Transcript Day-22

Astronomy 1010-H
Planetary Astronomy
Fall_2015
Day-22
Course Announcements
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How is the sunset/sunrise observing going?
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SW-chapter 6 posted: due Fri. Oct. 23
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Exam-2 will be returned on Friday
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1st Quarter Observing night: Tuesday, Oct. 20; 7:30pm
MATH TOOLS 6.1
 The light-gathering power of a telescope is
proportional to the square of the aperture
size.
 A telescope’s magnification depends on the
focal lengths of the objective lens or mirror
and the eyepiece.
 Focal length: distance between lens and the
image (longer = larger image).
 Aperture sets the light-collecting power.
 Focal length determines the image size.
 Reflectors have
advantages over
refractors.
 No chromatic aberration.
 Bigger telescopes due to
increased focal length in
the same amount of
physical space and no
need for massive lenses.
 The largest telescopes in
the world are reflectors.
Concept Quiz—Bigger Telescopes
Why do astronomers want to build bigger telescopes?
A. to eliminate the effects of astronomical seeing
B. to search for life on Mars
C. to observe fainter or more distant objects
 Resolution = smallest details that can be
separated.
 The longer the focal length, the better the
separation of two objects or features.
 Diffraction, or blurring of an image, sets
the best possible resolution.
 The diffraction limit depends on the ratio
of wavelength-to-telescope aperture.
The resolution of a telescope
depends on its size and the
wavelength of the light
Better resolution means smaller
angle 
  2.06 10
5

D
The atmosphere limits the
resolving power of a
ground-based telescope
Adaptive Optics can clear
up most of the distortions
caused by the atmosphere
The
distortions
are caused by
differences in
the air above
the telescope
MATH TOOLS 6.2
 The ultimate resolution of a telescope is set
by the diffraction limit.
 The angle subtended by the smallest
resolution, θ, is determined by the ratio of
the wavelength of light being studied to the
aperture diameter.
 1 arcsecond = 1/3,600 of a degree.
 Human eye:
 Earth’s atmosphere
degrades images.
 Astronomical seeing =
limit on resolution due
to the atmosphere.
 Space-based
telescopes do not have
this problem.
Turbulence
 Differences in the temperature and
density of small portions of Earth’s
atmosphere cause passing starlight to
quickly change direction, making stars
appear to twinkle.
Once a site with good “seeing” is
found everyone wants to use it
Kitt Peak, Arizona
Mauna Kea, Hawaii
Earth’s atmosphere hinders
astronomical research
Image of stars taken with
telescope on the Earth’s
surface
Same picture taken with Hubble
Space Telescope high above
Earth’s blurring atmosphere
Adaptive & Active optics
Adaptive optical systems
can overcome atmospheric
distortion
Active = quasi-static; 1 or
2 changes per minute.
Adaptive = rapid changes;
tens to hundreds of times
per minute
 Adaptive optics can help correct for this
atmospheric distortion.
 Earth-based image quality can compete with
the Hubble Space Telescope in the visible.
 Photography opened
the door to modern
astronomy.
 Captured images on
photographic plates.
 Increased integration
time comes with
longer exposures.
 Expensive, slow, and
messy.
CCD’s are much more sensitive
than photographic film