Telescopes and the Atmosphere

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Telescopes and the Atmosphere
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Attendance Quiz
Are you here today? (a) yes (b) no (c) here? do
you mean physically, mentally, or what?
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Midterm 1
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Exam Grades

• To help those of you who may have struggled on
  the first midterm, I am instituting the following
  exam grading policy
• The lower of your two midterms scores can be
  replaced by your final exam, if your final score
  is higher than either midterm
• Thus, if you get a final exam score higher than
  either midterm, the exam portion of your grade
  (which is 60 of your total grade) will be 40
  final exam, 20 higher midterm
• If you get a final exam score lower than both
  midterms, your exam grade will be 20 final exam,
  20 each of the two midterms
• Thus, even if you bombed the first midterm, you
  can still do well in this class
• This policy only applied to exams you take. If
  you skip the second midterm, it will count as a
  zero, no matter what!
• Remember, there is no curve in this class, so
  everyone wins with this policy!

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Todays Topics

• Telescopes
• Your eye v. a telescope
• Collecting area
• Angular resolution
• Reflecting v. refracting telescopes
• What are telescopes used for?
• Non-optical telescopes
• The Atmosphere
• Light pollution
• Twinkling
• Atmospheric transmission
• Telescopes in space

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Your Eye v. a Telescope I

• Fundamentally, your eye and a telescope work in a
  similar manner - light is bent by a lens (or
  mirror) to make an image

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Your Eye v. a Telescope II

• There are three major
• advantages that a telescope
• has over your eye
• A telescope has a larger area to collect light
  and therefore can see fainter objects
• A detecting device can be attached to a telescope
  to collect light for a longer time
• A telescope has better angular resolution than
  your eye

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Collecting Area

• The sensitivity to light of a telescope is a
  function of its collecting area
• This is the area of the primary light collecting
  surface
• Most telescopes are described by the diameter of
  their primary mirror (or lens)
• Area pr2 p(D/2)2 (p/4)D2 0.8D2
• For Gemini, the area is 0.8(8m)2 50m2
• (about 106 times your dilated pupil)
• Since collecting area is proportional to the
  square of the diameter, a telescope with twice
  the diameter will have four times the collecting
  area

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Collecting Area Quiz
One Keck telescope on Mauna Kea has a diameter of
10m, compared to 5m for the Palomar telescope.
The light gathering power of the Keck is larger
by a factor of a) 2 b) 4 c) 15 d) 50 e) There
is not enough information to tell
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Angular resolution

• Angular resolution refers to the ability to
  distinguish two objects that appear very close
  together in the sky
• In the absence of blurring effects from the
  atmosphere, the angular resolution of a telescope
  is determined by the wavelength of light and the
  diameter of the telescope
• A small angular resolution is good because it
  means you can separate objects that are very
  close together
• Angular resolution is determined by the formula ?
  ?/D

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Angular Resolution Quiz
The Hubble Space Telescope has a primary mirror
diameter of 2.4 m. The Keck telescopes, which
use adaptive optics to correct for atmospheric
effects, have a primary mirror diameter of 10 m.
The angular resolution (ability to see details)
of the Keck telescopes is smaller (better) than
the Hubble Telescope by a factor of about a)
2.4 b) 4 c) 10 d) 16 e) 24
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Refracting v. Reflecting Telescopes

• Refracting telescopes use a lens (like your eye
  but larger) to create an image
• Reflecting telescopes use a mirror to focus light
• Advantages of a reflecting over a refracting
  telescope
• A lens of a given size is heavier than a mirror,
  which can be made of a thin or honeycombed
  lightweight material with a thin aluminum coating
• The lens of a large refracting telescope will
  distort (sag) under its own weight
• The lens in a refracting telescope must be very
  high quality
• Different colors of light are bent differently,
  so colors are not all focussed together in a
  refracting telescope (chromatic aberration)

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What are telescopes used for? - Imaging

• The primary use of a telescope is to make an
  image of an object
• Originally, images were captured on photographic
  film modern detectors are electronic (like the
  CCD chip in your digital camera)
• Every image consists of only certain wavelengths
  of light
• Astronomers often use filters that only allow
  certain colors or wavelengths to pass through
• The image at right is actually a composite image
  of 3 colors
• Most color images in astronomy are made this way

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What are telescopes used for? - Spectroscopy

• Another common use of telescopes is to take a
  spectrum of an object, a process known as
  spectroscopy
• A spectrum consists of a plot of relative
  brightness of an object as a function of
  wavelength
• The resolution of a spectrum (the spectral
  resolution) determines how much detail can be
  seen (similar to how angular resolution
  determines how much spatial detail can be seen in
  an image
• As we will learn later, the spectrum of an object
  can be like a signature for certain elements
  being present in the object

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Electromagnetic Spectrum

• There are many types of EM radiation besides
  visible light
• Astronomers are interested in all types of EM
  radiation from astronomical objects
• Some things can only be learned by observing
  outside the visible portion of the spectrum

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Non-optical telescopes

• Telescopes, and detectors, can be built to detect
  radiation from throughout the EM spectrum
• Because of the spectral resolution equation (?
  ?/D), radio telescopes are often much larger than
  optical telescopes
• It is also possible to combine multiple
  telescopes to create a telescope with the angular
  resolution (but not collecting area) of a larger
  telescope

Arecibo Radio Telescope Diameter is 305 m (1000
ft)
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Very Long Baseline Array (VLBI)
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Non-optical telescopes

• As we will see shortly, the atmosphere only
  transmits certain wavelengths thus, to observe
  those wavelengths it is necessary to observe from
  balloons, airplanes, and space

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Telescopes and the Atmosphere

• The atmosphere is vital to our existence, not
  least because we breathe it!
• However, for astronomers, the atmosphere is a
  nuisance, and sometimes they need to work around
  its limitations
• One example of this is light pollution
• Notice the difference between this image of the
  sky from a city
• and this one from a dark site

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Telescopes and the Atmosphere

• The atmosphere is turbulent, causing light from
  space to appear to jitter or jump around
• This happens so fast, that our eyes perceive this
  as twinkling, and the image of a star is
  smeared out
• By going into space (or by using fancy correction
  schemes known as adaptive optics) it is possible
  to eliminate this problem, and reach the
  theoretical angular resolution of the telescope
  (? ?/D)

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Telescopes and the Atmosphere

• The atmosphere absorbs and scatters radiation
  from space
• This can be beneficial, as in the case of UV
  radiation
• However, astronomers see the atmosphere as a
  nuisance, and sometimes need to work around its
  limitations
• Some wavelengths can only be seen on mountaintops
  (e.g., IR), others from airplanes or balloons
  (far-infrared, gamma rays) and still others only
  from space (UV, X-rays)

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Non-ground-based telescopes

• As we have seen, there are many reasons for some
  telescopes to be placed in other places besides
  the ground
• Moving to a mountaintop can improve observing at
  visible wavelengths, and most major observatories
  are at high altitude
• Some wavelengths (e.g., far-infrared) can be
  observed from airplane altitudes
• Some wavelengths can only be observed from space
• In addition, there are advantages (but also
  costs) of observing all wavelengths in space
  (e.g., Hubble)

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Telescopes Quiz I
Which of the following wavelengths can be easily
observed by a telescope located on the Earths
surface? a) gamma ray b) X-ray c) ultraviolet
d) radio e) none of the above wavelengths
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Lecture Tutorial Telescopes Earths
Atmosphere, pp. 49-51

• Work with one or more partners - not alone!
• Get right to work - you have 15 minutes
• Read the instructions and questions carefully.
• Discuss the concepts and your answers with one
  another. Take time to understand it now!!!!
• Come to a consensus answer you all agree on.
• Write clear explanations for your answers.
• If you get stuck or are not sure of your answer,
  ask another group.
• If you get really stuck or dont understand what
  the Lecture Tutorial is asking, ask me for help.

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Telescopes Quiz II
Imagine that you are the head of a funding agency
that can afford to build one telescope. Which
of the following proposed telescopes would be
best to support? a) a radio telescope in orbit
around Earth b) an ultraviolet telescope in orbit
around Earth c) an X-ray telescope located in
New Mexico
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Telescopes Quiz III

• For the last question of the tutorial, you had to
  choose between
• Project Zeta, a radio telescope, on the desert
  floor, which will be used to detect potential
  communications from distant civilizations, and
• Project Epsilon, an infrared telescope, located
  in the high-elevation mountains of Chile, which
  will be used to view newly forming stars
• Which did you choose to fund?
• Project Zeta
• Project Epsilon

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Cal Poly Pomona students participate in SETI
researchAstrobiology and the Search for
Extraterrestrial Intelligence

• Research at the SETI Institute in Mountain View,
  California
• Projects include
• IR spectrocsopy of ethane/water mixtures to study
  early solar system (Ashley Curry)
• Planetary geology and geomorphology of Mars
  (Amber Butcher)
• Mapping of meteor showers to study matter in the
  Solar System (Steffi Valkov)

Part of the Allen Telescope Array (ATA)
SETI Gurls
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Real life v. the Movies
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Special Public Lecture

• Are We Alone?
• Dr. Jill Tarter
• Director for SETI Research
• SETI Institute
• Friday, May 6
• 7pm
• Ursa Major C
• 5 clicker points extra credit for attending

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SETI Gurls