CCDs

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CCDs
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CCDsthe good ()

• Linear response ? photometry is simple
• High efficiency, compared to other detectors
• Sensitive to many wavelengths
• 2-D arrays possible in large formats
• Can be shuttered, or frame transfer
• High dynamic range (i.e., contrast)

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CCDsthe bad (-)

• Read noise electrons not transferred perfectly
  (but pretty good)
• Dark current
• Temperature sensitive
• Coolant/cooler?
• Accumulates condensates (i.e., gook)

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CCDsthe ugly

• Electron wells are finite and imperfect
• Leakage
• Saturation blooms
• Cosmic rays
• Pixel-to-pixel variation
• Age-dependent

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Linearity (good)

• Expose to more light, get more electronslinearly
  increasing
• Photometry made easier because signal can be
  expressed as (data numbers per second),
  unambiguously
• Makes comparison of different images easier

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Linearityone more thing

• Allows straightforward normalization and addition
  of images

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Quantum efficiency

• Generally, higher than most other detection
  schemesthats good (especially photographic
  film)
• Wavelength dependent

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Exposure metering

• Can be shuttered, or
• Can be frame transfer

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Dark current

• Thermal motions of electrons produces a spurious
  signal that is not due to incident light
• Temperature-dependent, so most cameras are cooled
• The level of spurious signal is still linear
  w.r.t. temperature and exposure duration, so can
  be subtracted from the real images
• Examples

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Temperature dependence of dark current
Q Whyd we do this? A The camera is cold, so
any residue floating around in the telescope will
condense on the CCD. Yuck! Therefore, periodic
bakeouts to remove gook from the CCD.
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Pixel-to-pixel variation

• Differences in charge transfer efficiency
• Differences in well depth (less important)
• Shorted pixels continuously leaking charge
• Age-dependent (see example)
• Compensate via flat fielding and subtraction of
  dark frames

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CCD aging
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Pixel saturation

• Potential wells have a finite depth, can hold
  only a finite number of electrons
• 100,000 to 200,000 electrons is typical limit
• When the well is full, where do those electrons
  go?
• They spill over into neighboring pixels
• Example

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Composite Example
1. Raw data
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Composite Example
2. Subtract dark frame
3. Correct for stray light (no true flat fields
for X-rays)
4. Co-register the cleaned images, normalize for
exposure time 5. Replace saturated parts of
long exposure with pixels from short
exposure, to yield the final product
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Why composite?

• Trivial answer It looks nice.
• Less trivial answer Enhanced dynamic range. You
  get to see the faint parts and the brighter
  parts, with quantitative accuracy.

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One more thing cosmic rays

• Troublesome
• Aesthetically unpleasant
• Confuse morphology of imaged object
• How to remove?