Resolution)
 Diffraction is the spreading of light upon passing
the edge of an object. This also depends on the
color of light
 Resolving power (or angular resolution) is the
smallest angular separation detectable with an
instrument. It is a measure of an instrument’s
ability to see detail. This is often called the
diffraction limit of the telescope and is given by
(see Mathematical Insight 6.2):
 wavelength of light 
diffraction limit = 2.510  

diameter
of
telescope
in arcseconds


5
© Sierra College Astronomy Department
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Lecture 5b: Telescopes: Portals of Discovery
The Powers of a Telescope
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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
resolving power of between 1 and ½ arcsec.
Operating above the atmosphere, Hubble
Space Telescope has a resolving power of
0.1 arcsec or better.
Hubble
© Sierra College Astronomy Department
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Lecture 5b: Telescopes: Portals of Discovery
The Reflecting Telescope
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An inwardly curved - or concave 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 mirror placed in front of the
objective mirror to deflect light rays out
to the eyepiece.
© Sierra College Astronomy Department
Concave
Mirror
Focusing
Telescope
designs
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Lecture 5b: Telescopes: Portals of Discovery
Telescope
designs
The Reflecting Telescope
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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 Cassegrain focus reflecting telescope
has a secondary convex mirror that
reflects the light back through a hole in
the center of the primary mirror.
© Sierra College Astronomy Department
See next slide
18
Newtonian
Prime
Schmidt-Cassegrain
Cassegrain
Lecture 5b: Telescopes: Portals of Discovery
The Reflecting Telescope

Reflectors can be made larger (and less
expensively) than refractors because:
1. There are fewer surfaces to grind, polish,
and configure correctly.
2. Reflecting mirrors do not exhibit chromatic
aberration as do lenses.
3. Light does not transmit through a mirror so
imperfections in the glass are not critical.
4. Mirrors can be supported on their backs;
lenses must be supported along their rims
© Sierra College Astronomy Department
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Lecture 5b: Telescopes: Portals of Discovery
What to do with a Telescope?
Comic
So we have a telescope! What do we do
with it?
 Imagary: Most detectors are
monochromatic or are sensitive to a
wavelength regime, but by the use of
filters one can take pictures that only let
in red, green and blue light. Then by
combining these, one can get a color
picture in the end
© Sierra College Astronomy Department
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Lecture 5b: Telescopes: Portals of Discovery
Large Optical Telescopes
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Photometry is the measurement of light
intensity from a source, either the total
intensity or the intensity at each of various
wavelengths. Early photometers were like a
camera’s light meter; modern photometers
use a CCD for greater speed and accuracy.
Timing: one can measure the time it takes
for an object (e.g. star or asteroid) to
brighten and dim and produce a light curve,
a plot of the intensity vs. time.
© Sierra College Astronomy Department
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Lecture 5b: Telescopes: Portals of Discovery
Large Optical Telescopes
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Spectral analysis uses a spectrometer - an
instrument that separates electromagnetic
radiation according to wavelength. A
spectrograph is a visual record of the
spectrum taken by a spectrometer.
A spectrometer uses a diffraction grating a device that uses the wave properties of
EM radiation to diffract and separate the
radiation into its various wavelengths.
© Sierra College Astronomy Department
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Lecture 5b: Telescopes: Portals of Discovery
Large Optical Telescopes
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The best viewing conditions for large
telescopes is on top of mountains in dry,
clear climates and away from artificial
lights (i.e. light pollution). This also
reduces the amount of atmospheric
turbulence.
Active/Adaptive optics is a system that
monitors and changes the shape of a
telescope’s objective to produce the best
image. © Sierra College Astronomy Department
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Lecture 5b: Telescopes: Portals of Discovery
Radio Telescopes
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Radio waves have less intensity than light
rays and have much longer wavelengths
(which leads to less resolution).
To detect radio waves, very large
parabolic dishes are required. However,
the dish surfaces do not have to be as
smooth as glass mirrors because the
longer radio wavelengths diffract more
Windows
than do light rays.
© Sierra College Astronomy Department
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Lecture 5b: Telescopes: Portals of Discovery
Interferometry

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.
© Sierra College Astronomy Department
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Lecture 5b: Telescopes: Portals of Discovery
Interferometry

The Very Large Array (VLA) near
Socorro, New Mexico is the most famous
example of interferometry
– 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
– A radio antenna has been put in space to extend
the resolution further

Optical interferometers have been
constructed, but face far more engineering
challenges. The Very Large Telescope (VLT)
is one example, consists of four 8.2-m
© Sierra College Astronomy Department
telescopes
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The Very Large Array along one Arm
VLBA configuration
Very Large Telescope (VLT)
Lecture 5b: Telescopes: Portals of Discovery
Detecting Other EM Radiation
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Near infrared - 1200 nm to 40,000 nm can be detected from high, dry mountain
tops such as Mauna Kea in Hawaii.
Far infrared - greater than 40,000 nm can be detected from aircraft such as the
SOFIA, NASA’s Airborne Observatory.
Infrared telescopes must be cooled so heat
(IR radiation) from the surroundings does
not mask the signals received from space.
Currently, the Spitzer Space Telescope
studies objects in the infrared
© Sierra College Astronomy Department
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Lecture 5b: Telescopes: Portals of Discovery
Detecting Other EM Radiation
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Ozone is the chief absorber of
wavelengths shorter than about 300 nm.
Ultraviolet telescopes must be located
in space.
Likewise, X-ray telescopes and gammaray telescopes also must be placed
above the atmosphere is orbiting
satellites.
– Chandra is an X-ray telescope currently in
space
© Sierra College Astronomy Department
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Hubble Space Telescope (HST)
Fig.06.35
Lecture 5b: Telescopes: Portals of 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 and is
now functioning at design specifications.
HST will be upgraded and be in service for a few
more years.
The James Webb Space Telescope (JWST) will be
launched in 2012 and will replace HST
The End
© Sierra College Astronomy Department
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