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Telescopes
Astronomy 1-1
Lecture 06-1
Functions
Three basic functions.
Magnification
To increase the size of the object that is being
examined
Resolution
The ability to discriminate fine detail
Light Gathering Power
The ability to collect light
Astronomy 1-1
Lecture 06-2
Types of Optical Telescopes
Two basic types
Refractors
These are telescopes that use a lens to bring the
light to a focus
Reflectors
These are telescopes that use a mirror to bring the
light to a focus
Both type of telescopes can have problems associated with
them
Astronomy 1-1
Lecture 06-3
Refractors
Light is manipulated by lenses to form an image
Astronomy 1-1
Lecture 06-4
Refractor Problems
Chromatic aberration
•Lenses are used to bend the
light to bring it to a focus
•How much the light is bent is
dependent upon the wavelength
•So different colors are focused
at different points
•Problem can be corrected
Light gathering power
•For more light, need wider lenses
•Cost
•Weight
Astronomy 1-1
Lecture 06-5
Reflectors
Mirrors are used to manipulate the light
Astronomy 1-1
Lecture 06-6
Reflector Problems - I
Spherical Aberration
Different parts of a spherical
mirror have slightly different
focal lengths
This causes the image to be
fuzzy because the light is
focused over a range of
positions
The problem is corrected by
varying the curvature of the
mirror as function of position
This makes the mirror parabolic
Astronomy 1-1
Lecture 06-7
Spherical Aberration
Astronomy 1-1
0.5 wavelength
0.25 wavelength
0.125 wavelength
0.1 wavelength
Lecture 06-8
Reflector Problems - II
Coma
Parabolic mirrors will focus all
of the incoming light at one
point
However light that is reflected
off the sides will be distorted
in shape
A spot of light will be distorted
into a tear drop shape
High quality reflecting telescopes use a combination of a
spherical mirror and correcting lenses at the front of the
telescope
Astronomy 1-1
Lecture 06-9
Light Gathering
The larger the telescope - the better the
light gathering
Brightness is proportional to square of
radius of mirror
Astronomy 1-1
Lecture 06-10
Resolving Power
The larger the telescope also improves the
resolving power
The resolving power is the ability to discern two
closely spaced objects.
Astronomy 1-1
Lecture 06-11
Resolving Power
10'
1'
5"
1"
Astronomy 1-1
Lecture 06-12
Transparency of Earth's Atmosphere
Earth's atmosphere is not transparent to all the
radiation incident upon the Earth.
Wavelengths in two primary regions make it
through the atmosphere.
The visible wavelengths, and Radio wavelengths.
Earth based telescopes are therefore limited to
observing in these two bands of radiation.
To see in the other wavelengths requires space
based observation platforms.
Astronomy 1-1
Lecture 06-13
Transparency of Earth's Atmosphere
Astronomy 1-1
Lecture 06-14
Detectors
Image acquisition – no longer photographic
Charge-coupled devices (CCDs) are electronic
devices, can be quickly read out and reset
Astronomy 1-1
Lecture 06-15
Images and Detectors
Image processing by computers can sharpen images
Astronomy 1-1
Lecture 06-16
High-Resolution Astronomy
Air movements blur images
Astronomy 1-1
Lecture 06-17
High-Resolution Astronomy
Solutions:
Put telescopes on mountaintops, especially in
deserts
Put telescopes in space
Astronomy 1-1
Lecture 06-18
High-Resolution Astronomy
Active optics: Control mirrors based on
temperature and orientation
Astronomy 1-1
Lecture 06-19
High-Resolution Astronomy
Adaptive optics: Track atmospheric changes with
laser; adjust mirrors in real time
Astronomy 1-1
Lecture 06-20
High-Resolution
Astronomy
These images show
the improvements
possible with
adaptive optics:
Astronomy 1-1
Lecture 06-21
Radio Astronomy
Radio telescopes
Similar to optical reflecting telescopes
Prime focus
Less sensitive to imperfections (due to longer
wavelength)
Can be made very large
Astronomy 1-1
Lecture 06-22
Radio Astronomy
Astronomy 1-1
Lecture 06-23
Radio Astronomy
Longer wavelength means poor angular resolution
Advantages of radio astronomy
1. Can observe 24 hours a day
2. Clouds, rain, and snow
don’t interfere
3. Observations at an
entirely different frequency;
get totally different
information
Astronomy 1-1
Lecture 06-24
Interferometry
Combine information from several widely spread radio
telescopes as if they came from a single dish
Resolution will be that of dish whose diameter = largest
separation between dishes
Astronomy 1-1
Lecture 06-25
Interferometry
Interferometry involves combining signals from two
receivers; the amount of interference depends on the
direction of the signal
Astronomy 1-1
Lecture 06-26
Interferometry
Can get radio images whose resolution is close to
optical
Interferometry can also be done with visible light
but is much more difficult due to shorter
wavelengths
Astronomy 1-1
Lecture 06-27
Space-Based Astronomy
Infrared radiation can image where visible radiation is
blocked; generally can use optical telescope mirrors
and lenses
Astronomy 1-1
Lecture 06-28
Space-Based Astronomy
Infrared telescopes can also be in space; the image on
the left is from the Infrared Astronomy Satellite
Astronomy 1-1
Lecture 06-29
Space-Based
Astronomy
The Spitzer Space
Telescope, an infrared
telescope, is in orbit
around the Sun. These are
some of its images.
Astronomy 1-1
Lecture 06-30
Space-Based
Astronomy
Ultraviolet observing must
be done in space, as the
atmosphere absorbs almost
all ultraviolet rays.
Astronomy 1-1
Lecture 06-31
Space-Based Astronomy
X-rays and gamma rays will not reflect off mirrors as
other wavelengths do; need new techniques
X-rays will reflect at a very shallow angle and can
therefore be focused
Astronomy 1-1
Lecture 06-32
Space-Based Astronomy
X-ray image of
supernova remnant
Astronomy 1-1
Lecture 06-33
Space-Based Astronomy
Gamma rays cannot be focused at all; images are
therefore coarse
Astronomy 1-1
Lecture 06-34
Full-Spectrum Coverage
Much can be learned from observing the same
astronomical object at many wavelengths. Here, the
Milky Way:
Astronomy 1-1
Lecture 06-35