Ideal Detector Characteristics

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Ideal Detector Characteristics

• Characteristics of detectors
• High quantum efficiency
• QE of detections/ of incident photons
• Detections may be
• Crystals formed on a photographic plate (few --
  thus the end of chemical phy)
• Photoelectrons released (5-10 in photomultiplier
  tubes, 90 in PIN diodes)
• Charge-pairs created (gt 50 In CCDs thus
  telescope size evolution)

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2. Low noise

• Want photon-limited noise (photon arrival rate,
  by Poisson statistics, goes as sqrt(n), n
  number of photons)
• Usually actually limited by sky transparency
  changes
• At low photon arrival rates (i.e, faint objects),
  the limit is Johnson noisethermal agitation of
  electrons in the detector.
• Reduce thermal noise by cooling

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• Liquid nitrogen often used in dewars to cool PMTs
  and CCDs.
• Achieves 77 kelvins, lowest possible temperatures
  without extreme efforts (Liquid helium, hydrogen)
  HOW DOES IT DO IT?
• Dangers (explosions if vent becomes clogged)
• Requires keeping topped off as it evaporates.
• May be too cold for PMTs (so, use dry ice
  sublimation 195K)
• NICMOS being repaired solid N ? liquid neon

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• Thermoelectric cooling (the Peltier effect), in
  which DC current passed through a semiconductor
  device makes one side hot and the other cold.
• Can achieve 50-100C below reservoir
• Reservoir may be room air or re-circulated
  coolant (with or without refrigeration).

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• 3. Linear response
• Want response linear with exposure which is the
  product of light flux and exposure time
• Failure of the reciprocal relation R Ft is
  reciprocity loss and is a characteristic of
  chemical photography (note pre-flashing, log-log
  scale).
• CCDs and PMTs are linear over a large range.

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4. Fast time response

• Rise time is the response time to an incident
  photon.
• This important mainly in high-speed photometry
  (occultations, for example), or pulse-counting
  photometry
• Photo at right is oscilloscope trace showing 2ns
  rise time to a single electron generated in a
  2-inch diameter PMT.

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5. Wide dynamic range

• Response to a wide range of light with enough
  resolution of measurement
• Photographic plates, the eye, have a wide range.
  Others limited.
• With PMTs we resolve it by having variable gain
  (amplification) amplifiers
• With CCDs the problem is in avoiding saturation
  while having enough resolution. Need large
  number of bits in A/D converter.

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6. Wide spectral bandwidth

• Limited by physics of detection
• Peak may be set by choice of semiconductors used
• Photographic plates have different emulsion
  formulas to yield different color response
  (Panochromatic..)
• Modified by filters used and reflectivity of
  mirrors.

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7. Ability to integrate or amplify

• PMTs and PIN diodes need amplification. PMTs
  internal gain of 106 needs external
  amplification. Diodes have no internal gain
  (more later)
• CCDs have unity gain but can integrate.
• We effectively integrate PMT measurements by
  averaging the signal or counting photons.

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8. For 2-D detectors good spatial resolution

• Want a lot of pixels across the image.
• Photography good.
• CCDs small and few pixels
• For science want gt 2 pixels per seeing disk FWHM
  and image scale (chalk talk)
• For pretty pictures need 300 dpithats a lot
  of pixels!

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