Astronomical Imaging

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Astronomical Imaging

• Telescopes and Detectors

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Astronomical Imaging

• GOAL image large objects at VERY large distances
  (typically measured in light years, ly)
• Nearest star alpha Centauri, 4.3 ly
• Nearest galaxy Andromeda, 3 million ly
• Edge of universe 15 billion ly
• REQUIREMENTS
• High angular resolution (where possible)
• High telescope/detector sensitivity

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Angular Resolution

• Angular resolution ability to distinguish
  detail
• Easy yardstick for grasping resolution the Moon
• Moons disk 1/2 degree across (same for Sun)
• 1 degree 60 arc minutes 1 arc minute 60 arc
  seconds
• unaided eye can distinguish shapes/shading on
  Moons surface (resolution 1 arc minute)
• w/ small telescope can distinguish large craters
  (resolution a few arc seconds)
• w/ large telescope can see craters 1/2 mile (1
  arc second) across

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Angular Resolution

• Factors determining angular resolution
• Diameter of main light collecting surface (mirror
  or lens) of telescope
• determines diffraction limit of telescopic
  imaging system
• Quality of telescope collecting surface
• smoother surface better resolution
• Atmospheric effects
• turbulence smears image
• essentially same effect as stars twinkling

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Sensitivity

• Sensitivity ability to detect faint sources of
  electromagnetic radiation
• Telescope sensitivity proportional to its light
  collecting area (area of mirror or lens surface)
• Detector sensitivity measured by its quantum
  efficiency (fraction of input photons that
  generate signal in detector)
• Also, need the ability to expose the detector
  (integrate) for very long periods of time

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Telescopes Basic Flavors

• Refractor telescopes
• exclusively use lenses to collect light
• have big disadvantages aberrations sheer
  weight of lenses
• Reflector telescopes
• use mirrors to collect light
• relatively free of aberrations
• mirror fabrication techniques steadily improving

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Optical Reflecting Telescopes

• Use parabolic, concave primary mirror to collect
  light from source
• modern mirrors for large telescopes are
  lightweight deformable, to optimize image
  quality

3.5 meter WIYN telescope mirror, Kitt Peak,
Arizona
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Optical Reflecting Telescopes

• Basic optical designs
• Prime focus light is brought to focus by primary
  mirror, without further deflection
• Newtonian use flat, diagonal secondary mirror to
  deflect light out side of tube
• Cassegrain use convex secondary mirror to
  reflect light back through hole in primary
• Nasmyth focus use tertiary mirror to redirect
  light to external instruments

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Optical Reflecting Telescopes
Schematic of 10-meter Keck telescope
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Big Optical Telescopes
Keck telescope mirror (note person)

• Largest telescopes in use or under construction
• 10 meter Keck (Mauna Kea, Hawaii)
• 8 meter Subaru (Mauna Kea)
• 8 meter Gemini (Mauna Kea Cerro Pachon, Chile)
• 6.5 meter Mt. Hopkins (Arizona)
• 5 meter Mt. Palomar (California)
• 4 meter NOAO (Kitt Peak, AZ Cerro Tololo, Chile)

Summit of Mauna Kea, with Maui in background
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Radio Telescopes

• Usually Cassegrain in design
• primary mirror is replaced by parabolic
  reflector dish
• secondary is called subreflector

12 meter radio telescope, Kitt Peak, Arizona
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Radio Telescopes

• Since wavelength of interest is longer, must
  increase telescope aperture to achieve good
  angular resolution
• alternative is to use an array of radio
  telescopes

Very Large Array, New Mexico
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X-ray Telescopes

• Use grazing incidence optics to defeat tendency
  for X-rays to be absorbed by mirrors
• Tiny wavelength, so exceedingly difficult to
  produce smooth mirrors for tight focus
• Chandra is first X-ray telescope to achieve lt1
  arcsecond resolution

Chandra X-ray telescope mirror design
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Detectors

• Optical CCDs rule
• film replaced by CCDs by early 80s
• detector formats (sizes) continually growing
• 1024x1024 industry standard
• 4096x4096, CCD arrays no longer uncommon
• IR CIDs (near-IR), bolometers (far-IR)
• CIDs similar to CCDs but each pixel addressed
  independently
• bolometers directly measure heat input

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Detectors

• Radio receivers
• original (50s-60s) technology similar to that
  of home stereo use
• now emphasize extremely high sensitivity and
  extremes in radio frequency range
• X-ray proportional counters, CCDs
• prop. counters efficiently convert X-ray energies
  to voltages
• CCDs provide better X-ray position energy
  determination

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Observatory Sites

• The best telescope/detector is useless at a bad
  site!
• Factors for consideration of appropriate site
• atmospheric transparency at wavelength of
  interest
• atmospheric turbulence
• sky brightness
• accessibility

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Observatory Sites

• Optical work
• need dark, cloud-free site
• helps to remove atmosphere from system (e.g.,
  Hubble)!
• IR work
• need cold site
• dry site very important at certain wavelengths
• radio work
• need dry site (shorter wavelengths)
• need interference-free site (longer )
• X-ray work
• need to be above atmosphere

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Optical/IR Telescopes

• Dark, high, dry most big optical/IR telescopes
  are placed on mountaintops in deserts

Kitt Peak, Arizona
Mauna Kea, Hawaii
Gemini South, Chile
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IR Telescopes

• For optimum IR work, need high, dry, cold site
• South Pole works well, but accessibility an issue

Center for Astronomical Research in Antarctica
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IR Telescopes

• Helps to go into space, or at least above the
  bulk of the atmosphere

SIRTF NASAs Space Infrared Telescope Facility
SOFIA NASAs Stratospheric Observatory for IR
Astronomy
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X-ray Telescopes

• Must go above atmosphere to detect celestial
  objects! (X-rays are absorbed by Earths
  atmosphere)

Chandra is in high Earth orbit