Why Use a Telescope?

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Why Use a Telescope?

• All astronomical objects are distant so a
  telescope is needed to
• Gather light -- telescopes sometimes referred to
  as light buckets
• Resolve detail
• Magnify an image (least important of the three)
• Uses combination of lenses and/or mirrors

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Refracting Telescope

• Refraction as light passes from one medium to
  another (e.g. air to glass) it is bent
• Light is gathered and focused by a curved lens.

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Refracting Telescope

• First telescopes were of this type
• Not made for astronomical use any more
• Very difficult to make large, defect-free lenses
• Weight of large lenses make them deform over
  time.

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Reflecting Telescope

• A curved mirror is used to collect and focus the
  light.

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Reflecting Telescope

• Used in modern telescopes
• Mirror can be supported from the back.
• Do no need large, defect free glass since surface
  is coated with reflective material.

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Focal Length

• Distance from lens or mirror to focus focal
  length.
• Rarely will put and eye or detector at the prime
  focus.
• An eyepiece (usually a lens) will be used to
  magnify the image.

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Eyepiece and Magnification
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Telescope Design

• For a reflecting telescope, a secondary mirror is
  used to reflect the image to a detector outside
  of the telescope.

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Observing the Image

• On research telescopes astronomers rarely look
  through the telescope
• Detector is put at the focus to record a digital
  image.
• Data is processed on a computer

CCD Chip
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Telescope Properties Gathering Light

• The larger the area of the primary mirror, the
  more light can be collected and the fainter the
  object we can detect.

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Gathering Light

• Can combine many smaller mirrors to make one
  large area
• Keck telescope primary mirror made of 36
  hexagonal mirrors

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Telescope Properties Angular Resolution

• The smallest separation in angle which can be
  observed by the telescope

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Angular Resolution

• Absolute limit of angular resolution

• best angular resolution in arcseconds
• ? wavelength (meters)
• D diameter of mirror (meters)

• Want ? to be small
• Smaller wavelengths better resolution
• Larger mirror better resolution

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Angular Resolution
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Angular Resolution

• Resolution usually limited by motions in the
  atmosphere (twinkling)
• Need site with calm, dry weather, little
  atmosphere above the telescope to reduce effect.
• Adaptive optics sensors monitor distortions due
  to atmosphere and correct shape of mirror 10-100
  times per second
• Very Large Telescope (8.2 meters)

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Adaptive Optics
Object viewed with adaptive optics
Object viewed through typical telescope
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Hubble Space Telescope
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Spectroscopy

• Light coming though telescope is separated by
  prism or diffraction grating to produce a
  spectrum.

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Observing at Other Wavelengths

• Wavelengths other than visible are very useful
  since they are often produced by different
  objects or processes than optical light
• Atmosphere blocks some types of light
• Low opacity Optical and Radio
• Medium opacity Infrared and UV
• High opacity Gamma Rays, X-rays some UV

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Infrared Telescopes

• Telescope design much like optical telescope, but
  with different detector.
• IR light can pass through dust
• Used to observe star formation, center of
  galaxies, low T objects (i.e. planets)
• Best if observed above much of the atmosphere.

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IRAS
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Radio Telescopes

• Can observe day or night
• Not affected by Earths atmosphere
• Radio light can pass though dust in space
• Because wavelength is long, we need large
  telescope to get good resolution

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64 m telescope at Parkes Obs. in Austrailia
305 m telescope at Arecibo Observatory in Puerto
Rico
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Interferometers

• More than one radio telescope is used to increase
  resolution.
• Large effective diameter
• Image made only after much computer processing

Very Large Array in New Mexico
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Ultraviolet, X-rays and Gamma Rays

• All blocked by atmosphere
• Telescopes must be above atmosphere

Compton Gamma-ray Observatory
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Sky at Many Wavelengths