Announcements Wed, Feb 15

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AnnouncementsWed, Feb 15

• Homework 5
• Tutorial Make sure you click Q, quit and submit,
  submit problem. (Demo)
• Numerical question no commas or units e.g.
  11000 not 11,000 km. (Demo)
• PRS
• Problem was RF interference (turn off cell
  phones)
• Everyone who registered received 6 EC pts
• New hub with fix will be installed, probably
  Friday.
• Will try EC questions today
• Tentative PRS quiz next Wednesday

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A lens creates an extended image of an extended
object.lenses reverse images
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Three main functions of a telescope

• brighten
• called light gathering power
• proportional to the diameter of the objective
  lens. (pr2)

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Light gathering power Comparing two telescopes

• Effective collecting area area (d telescope
  diameter
• Compare two telescopes Hubble (d 2.4m) and
  Keck (d 10m)

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Q. Two telescopes have 1m and 4m diameters. The
larger telescope has

• 2x light gathering power
• 4x light gathering power
• 8x light gathering power
• 16x light gathering power
• Same light gathering power depends only on ratio
  of focal lengths

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Three main functions of a telescope

• Brighten faint objects
• called light gathering power
• directly related to the diameter of the objective
  lens.
• See fine detail
• called angular resolution

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Poor and Great Angular Resolution
Telescope images are degraded by the blurring
effects of the atmosphere and by light pollution
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Computing the angular resolution of a telescope
(ignores blurring of atmosphere)
where ?, d are in same units (e.g. meters) and ?
is in arcseconds
Example HST, yellow light (? 550 nm, 1nm 10-9
m)
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Seeing Through the Atmosphere

• Earths atmosphere causes problems for
  astronomers on the ground.
• Bad weather makes it impossible to observe the
  night sky.
• Air turbulence in the atmosphere distorts light.
• That is why the stars appear to twinkle.
• Angular resolution is degraded.
• Man-made light is reflected by the atmosphere,
  thus making the night sky brighter.
• this is called light pollution

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Atmospheric Blurring
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Lunar images every 1/30 sec
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Adaptive Optics (AO)

• It is possible to de-twinkle a star.
• The wavefronts of a stars light rays are
  deformed by the atmosphere.
• By monitoring the distortions of the light from a
  nearby bright star (or a laser)
• a computer can deform the secondary mirror in the
  opposite way.
• the wavefronts, when reflected, are restored to
  their original state.

• Angular resolution improves.
• These two stars are separated by 0.38?
• Without AO, we see only one star.

AO mirror off
AO mirror on
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Using AO at Keck ObservatoryUsed to discover
Gabrielle, moon of Xena (tentatively, the 10th
planet)
Xena
Gabrielle
Where did the names come form?
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Three main functions of a telescope

• brighten
• called light gathering power
• directly related to the diameter of the objective
  lens.
• see fine detail
• called angular resolution
• and least important,
• magnify
• magnification (objective lens focal length /
  eyepiece lens focal length).

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Refracting telescopes have drawbacks

• Spherical aberration
• Chromatic aberration
• Lens is achromatic if it bends light at same
  angle independent of wavelength
• Expense! Very difficult to make large achromatic
  lenses
• All large ( gt1 m diameter) telescopes are
  reflectors, not refractors)

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Special achromatic compound lenses and lens
coatings can often fix this aberration.
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Refractor Galileos original telescopes (Museo
di Storia del Scienza in Florence)
Where you may also view Galileos middle finger.
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Refracting telescopes have drawbacks

• Spherical aberration
• Chromatic aberration

• Sagging due to gravity distorting the lens
• Unwanted refractions
• opaque to certain wavelengths of light

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A reflecting telescope uses a mirror to
concentrate incoming light at a focus.
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The secondary mirror in the tube does not cause a
hole in the image. It does however make it ever
so slightly dimmer because it reduces the total
amount of light reaching the primary mirror.
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• Drawback of Using Spherical Mirrors in Reflecting
  Telescope
• Spherical Aberration
• (can be corrected with a correcting lens)

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The largest research telescopes in the world are
ALL reflectors. The Keck I telescope on Mauna
Kea on the Big Island of Hawaii uses 36 hexagonal
mirrors to make a total diameter of 10 m. (Note
the astronomers standing on either side of the
platform.)
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University of Iowas Rigel Telescope in Arizona
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Reflector Iowa Robotic Telescope (near Tucson
Arizona)
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An electronic device called a CCD (charge coupled
device) is commonly used to record the image at a
telescopes focus

• A Charge-Coupled Device (CCD)

Hubbles ACS camera (CCD)
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Inside a digital camera (same CCD technology as
astronomers use)
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Ordinary Photographs vs. CCDs
CCD sensors are 100x more sensitive to light
than photographic film
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Spectrographs record the spectra of astronomical
objects.
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Spectrographs record the spectra of astronomical
objects.
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Spectrum of Vega (Rigel Telescope)
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Q. A drawback of simple refractor telescopes
compared with a reflector of the same size is

• Spherical aberration
• Chromatic aberration
• Reduced light gathering power
• Worse angular resolution
• Less ability to detect faint objects

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Q. Compared with the best photographic film, the
most important advantage of CCD sensors for
astronomy is

• Less expensive
• Better angular resolution
• Less chromatic aberration
• More collecting area
• More sensitive to light

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Nonvisible light (Photons)

• Most light is invisible to the human eye.
• Special detectors/receivers can record such
  light.
• Digital images are reconstructed using
  false-color coding so that we can see this light.

Chandra X-ray image of the Center of the Milky
Way Galaxy
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Radio Telescopes

• The wavelengths of radio waves are long.
• So the dishes which reflect them must be very
  large to achieve any reasonable angular
  resolution.

305-meter radio telescope at Arecibo, Puerto Rico
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Interferometry

• Two (or more) radio dishes observe the same
  object.
• Their signals are made to interfere with each
  other.
• An image is reconstructed with the angular
  resolution one would get from a dish the size of
  the distance between them.
• The light-collecting area is still only the sum
  of the areas of the individual dishes.

Very Large Array, New Mexico
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Very Long Baseline Array (VLBA)
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Angular resolution of an interferometer
where D is the largest separation between
telescopes.
For example the VLBA has telescopes in Hawaii and
Virgin Islands (8000 km). At a typical radio
wavelength ? 1 cm the angular resolution is
This is the size of a basketball at the distance
to the Moon!
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North liberty Iowa VLBA Radio Telescope (live
webcam)
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Observations at wavelengths other than visible
light are revealing previously invisible sights.
Visible light image
radio wavelength image
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Jupiter Seen with optical and radio telescopes
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Q. The angular resolution of an interferometer is
given by

• The ratio of observing wavelength to largest
  distance between telescopes
• The ratio of observing wavelength to largest
  telescope diameter
• The ratio of smallest telescope diameter to
  observing wavelength
• The ratio of smallest distance to largest
  distance between telescopes
• The ratio of smallest to largest observing
  wavelength

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Observations at other wavelengths are revealing
previously invisible sights.
UV
infrared
Map of Orion region
Ordinary visible
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Telescopes in orbit around the Earth detect
radiation that does not penetrate the atmosphere.

• X-rays, UV, Infrared Atmosphere is opaque (rays
  are absorbed)
• Optical, Radio Atmosphere is transparent

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Other Space ObservatoriesChandra X-ray
ObservatoryXMM-Newton X-ray ObservatoryCompton
Gamma Ray Observatory
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X-ray Telescopes

• Different types of photons behave differently.
• X-rays will pass right through a mirror.
• They can only be reflected/focused at shallow
  angles
• like skimming stones

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The Entire Sky at the Visible Wavelengths
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The Entire Sky at the Radio (21-cm) Wavelengths
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The Entire Sky at the Infrared Wavelengths
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The Entire Sky at the X-ray Wavelengths
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The Entire Sky at the Gamma Ray Wavelengths
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Q. Which type(s) of radiation must be studied
from space?

• Radio
• Optical
• X-ray
• Infrared
• Both X-ray and infrared

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Atmospheric Effects on Observations
Key points

• What is light pollution?
• Do stars really twinkle?
• What atmospheric problems for astronomy can not
  be solved with technology on the ground?