The Imaging Chain for Optical Astronomy

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The Imaging Chain for Optical Astronomy
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Review/overview

• The imaging chain typically includes the
  following elements
• energy source
• object
• collection
• detection
• processing
• display analysis

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Source/object

• In astronomy, the source of energy (light) is
  almost always also the object of the imaging
• exceptions planets, dust reflecting starlight
• Astronomical sources place specific requirements
  on astronomical imaging systems
• requirements are often conflicting
• excellent angular resolution wide field of view
• high sensitivity large dynamic range
• broad wavelength coverage spectral lines

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More luminous objects can be detected out to
larger distances
Lines of constant apparent brightness
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More distant objects are usually larger in
physical size
Lines of constant angular size
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Angular sizes span a wide range
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Atmosphere modifies source

• For ground-based optical astronomy, Earths
  atmosphere plays a large role in determining the
  character of the source
• scintillation modifies source angular size
• twinkling of stars smearing of point sources
• extinction cuts down on light intensity
• atmosphere scatters a small amount of light,
  especially at short (bluer) wavelengths
• water vapor blocks out specific wavelengths, esp.
  in near-IR
• scattered light produces interfering background
• astronomical images are never limited to light
  from source alone always include source
  background sky
• light pollution worsens sky background

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Collection Telescopes

• 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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Why build big telescopes?

• Larger aperture means more light gathering power
• sensitivity goes like D2, where D is diameter of
  main light collecting element (e.g., primary
  mirror)
• Larger aperture means better angular resolution
• resolution goes like lambda/D, where lambda is
  wavelength and D is diameter of mirror

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Why build small telescopes?

• Smaller aperture means less chance of saturation
  (overexposure) on bright sources
• Smaller aperture generally means larger field of
  view
• recall F ratio, Ff/D, where f is focal length of
  collecting element and D is diameter of aperture
• for two reflecting telescopes with same F ratio
  and the same size detector, the telescope with
  smaller D produces images that cover a wider angle

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Detection Cameras for Astronomy

• Camera usually includes
• filters
• most experiments require specific wavelength
  range(s)
• broad-band vs. narrow-band
• reimaging optics
• enlarge or reduce image formed by primary
  collecting element
• detector
• Most common detectors
• The eye
• Photographic emulsion
• film
• plates
• CCDs

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The eye as astronomical detector

• Must reimage the image formed by the primary (or
  objective) such that the light rays are parallel
  as they enter the eye (i.e. rays appear to come
  from infinity)
• reimaging is accomplished by the eyepiece
• Point sources (stars) appear brighter to the eye
  through a telescope by a factor D2/P2 , where D
  is telescope diameter and P is the diameter of
  the eyes pupil
• for maximum effect, magnification has to be
  sufficient for light to fill pupil
• Extended sources (for example, nebulae) do not
  appear brighter through a telescope
• Gain in light gathering power is exactly
  compensated by magnification of image, which
  spreads light out

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Photographic techniquessilver halide

• film
• large amount of work is still done by amateurs
  using highly sensitive BW and color film
• plates
• from the earliest development of AgX techniques
  until advent of CCDs in late 70s, most images
  were captured on photographic plates
• panes of glass overlaid with silver halide
  emulsion

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CCDs

• charge coupled devices (CCDs) are now standard
  light detection medium for professional and
  amateur astronomical imaging systems alike
• numerous advantages over film
• high quantum efficiency (QE), meaning most
  photons incident on a CCD are detected
• linear response, meaning signal builds up in
  direct proportion to number of photons collected
• fast processing turnaround (CCD readout speeds 1
  sec)
• regular grid of pixels (as opposed to random
  distribution of AgX grains)
• image delivered in computer-ready form

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Image processing

• Once images are collected, they need to be
  corrected for
• Atmosphere (to the extent possible)
• e.g., sequence of images obtained at a variety of
  telescope elevations usually can be corrected for
  atmospheric extinction
• CCD defects and artifacts
• dark current
• CCD pixel reports a signal even when not exposed
  to light
• bad pixels
• some pixels will be dead, hot, or even
  flickering
• variations in pixel-to-pixel sensitivity
• every pixel has its own QE
• can be characterized by flat field

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Image display analysis

• Often, this step in the imaging chain is where
  the astronomy really begins.
• Type and extent of display and analysis depends
  on purpose of imaging experiment
• Common examples
• evaluating whether an object has been detected or
  not
• determining total CCD signal (counts) for an
  object, such as a star
• determining relative intensities of an object
  from images at two different wavelengths
• determining relative sizes of an extended object
  from images at two different wavelengths